WTP-64WORLD BANK TECHNICAL PAPER NUMBER 64

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The Efficient Use of Water in IrrigationPrinciples and Practices for Improving Irrigationin Arid and Semirid Regions

Daniel Hillel

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WORLD BANK TECHNICAL PAPER NUMBER 64

The Efficient Use of Water in IrrigationPrinciples and Practices for Improving Irrigation

in Arid and Semiarid Regions

Daniel Hillel

The World BankWashington, D.C.

The International Bank for Reconstructionand Development/THE WORLD BANK1818 H Street, N.WWashington, D.C. 20433, U.S.A.

All rights reservedManufactured in the United States of AmericaFirst printing June 1987

Technical Papers are not formal publications of the World Bank, and are circulatedto encourage discussion and comment and to communicate the results of the Bank'swork quicldy to the development community; citation and the use of these papersshould take account of their provisional character. The findings, interpretations, andconclusions expressed in this paper are entirely those of the author(s) and should notbe attributed in any manner to the World Bank, to its affiliated organizations, or tomembers of its Board of Executive Directors or the countries they represent. Any mapsthat accompany the text have been prepared solely for the convenience of readers; thedesignations and presentation of material in them do not imply the expression of anyopinion whatsoever on the part of the World Bank, its affiliates, or its Board or membercountries concerning the legal status of any country, territory, city, or area or of theauthorities thereof or concerning the delimitation of its boundaries or its nationalaffiliation.

Because of the informality and to present the results of research with the leastpossible delay, the typescript has not been prepared in accordance with the proceduresappropriate to formal printed texts, and the World Bank accepts no responsibility forerrors. The publication is supplied at a token charge to defray part of the cost ofmanufacture and distribution.

The most recent World Bank publications are described in the catalog NewPublications, a new edition of which is issued in the spring and fall of each year. Thecomplete backlist of publications is shown in the annual Index of Publications, whichcontains an alphabetical title list and indexes of subjects, authors, and countries andregions; it is of value principally to libraries and institutional purchasers. The continuingresearch program is described in The World Bank Research Program: Abstracts of CurrentStudies, which is issued annually. The latest edition of each of these is available free ofcharge from the Publications Sales Unit, Department F, The World Bank 1818 H Street,N.W, Washington, D.C. 20433, U.S.A., or from Publications, The World Bank, 66,avenue d'lena, 75116 Paris, France.

Daniel Hillel is Professor of Plant and Soil Sciences at the University ofMassachusetts and a consultant to the World Bank.

Library of Congress Cataloging-in-Publication Data

Hillel, Daniel.The efficient use of water in irrigation.

(World Bank technical paper, ISSN 0253-7494 ; no. 64)Bibliography: p.1. Irrigation efficiency. 2. Irrigation.

I. Title. II. Series.S619.E34H55 1987 631.5'87 87-14281ISBN 0-8213-0914-5

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ABSTRACT

This book is in the nature of a primer, providing abasic review and analysis of the principles governing soil-crop-water-climate relationships, irrigation, and theefficient utilization of water in arid and semiarid regions.It presents a critique of traditional and of currentirrigation concepts and practices, pointing out the needs andpotentialities for improving the efficiency of land and wateruse in developing countries. Starting from a basic analysisof the environmental, physiological, and agronomic factorsaffecting irrigation, the book contrasts historical andmodern approaches to management. It then describes methodsof scheduling irrigation and of measuring irrigation water,and compares alternative irrigation systems. It alsospecifies the requirements and methods of drainage andsalinity control. Finally, this book discusses some of thehuman considerations involved in the vital task of developingsound, appropriate, and sustainable irrigation systems.

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CONTENTS

Preface and Acknowledgments ........... .. ................ vii

1. The Task of Irrigation Development .................. 1

2. Environmental Processes ............ 5The Field Water Cycle and Water Balance ... ........ 5Radiation Exchange and Energy Balance .... ......... 11Potential Evapotranspiration and Pan Evaporation .. 14

3. Crop - Water Relations .............................. 19Water in the Physiology of Crops .................. 19Crop Evapotranspiration and Water Requirements .... 21Water Use - Yield Relationships ................... 25

4. Irrigation Management ............................... 29Historical Concepts of Irrigation Management ...... 29Modern Concepts of Irrigation Management ... ....... 31Irrigation Scheduling ............................. 34Efficiency of Water Use and Water Conservation .... 42Measurement of Irrigation Water ................... 45

5. Irrigation Methods .................................. 50Surface Irrigation ................... ............. 51Sprinkle Irrigation ............................... 61Drip Irrigation ................................... 69Micro-Sprayer Irrigation .......................... 75Low-Head Bubbler Irrigation ....................... 76Considerations and Comparative Costs ............... 78

6. Drainage and Salinity Control ....................... 81Drainage Needs and Criteria ....................... 81Soil Salinity and Leaching Requirements ... ........ 86Quality of Irrigation Water ....................... 89

7. Issues and Implications ......................... 95Human Aspects of Irrigation Development ... ........ 95A Look Back ....................................... 97A Look Ahead ....................................... 99

References .............................................. 101

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PREFACE AND ACKNOWLEDGMENTS

Irrigation is playing an increasingly important role inthe agricultural economy of drought-prone regions. Aspracticed in many places, however, it is still based largelyon antiquated methods of distribution and application whichfail to measure and optimize the supply of water needed tosatisfy the time variable demands of different crops.Unmeasured irrigation tends to waste water, nutrients, andenergy, and may cause soil degradation by water-logging andsalinization, particularly where the necessity for drainageis neglected.

The vital task of increasing agricultural production inthe less developed countries must include a concerted effortto modernize existing tradition-bound irrigation schemes soas to achieve higher levels of profitable and sustainableproduction. New schemes being planned should likewise bebased on sound principles and techniques for attaininggreater control over the soil-crop-water regime and foroptimizing irrigation in relation to all other essentialagricultural inputs and operations. Water and soil must berecognized as the precious and fragile resources that theyare, and managed accordingly.

In recent years, revolutionary developments have takenplace in the science and art of irrigation. A morecomprehensive understanding has evolved of the interactiverelationships governing the soil-crop-water regime asaffected by climate and irrigation methods. These scientificdevelopments have been paralleled by a series of technicalinnovations in the methodology of water control which havemade it possible to establish and maintain nearly optimalsoil moisture conditions practically continuously. Foremostamong these innovations are techniques for high-frequency,low-volume applications of water (and nutrients) in preciseand timely response to changing crop needs. The advent ofrelatively inexpensive, permanently or seasonally installed("solid set") water application systems, and the developmentof self-controlling ancillary devices, have apparentlyremoved some of the prior economic constraints to the wide-spread adoption of high-frequency irrigation.

Properly applied, the new irrigation methods can raiseyields while minimizing waste (by runoff, evaporation, andexcessive seepage), reducing drainage requirements, andpromoting the integration of irrigation with essentialconcurrent operations (e.g., fertilization, tillage, and pestcontrol). The use of brackish water has become morefeasible, as has the irrigation of coarse-textured soils andof steep, sandy, or stony lands previously considered totallyunproductive. Such advances and their consequences couldhardly have been foreseen in the irrigation literature ofprior decades.

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Despite all the advances, old and inefficient ways stillpersist throughout many irrigated areas. On a world-widebasis, the modern technologies have been applied to only some3% of the land under irrigation. In too many places,inefficiency is perpetuated by institutionally imposedstandards based on excessive and hence wasteful applicationsof water. However, institutional inertia and conservativeattitudes are only part of the problem. Some of the newirrigation systems developed in the industrialized countriesare indeed too highly mechanized, complex, energy intensive,and large in scale to be directly applicable to the low-capital, low-technology circumstances of unindustrializedcountries, where farming is often practiced on a small scaleand the relative costs of labor and capital are verydifferent. Hence ready-made modern technology often failswhen introduced in toto arbitrarily into developingcountries. Elaborate and expensive systems, imported in thegrand hope of achieving instant modernization, can quicklybecome "white elephants," monuments to hasty "progress" basedon inappropriate technology. The best principles of modernirrigation should be disseminated, not necessarily the mostelaborate machinery. Rather than simply transfer technology,we need to adapt or redesign it to suit differing conditions.

irrigationists engaged in development projects needsound, periodically updated information on developments inthe science and art of modern irrigation, as an aid inevaluating and selecting modern irrigation systems which canbe tailored to different combinations of soils, crops, andclimates. However, the available literature on modernirrigation often seems too voluminous and fragmented. In arapidly changing field, the formerly up-to-date references ofyester-decade inevitably diminish in relevance as subsequentinnovations multiply.

This publication is an attempt to distill and summarizecurrently available information on modern irrigation and topresent it in a fundamental, yet simplified, form. An efforthas been made to keep the presentation as readable aspossible so as to reach a variety of readers, from centralplanners and design engineers to agronomists and projectmanagers. In keeping with the limited scope of this book,which is meant to be expository rather than encyclopedic,only selected references are given to illustrate thehighlights of the subject. Even so, some readers may findthe presentation to be too theoretical, while others mayconsider it too simplistic. That is the risk one must takein any attempt to communicate a comprehensive body ofknowledge to a diverse readership.

The main aim of this book is to present an integratedconception of the composite soil-crop-climate system, whichis the scientific basis of modern irrigation development.Therefore, it is not designed to be a directly utilitarian or

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technical "how-to" handbook, but rather a "what" and "why"discourse, laying the factual and conceptual foundations forindependent and flexible decision-making in the area ofirrigation development. The author believes that acquiringfundamental knowledge of how a system works is, in the longrun, the most practical approach toward the system'smanagement. A basic understanding of principles andprocesses should enable practitioners to adjust theirthinking to unforeseen situations, whereas ready-madeprescriptions are necessarily specific and inflexible andtherefore rarely apply as new problems arise in varyingcircumstances.

Above all, the author hopes that the informationcontained herein will contribute to a better and widerawareness of both the potentialities and the limitations ofmodern irrigation methods, and may thus promote theknowledgeable selection and adaptation of appropriatetechnologies for greater sustainable production and forbetter resource utilization and conservation. Obviously, nosingle work by one or even several authors can completelyencompass so important, complex, and active a field as moderniLrrigation. However, even if it does no more than suggestthe need to re-examine traditional practices and to questionsome of the premises on which irrigation projects were basedin the past, this book will have served its purpose.

I am grateful to W. David Hopper, Vice President of theWorld Bank, for his personal interest and inspiringprofessional vision; to Guy Lemoigne, Irrigation Advisor tothe World Bank, who commissioned this book; and to ShawkiBarghouti, Irrigated Crops Advisor in the same Department,for his wise and sympathetic counsel and for his brotherlyencouragement. My students and colleagues Nadim Khouri,Ralph Baker, and David Edelstein, of the University ofMassachusetts, reviewed the manuscript and helped to improveit, as did Editor Virginia Hitchcock of the World Bank'sPublications Department. Finally, I am profoundly gratefulto my wife Michal, who restored my spirit and made thewriting of this book possible.

1. THE TASK OF IRRIGATION DEVELOPMENT

The needs of growing populations and developing nationaleconomies demand intensification of land and water usefor the purpose of increasing and stabilizing agriculturalproduction. These needs are most acute in the arid and semi-arid regions, where, by a cruel stroke of nature, the waterrequirements of crops are greatest even while the suppliesby rainfall are least. Such regions occupy over half ofAfrica and sizable portions of South America and of western,southern, and central Asia (as well as most of Australia andparts of North America). Over 500,000,000 of the earth'speople live in these regions, many of them barely able to ekeout a living from the limited resources of land and wateravailable to them. Under such conditions, even a slightimprovement of the water economy may spell the differencebetween marginal subsistence and profitable production.

Water constitutes a major constraint to increasing cropproduction in our hungry world. To grow successfully, eachcrop must achieve a water economy such that the demand madeupon it by the climate is balanced by the supply available toit. The problem is that the evaporative demand of thealtmosphere is practically continuous, whereas the supply ofwater by natural precipitation is only sporadic. To surviveduring dry spells between rains, the crop must rely on thelimited reserves of extractable moisture temporarily presentin the pores of the soil. So tenuous and delicate is thewater economy of most crop plants that even short-termdeprivation can cause sufficient stress to impair normalphysiological functions and reduce yields.

How efficient is the soil as a water reservoir forcrops? How readily can crop plants draw water from the soilin varying climatic conditions and to what limit can soilmoisture continue to sustain adequate crop growth? What ist]he functional dependence of crop yield on water supply?What are the optimal water requirements of various crops indifferent regions and how can they be determined in practice?How can soil moisture (and associated factors) be controlledin an effort to optimize soil conditions affecting cropyields? Given a limited water supply, how much land can beirrigated and what are the best crops to raise, consideringthe nature of the soil, the climate, and the quality of thewater? How can current water management be improved and howshould new agricultural development projects be planned andmanaged so as to maximize the net returns under differentsets of conditions? These and similar questions continue tochallenge agriculturists in arid and semiarid regions.

Irrigation is the supply of water to agricultural cropsby artificial means, designed to permit farming in aridregions and to offset drought in semiarid or semihumidregions. As such, it already plays a key role in feeding an

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expanding population and seems destined to play an evengreater role in the foreseeable future. Even in areas wheretotal seasonal rainfall may seem ample on the average, it isoften unevenly distributed during the year and variable_fromyear to year so that traditional dryland farming is a high-risk enterprise and only irrigation can ensure a stablesystem of production. Irrigation can prolong the effectivegrowing period in areas with dry seasons, thus permittingmultiple cropping (two or three -and sometimes four - cropsper year). With the security of cropping under irrigation,additional inputs needed to increase production (fertilizers,improved varieties, pest control, and better tillage) becomeeconomically feasible. Irrigation reduces the risk of theseexpensive inputs being wasted by drought.

The process of irrigation consists of introducing waterinto the part of the soil profile that serves as the rootzone, for the subsequent use of the crop. A well-managedirrigation system is one that optimizes the spatial andtemporal distribution of water so as to promote crop growthand yield, and to enhance the economic efficiency of cropproduction. The aim is not necessarily to obtain the highestyields per unit area of land, or even per unit volume ofwater, but to maximize the net returns, not just for a givenseason but in the long run. Since the physical circumstancesand the socio-economic conditions are site-specific (andoften season-specific) in each case, there can be no singlesolution to the problem of how best to develop and manage anirrigation project. In different combinations, however, thefactors and principles involved are universal.

From its early and primitive antecedents in the rivervalleys of the Middle East some seven millennia ago, thepractice of irrigation has evolved gradually in the directionof increasing the farmer's control over crop, soil, and evenweather variables. Although the degree of control possibleeven today is only partial, as the open field remains eversubject to unpredictable vagaries, modern irrigation is ahighly sophisticated operation, involving the simultaneousmanipulation of numerous factors of production. And yetprogress continues. Along with the rising costs of energy,scarcity of good land and water, and increasing demand foragricultural products, the search for new knowledge of how toimprove the efficiency of irrigation, and the imperative todisseminate and apply the knowledge gained to date, havebecome more urgent than ever.

Unfortunately, the history of civilization is repletewith examples of how irrigation development had in manyplaces become self-destructive. All too often, the short-term gain in production resulting from irrigation led tointensive settlement and resource exploitation. This, inturn, was followed inexorably by long-term loss in the formof water resource depletion and pollution, as well as of soil

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degradation. These insidious trends are not confined to thepast, but in fact are prevalent at present no less than inhistory, in developed as well as in less developed countries.Continuing wasteful practices of water and soil managementresult in squandering of these precious resources and loss oftheir potential productivity.

Why is it that irrigated farming in so many areas failsso woefully to achieve its potential? The problem is notinherent in the principle of irrigation per se, but in itsfrequently careless practice. What is at fault is theunmeasured and generally excessive application of water toland, with little regard either for the real cost of thewater (in contrast with its often arbitrarily set price) orfor the potentially insidious processes thereby set inmotion. Another frequent fault is the neglect to provide fordrainage as well as for salinity and erosion control.

It is the universal fallacy of man to assume that if alittle of something is good, then more must be better. Inirrigation (as indeed in many other activities), just enoughis best, and by that we mean a controlled quantity of waterjust sufficient to meet the requirements of the crop and toprevent accumulation of salts, no less and certainly no more.The application of too little water is an obvious waste, asit might fail to produce the desired benefit. Excessiveflooding of the land is, however, likely to be still moreharmful, as it might saturate the soil for too long, impedeaeration, leach nutrients, induce greater evaporation andsalinization, and ultimately raise the water-table to a levelthat suppresses normal root and microbial activity and thatcan only be drained at great (and sometimes prohibitive)expense. Even where groundwater drainage of irrigated landis feasible, there remains the problem of its disposal.Dumping the drainage back into the stream merely serves tosalinize the water supply - diminished in any case - uponwhich depend other water users downstream.

Control of irrigation should properly begin at thesource - the river, reservoir, or aquifer. Withdrawal ofwater from a source in excess of the rate of replenishmenteventually depletes the source and might deprive the crops oftheir water supply at the very time of their greatest need.The measured withdrawal of water should be calibrated toanswer continuous crop needs on a time-variable and space-variable basis in accord with climatic evaporative demand,crop growth stages, and heterogeneous soil conditions. Toomany irrigation projects have been designed or are operatedmerely as water delivery systems, without sufficient regardfor how the water is to be utilized in the field. Ideally,water should be available on demand and be properly priced(in proportion, or even in progressive disproportion, to thequantity used per unit area of land) so as to create a strongincentive toward conservation.

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Various means are being developed and improved in thecontinuing effort to achieve a higher level of control inirrigation. Among these are the use of laser guidancesystems for precision shaping of the land surface; precisiontillage and mulching to promote infiltration and reduceevaporation while minimizing the expenditure of energy;flumes and weirs to measure the amount of water used, andmetering valves to limit that amount; as well as newirrigation techniques tailored to specific combinations ofcrop, soil, climate, and water quality. Especiallypromising is the recent advent of entirely new irrigationtechniques based on the principles of high-frequency, low-volume applications of water in precise, continuous responseto crop needs. (These techniques will be detailed in latersections of this booklet.) Where rainfall occurs during orjust prior to the growing season, the conjunctive use of rainand irrigation can result in a large savings of water and anincrease of the land area which can be effectively irrigated.

No irrigation system can be considered sustainable,particularly in river valleys prone to high water-tableconditions, without a complementary drainage system.Irrigation without appropriate drainage can result indisaster. Hence drainage must be an integral part of theoverall irrigation design from the very outset. Soilsalinity must also be monitored continuously to prevent itsgradual accumulation in the root zone.

Last but not least: Irrigation is not simply anexercise in mechanics or in economics. It is a humanactivity and a social undertaking. Attention to the humanaspects of irrigation is a vital part of proper development,and a prerequisite to its success.

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2. ENVIRONMENTAL PROCESSES

'rhe Field Water Cycle and Water Balance

Any attempt to control the supply of water to crops mustbe based on a thorough understanding of the variable state ofwater in the soil and of its cyclic movement into, within,and out of the root zone. The cycle of water in the fieldconsists of a series of sequential or concurrent dynamicprocesses, beginning with the entry of water into the soil(called infiltration), continuing with its redistribution anddownward drainage within the soil, and culminating with itsuptake by plants and its return to the atmosphere in the twinprocesses of transpiration and evaporation.

The rate of infiltration can be governed by the rate atwhich water is applied to the surface, as long as theapplication rate does not exceed the maximum rate which thesoil can absorb through its surface. That limiting rate,called the soil's "infiltrability," is relatively high forinitially dry and coarse-textured (sandy) soils and low forwet: and fine-textured (clayey) soils. Infiltrability isespecially low where the soil surface has been compacted byItraffic or by the beating action of raindrops. An important,design criterion for a sprinkle or drip irrigation system isto deliver water only at the rate which the soil surface canabsorb, since an excessive rate of water application cancause ponding, restriction of aeration, runoff, and erosion.The typical variation of infiltration rate with time fordifferent soils is illustrated in Figure 2.1.

-$ 50

i 40

h 30

Sandy loam0" 20 _

4J ~ ~ ~~Slt loam

4a ~vCla_ loam

H0 -~~~~~~~~~~

1 2 3Time (hours)

Figure 2.1. Typical infiltrability curvesfor various soils.

The water which has entered the soil during infiltrationdoes not remain immobile after the infiltration event hasended. Because of gravity and tension gradients in the soil,this water generally continues to move downward, albeit at adiminishing rate, in a process called redistribution. In thecourse of this process, the relatively dry deeper zone of thesoil profile absorbs water draining from the infiltration-wetted upper part. Within a few days, however, the rate offlow can become so low as to be considered negligible, atwhich time the remaining water content in the initiallywetted zone is termed the "field capacity" and is often takento represent the upper limit of the soil's capacity to storewater. The redistribution process depends on the antecedent(pre-infiltration) soil moisture content, the amount of waterinfiltrated, and - primarily - the composition and structureof the soil profile. Field capacity tends to be higher inclayey than in sandy soils. Moreover, it is generallygreater in layered than in uniform soil profiles of similartexture, as layering inhibits the internal drainage of water.As a rule, a soil's moisture content at field capacity isconsiderably lower than at saturation (a condition in whichall pores are completely filled with water, to the exclusionof air), being roughly 50% of saturation in a medium-texturedsoil but much less in a coarse-textured soil (Figure 2.2).

50% -

(fl\

d 40% -

Clay loam4J3

W 20% "Field capacity"

o Sandy loam

ci 10 ______

1 2 3 4 5 6 7Time (days)

Figure 2.2. Decrease of soil wetness duringredistribution following infiltration.

Drainage out of the root zone is generally considered tobe a loss from the standpoint of crop water use. It is notnecessarily a complete loss, however. If the area isunderlain by an exploitable aquifer, the water drained fromthe root zone may eventually recharge the groundwater and can

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be recovered by pumping. Where the water table is close tothe surface, some water may enter the root zone directly bycapillary rise and may supply a portion of the water needs ofthe crop. This process of "subirrigation" is not, however, anunmixed blessing, as the continued upward draw of groundwatermay eventually infuse the root zone with harmful salts.

The pattern and rate of evaporation from bare soilsurfaces depends on the external climate as well as on theinternal movement of soil moisture and heat. Soon after aninfiltration event, while the soil surface is still wet, itis primarily the climate which dictates the evaporation rate.But as the surface zone desiccates (generally within a fewdays), the evaporation rate necessarily diminishes to becomevery slow. Soils which crack as they desiccate may, however,continue to lose water at an appreciable rate for many days.Soils with a high (near-the-surface) water-table can sustaina high evaporation rate still longer. Such soils may becomesterile as the evaporating groundwater deposits its salts atth,e soil surface (Hillel, 1980b).

Transpiration from plant canopies rather than directevaporation of soil moisture becomes predominant when a cropcovers the greater part of the surface. In arid environments,situations may develop in which the plants cannot draw waterfast enough to satisfy the climatically imposed demand.Under such conditions, plants experience stress and mustlimnit transpiration if they are to avoid dehydration. Theycan do this, to a limited degree and for a limited time, byclosing the stomates in their leaves (Kramer, 1983). Theinevitable price of this limitation of transpiration is areduction of growth, since the same stomatal openings whichtranspire water also serve for the uptake of carbon dioxideneeded in photosynthesis. While the relative effects ofstomatal closure on transpiration and on photosynthesis fordifferent types of crops is still a topic for research (Hanksand Hill, 1980), it is clear that conditions of stress limityield in any case and should be avoided, to the extentpossible, in irrigation management (Rawlins and Raats, 1975).

The root-zone soil moisture content at which crop stressbe,comes apparent is termed the wilting point. Typically, thefirst signs of temporary wilting occur at midday, during thehours of peak evaporative demand, following which the plantstend to recover as evaporation decreases toward evening andis practically nil during the night. Continued extraction ofsoil moisture without replenishment, however, eventuallyleads to permanent wilting, a phenomenon long believed totake place at a particular and characteristic soil moisturecontent. The soil moisture content at which pot-grown plantswilt so severely that they can no longer recover during thenight has been called the "permanent wilting point" (PWP).Traditionally, the range of moisture content between fieldcapacity and PWP has long been termed the "available" water

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content, taken to represent to effective or useful waterstorage capacity of different soils (Table 2.1). We now knowthat PWP is not a constant dictated by the soil alone but avariable which is affected in a combined and interactive wayby the soil, the crop, and the weather. When the crop andweather factors are standardized, however, a characteristicsoil moisture content can be measured and used as an index ofwhen irrigation must be applied to prevent crop failure. Informer decades, irrigators often allowed this degree of soilmoisture depletion, but the contemporary approach to crop-water management is to irrigate considerably before"permanent" (or even temporary) wilting is reached so as toavoid subjecting the crop to any yield-reducing stress.

Table 2.1.Moisture contents (by volume) of various soils types

Soil Satura Field Perm.Wilt. Avail.Moist. Steady infilt.tion % Cap.% Point % % mm/m mm/hr

Clay 60 40 20 20 200 3Clay Loam 50 30 15 15 150 3- 7Silt Loam 45 22 12 10 100 7-12Sandy Loam 42 14 6 8 80 12-20Loamy sand 40 10 4 6 60 20-30Sand 38 6 2 4 40 30

The field water balance, illustrated in Figure 2.3, isan account of all quantities of water added to, subtractedfrom, and stored within the root zone during a given periodof time. The difference between the total amount added andthat withdrawn must equal the change in storage. When gainsexceed losses, storage increases; conversely, when lossesexceed gains, storage decreases. Thus:

(Accretion) = (Gains) - (Losses).

This general statement can be amplified as follows:

(LS + AV) = (P + I + U) - (R + D + E + T) (2.1)

wherein AS is accretion of water in the root zone, AV theincrement of water incorporated in the vegetation, P theprecipitation, I irrigation, U the upward capillary flow intothe root zone from below, R runoff, D downward drainage outof the root zone, E direct evaporation from the soil surface,and T transpiration by plants. The last two variables aredifficult to separate and are therefore generally lumpedtogether and termed "evapotranspiration."

All quantities included in the field water balance areexpressed in terms of volume of water per unit area (i.e.

eFqLivalent depth units) during the period considered. It isconvenient to report water quantities in terms of millimeters(ecqivalent to cubic meters per 1,000 square meters of land).

TRANSPIRATION

EVAPORATION

= SLl _} t b RL~~~~~~~~~~~~IUNOFf

<z o 4 <gg D g ~~~~~~IFILTRtATION

., 8 ffi 22%92t ~XTRACTION

4DEEP PERCOLATION

CAPILLARY RISE

= - - -t-l -X- - tt--X' CAPILLARY FRINGE v WATERo "*.i -. ~ -. ~ -.. -. TABLE

o GROUNDWATER REChARGE AND FLOW

I.-

Figure 2.3. The water balance of a root zone (schematic).

Simple and readily understandable though the field waterbalance may seem in principle, it is still rather difficultto measure in practice. Often the largest component on the"losses" side of the ledger, and the one most difficult tomeasure directly, is the evapotranspiration (E+T), simplydesignated ET. To obtain ET from the water balance we musthave accurate measurements of all other terms of theequation. It might seem relatively easy to measure theamount of water added to the field by rain and irrigation(P+I), but this is seldom done on a field-by-field basis,either because of a lack of equipment or trained personnel,or simply through inattention. Even where the input ismeasured, there remains the problem of how to account fornonuniformities in areal distribution. The amount of runoffgenerally is (or at least should be) small in agriculturalfields, particularly in irrigated fields, so that, rightlyor not, it is most often ignored. So is the change in watercontent of the vegetation.

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For a long period, such as an entire season, the changein water content of the root zone is likely to be small inrelation to the total water balance. In this case, the sumof rain and irrigation (P+I) is approximately equal to thesum of evapotranspiration ET and deep percolation D. Forshorter periods, the change in soil-water storage &S can berelatively large and must be measured. This measurement canbe made by sampling periodically, or by use of specializedinstruments. During dry spells without rain or irrigation,the reduction in root-zone water storage must equal the sumof evapotranspiration and "net drainage" (the latter beingthe excess of downward percolation from the root zone overthe upward capillary rise into the root zone).

Common practice in irrigation is to measure the totalwater content of the root zone prior to an irrigation, and tosupply the amount of water necessary to replenish the soilreservoir to some maximal water content, generally taken tobe the "field capacity." This measurement is laborious, timeconsuming, and fraught with errors of sampling and ofinterpretation. Some irrigationists have tended to assumethat the deficit of soil moisture which develops betweenrains or irrigations is due entirely to evapotranspiration,thus disregarding the amount of water which may percolatethrough the bottom of the root zone. This flow is invisibleand difficult to measure, but may constitute a sizablefraction of the water balance. Percolation out of the rootzone is necessary to prevent an accumulation of salts, butthe rate of this drainage is difficult to measure or control.

The most direct method for measurement of the fieldwater balance is by use of a lysimeter (Figure 2.4), which isa large container of soil set in the field to represent theprevailing soil and climatic conditions of a crop and toallow more accurate measurement of evapotranspiration andpercolation than can be carried out in the open field itself.

Manometer

Container

Pressure sensors(hydraulic)

Figure 2.4. A weighing lysimeter (schematic).

- 11 -

Lysimeters are expensive devices to install, maintainand monitor, so their use is limited to specializedexperimental fields in regional research stations. Lessexpensive and formidable than lysimeters are soil moisturemonitoring devices such as the neutron moisture meter andtensiometers. The former measures the volume content ofwater in the soil as it varies in time and in depth, whereasthe latter measure the tension of soil moisture (i.e. thetenacity with which water is held by the soil). Such devicescan serve to guide irrigation scheduling, which we shalldiscuss in a later section, but they require trained manpowerand continuous maintenance.

Radiation Exchange and Energy Balance

Since the principal process involving the fate of waterin the field, and hence the water requirements of crops, isevapotranspiration, it is important to understand how thisprocess takes place and how it is affected by the crop andits environment. The process of evapotranspiration is drivenby a constant inflow of energy. In fact, the entire fieldwater balance (just described) is intimately and reciprocallyrelated to the cycle and balance of energy, since tha stateand content of water in the soil and its vegetative cover isaffected by, and in turn affects, the way the energy fluxreaching the field is partitioned and utilized. Hence controlof the soil-plant-atmosphere system must be based on thewater balance and energy balance considered simultaneously.

Solar radiation reaches the outer suirface of theatmosphere at a nearly constant incident flux (about 2cal/min/cm2). Practically all of this radiation is of thewavelength range of 0.3-3 microns (termed "shortwave"), andabout half of it consists of visible light (in the range of0.4-7 microns). In passage through the atmosphere, the fluxof solar radiation is reduced. On average, the atmospherereflects about one-third (and as much as 80% when the sky isovercast) of solar radiation back to space. In addition, theatmosphere absorbs and scatters part of the radiation, sothat only about half of the original direct flux generallyreaches the ground. A part of the scattered radiation alsoarrives at the filed indirectly as diffuse radiation from thesky. The sum of direct solar and diffuse sky radiation istermed "global radiation."

A significant fraction of the short-wave radiation fluxreaching the ground is reflected upward by the surface. Thatfraction, called "albedo," varies according to the color,roughness, and inclination of the surface. Albedo is of theorder of 5-10% for water, 10-30% for vegetation, and 15-40tfor bare soil (being lower for wet dark clays and higher fordry light-colored sands). In addition to reflecting incomingshortwave radiation, the earth's surface emits its own

- 12 -

radiation, but at a much lower intensity and much higherwavelength than solar radiation (i.e. in the range of 3-50microns, known as the range of infrared, termed "longwave,"radiation). Between the two spectra, the sun's (shortwave)and the earth's (longwave), there is very little overlap.The overall difference between the total incoming andoutgoing radiation fluxes (including both the shortwave andlongwave components) is termed "net radiation" (NR) and itrepresents the radiant energy absorption by the field:

NR = (SWI - SWO) + (LWI - LWO) (2.2)

where SWI is the incoming flux of shortwave radiation fromsun and sky, SWO the outgoing shortwave radiation (reflectedby the surface), LWI the incoming longwave radiation from thesky, and LWO the outgoing longwave radiation emitted by thesurface. During daytime, the incoming shortwave fluxgenerally dominates the radiation balance and the netradiation is positive, but during the night (in the absenceof direct solar radiation) the longwave radiation emitted bythe surface naturally exceeds that received from the sky andthe net radiation flux is negative.

We next consider the disposition of the radiant energyabsorbed by the field. Part of it is transformed into heat,which warms the soil, plants, and atmosphere. Another partis taken up by the plants in their metabolic processes (e.g.photosynthesis). Finally, a major part is generally absorbedas latent heat in the process of evapotranspiration. Thus:

NR = S + A + LE + M (2.3)

where S is the rate at which heat is stored in the soil,water, and vegetation; A is the energy flux that goes intoheating the air; LE is the rate of energy utilization inevapotranspiration (a product of the rate of evaporation Eand the latent heat of vaporization L); and M represents themiscellaneous metabolic energy terms such as photosynthesisand respiration. The heat stored in the soil may be asignificant portion of the net radiation flux at any momentduring the day, but the net storage over a diurnal period isusually small, since the nighttime loss of soil heat largelynegates the daytime gain. For this reason, mean soiltemperature generally does not change appreciably from oneday to the next. However, soil heat storage varies with theseason: in spring and summer it is positive as the soil warmsup, whereas in fall and winter it is negative as the soilcools down. The energy uptake of photosynthesis generallydoes not exceed 3% of the daily net radiation. Typically,the major share of the net radiation goes into the latentheat of evaporation and into heating of the air. Theproportionate allocation between these terms depends on theavailability of water for evaporation. In most agriculturallyproductive fields the latent heat term predominates over the

- 13 -

sensible heat, whereas in dry habitats (e.g., deserts) thelatter term predominates.

The relationships described for the energy balance applyto extensive uniform areas in which all fluxes are vertical.However, any small field differing from its surroundings issubject to lateral effects. Specifically, winds sweepingover a small field can transport energy into or out of it.This phenomenon, called "advection," can be especiallyimportant in arid regions, where small irrigated fields areoften surrounded by an expanse of dry land. Under suchconditions, the warm and dry incoming air can transfersensible heat to the crop and thus increase the rate ofevapotranspiration relative to that from a large fieldsubject to the same radiation regime. A common sight in aridregions is the poor growth of the plants near the windwardedge of an irrigated field, where penetration of warm drywind enhances evaporation. When advective heat inflow islarge, the energy absorbed in evapotranspiration from alimited and sparse stand of vegetatiow-(e.g., a plot of awidely spaced row-crop or a small grow'e of trees) can greatlyexceed that from an extensive stand of-smooth and densevegetation and may even exceed the energy input from netradiation. Hence, measurements of water consumption and ofapparent irrigation requirements obtained from small trialplots may not truly represent large fields, unless theseplots are buffered in the upwind direction by an expanse, or"fetch," of vegetation with similar characteristics and waterregime. The energy balance of an irrigated field isillustrated in Figure 2.5.

Water (TVapor (ET)

Sensiblesi ti On mea t

Wi nd

Direction Water

Water Vapor

______y_______________ t Sensibe Heat

SensibleHeat

z t ~~~~Soil Heat /

Dry AreaIrrigated Vegetative Area

Figure 2.5. Energy balance of an irrigated field (schematic).

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Potential Evapotranspiration and Pan Evaporation

The process of evapotranspiration obviously depends onboth the external meteorological regime (radiation,atmospheric humidity and temperature, wind, etc.) and theinternal state of the field itself, particularly its degreeof wetness and surface properties. Conceptually, therefore,one might suppose that there ought to be a definable rate ofevapotranspiration for the special case in which the field ismaintained perpetually wet, and that this rate should dependonly on the meteorological regime. The concept of "potentialevapotranspiration" (PET) is an attempt to characterize theclimatic environment in terms of its evaporative power, i.e.the maximal evaporation rate which the atmosphere is capableof extracting from a well watered field under given climaticconditions. PET is thus said to represent the externallyimposed "evaporative demand." Whether or not any particularfield can indeed meet that demand and deliver the full rateof PET depends, presumably, on whether it is kept wet enough.

Penman (1956) defined potential evapotranspiration morerestrictively as "the amount of water transpired in unit timeby a short green crop, completely shading the ground, ofuniform height and never short of water." As such, PET is auseful standard of reference for the comparison of differentclimatic regions or seasons. PET is conditioned, first ofall, by the flux of energy reaching the surface via solarradiation. This, in turn, depends on permanent attributes ofthe field such as latitude, slope, and aspect; as well as onseason of the year and other variables including atmosphericcloudiness or dustiness. The seasonal pattern and totalquantity of solar radiation at any particular location varylittle from year to year. However, the fraction of solarradiation utilized in evapotranspiration is affected by suchtime-variable factors as the field's albedo, surfacetemperature, soil thermal conductivity, and emissivity. Theenergy balance also depends on atmospheric advection, whichis related to windiness, air temperature and humidity, sizeand orientation of the field and of its upwind "fetch,"surface roughness, et cetera. Despite the field-specificnature of several of the variables affecting the energybalance, PET is often assumed to depend entirely on theclimatic inputs, regardless of specific field properties.This is obviously a simplistic concept, yet experience showsthat it can be a useful one.

Various empirical approaches have been proposed for theestimation of potential evapotranspiration. The simplestmethods are based on air temperature, since this climaticvariable is readily available and, in both moist and aridregions, is practically independent of the wetness of thesurface (Sellers, 1965). The earliest method for computingpotential evapotranspiration using air temperature alone wasdeveloped by Thornthwaite (1948). Using all the data

- 15 -

available to him, he obtained the following empiricalrelationship between PET and the mean monthly temperature T(degrees Celsius):

PET = 16(10T/I) mm/month (2.4)

where a=(0.675I -77.11 +17,902I+492,390)x10 6, and I= E (T/5)l- 5.The summation is over the 12 months. These relationships arevalid only when air temperature is between 0 and 26.5 C. Forlower temperatures, Thornthwaite assumed that PET is zero.For higher temperatures, the rate is considered to increasewith temperature, being independent of the heat index I. Thevalues of PET obtained with the Thornthwaite formulation mustbe adjusted to account for the variation of day length.

The formulation of Blaney and Criddle (1950) is stillwidely used to estimate crop water requirements as related totlhe concept of a climatically imposed evapotranspirationaldemand. Their method requires only data on air temperatureand day length:

ET=d(0.46T+8.13) (2.5)

wherein T is the mean of daily maximum and minimum airtemperatures (Celsius), and d is mean daily percentage ofannual daytime hours (a function of latitude and season). ETis expressed in mm/day, as a mean for the given month.

The uncertainty involved in any ET prediction using onlyone or two weather factors is high. Evapotranspiration mayvary widely between climates having similar air temperaturesif there are differences in atmospheric humidity, windiness,or sunshine. Thus, the effect of climate on crop water useis not fully defined by temperature and day length alone. Themethod due to Penman (1948) is physically based, hence it isinherently more meaningful than the strictly empiricalmethods. His equation, derived from a combination of theenergy balance and aerodynamic transport formulation, is:

PET=[(DELT/PC)NR+0.35(SVP-MVP)(0.5+U2/100)]/(DELT/PC+1) (2.6)

wrhere DELT is the slope of the saturated vapor pressure vs.temperature curve, PC is the psychrometric constant, NR isthe net radiation, SVP is the saturated vapor pressure atmean air temperature, MVP is the mean vapor pressure in theaLir, and U2 is the mean wind speed at 2m above the ground.As can be seen from Penman's equation, the major factorsgoverning PET are net radiation, temperature (affecting bothDELT/PC and SVP), vapor pressure deficit of the air, and windspeed. These variables can be obtained from standardmeteorological measurements taken at one level. The Penmanformulation avoids the necessity of determining the value ofST, the surface temperature, just as it disregards thepossible fluctuations in the direction and magnitude of the

- 16 -

soil heat flux term. Moreover, it makes no provision forsurface roughness or air instability or canopy resistanceeffects. Finally, the Penman theory takes no explicitaccount of advection. To correct for the differences betweenPET from a rough vegetated surface and potential evaporation(PE) from a smooth water surfaces, Penman used the followingempirical factors determined in southern England:

PE(wet bare soil)/PE(water) = 0.9 (2.7)PET(turf)/PE(water)=0.6(winter), 0.8(summer).

Table 2 summarizes PET values obtained in various regions.

Table 2.2.Typical daily PET (mm/day) for different agro-climatic zones

Mean daily temperatureClimatic zones

Cool (below 20C) Warm (above 20C)Tropics & subtropics

Humid & subhumid 3 - 5 6 - 8Arid & semiarid 5 - 7 8 -10

Temperate zonesHumid & subhumid 2 - 4 5 - 7Arid & semiarid 3 - 5 6 - 9

It should be emphasized again that the representation ofpotential evapotranspiration purely as an externally imposed"forcing function" (like, say, solar radiation) is only anapproximation. In actual fact, each field interacts with themeteorological regime in determining its evapotranspirationrate even when the field is well endowed with water. Thatinteraction is influenced by each field's own (and oftenvariable) values of surface reflectivity, aerodynamicroughness, thermal capacity and conductivity, crop phenology,canopy resistance, et cetera. The oft repeated principlethat under the same climate all well-watered fields exhibitthe same rate of evapotranspiration, regardless of specificcharacteristics, is only "more or less" true.

In recent decades, various modifications have beensuggested for the Penman formulation (e.g., Van Bavel, 1966;Szeicz et al., 1973; Monteith, 1980) to allow for sucheffects as short-term variations in soil heat flux, surfaceroughness, air instability, and canopy resistance; i.e. toaccount for the fact that the potential evapotranspirationof an orchard can be expected to differ from that of a cornfield, which, in turn, might differ from that of a lawn or asmooth bare soil. The advent of remote-sensing infraredthermometry has made possible continuous monitoring of

- 17 -

surface temperature, and hence also allows a betterestimation of the vapor pressure at the surface. With eachdevelopment, however, the theory may tend to become morecomplex and difficult to use in ordinary irrigation practice.

In view of the complexity of the physically basedestimations of potential evapotranspiration (the Penmanmethod and its derivates), it is not surprising that manypractitioners continue to prefer the simplified empiricalmethods which depend on correlation with past records ratherthan on explicit formulation of ongoing processes ormechanisms. Some of these methods provide good estimationsfor a season or a month but can be grossly inaccurate forshorter periods. For example, the Blaney-Criddle method canbe useful for estimations pertaining to a month or more, butis less reliable than the physically based (Penman-type)methods for shorter times (Campbell, 1977).

Various evaporation measuring devices (calledevaporimeters) have been proposed and tried for the purposeof obtaining an estimate of the climatically driven potentialevapotranspiration. Of these, the most frequently used areevaporation pans. They give an indication of the integratedeffect of radiation, wind, temperature, and humidity onevaporation from an open water surface. The most widelyadopted type is the "Class All pan (introduced by the U.S.Weather Bureau). It is a circular container, 121cm acrossand 25.5cm deep, placed on a slatted wooden frame restingover the ground. The pan is filled with water to a heightbetween 5 and 15cm below the rim. Pans are relativelyinexpensive and are easy to install, maintain, and monitor.They do, however, have several important shortcomings.

Although a vegetated field responds to the same climaticvariables as does a pan, it does not necessarily respond inthe same way. A field generally differs from a free watersurface in reflectivity, thermal properties, canopyresistance, and aerodynamic characteristics. Hence theprocess of evaporation from a water-filled pan is not a trueportrayal of evapotranspiration from plants and soil. Thedaytime storage of heat within the pan can cause considerableevaporation at night (10-40% of the diurnal total). Incontrast, nighttime transpiration from crops is generallybelow 5% of the diurnal because of the much smaller heatstorage and particularly because of the increase in canopyresistance due to the closure of stomates in the dark. Also,the color of the pan, heat transfer through the sides,turbidity of the water, and possible shading from screens ornearby plants - all affect the measurement. Pan evaporationdepends greatly on the exact placement of the pan relative towind exposure and advection from outside the field. Panssurrounded by a tall crop may evaporate 20-30% less than pansplaced in a fallow area, especially if the climate is dry andwindy. To avoid water loss to drinking animals (especially

- 18 -

birds), pans are often covered by screens. This reduces theevaporation rate (generally by about 10% to 20%), thusrequiring the use of a correction factor, and also interferessomewhat with the measurements and the servicing of the pan.

All of these shortcomings notwithstanding, panevaporimeters, if properly sited and maintained, can indeedbe used to assess potential evapotranspiration. A correctionfactor is needed, however, to account for environmentalconditions. For pans placed in the open, this factorgenerally varies from 0.85 for conditions of high humidityand light winds (below 175km per day) to about 0.5 for lowhumidity and very strong winds (above 700km per day). Theaverage is about 0.7. If, for example, the measured panevaporation rate (EPAN) is 10mm per day, and the appropriatepan coefficient (KPAN) is 0.7, then the estimated potentialevapotranspiration PET(est)=KPAN x EPAN, i.e. PET=7mm/day.

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3. CROP - WATER RELATIONS

Water in the Physiology of Crops

Green plants are autotrophs, able to create livingmatter from purely inorganic raw materials. Terrestrialplants do this by combining atmospheric carbon dioxide withsoil-derived water while converting solar radiation intochemical energy in the process of photosynthesis. Thisprocess not only produces food for all animals, includinghumans, but also releases into the atmosphere the elementaloxygen needed for respiration by animals and plants alike.

Water plays a central role in the metabolism of plants,as a source of hydrogen atoms for the reduction of carbondioxide in photosynthesis and as a product of respiration.Moreover, water is the solvent, and hence the conveyor, oftransportable ions and compounds into, within, and out of allliving plants. It is also a major structural component, oftenconstituting more than 90% of the vegetative biomass. Only asmall fraction of the water absorbed by plants is used inphotosynthesis, while most (often as much as 99%) escapes asvapor in the process of transpiration from plant canopies.TEranspiration is made inevitable by the exposure to the dryatmosphere of a large area of moist cell surfaces, necessaryIto facilitate absorption of carbon dioxide. Hencetranspiration has been described, from the point of view ofcrop growers in arid regions, as a "necessary evil." Mostcrop plants are extremely sensitive to lack of sufficientwater to replace the amount they must transpire.

Terrestrial plants live simultaneously in two verydifferent realms, the atmosphere and the soil, in each ofwhich conditions vary constantly, and not necessarily incoordination. Mesophytes, including most crops, controltheir water economy by developing extensive root systems toextract water from the soil, as well as by regulating theaperture of their stomates so as to limit water loss from theleaves. (Stomates are narrow perforations in the surfaces ofleaves that serve as entry points for carbon dioxide whilesimultaneously releasing transpired water vapor.) Wheneverplants experience moisture stress, stomates begin to close,thus curtailing the excessive loss of water even at theexpense of decreased absorption of carbon dioxide. Closureof stomates cannot, however, stop transpiration entirely, asleaves continue to lose some water (albeit at a reduced rate)through their cuticular surfaces. Sustained transpirationwithout sufficient replenishment causes dehydration, of whichloss of foliar turgidity (wilting) is the most visible sign.

Many physiological processes are affected by waterstress some time before a plant actually wilts (Hsiao, 1973).By the time the plant approaches the permanent wilting point,

- 20 -

cell expansion has long since ceased and the tightly closedstomates severly restrict carbon dioxide entry as well aswater loss. Since transpiration rate may not decreaseappreciably before wilting, some researchers in the early1930's concluded erroneously that production remainsunaffected by moisture stress until soil moisture falls to avalue near the permanent wilting point. The later observationthat growth decreases earlier than does transpiration negatessuch a conclusion. It now appears that in most crops growthcan proceed unimpaired and yield maximized only when the soilmoisture potential remains high (and water remains readilyavailable) continuously throughout the growing season.

When plants are stressed and transpiration is curtailedeven temporarily (e.g., as a result of stomatal closure atmidday), crop canopy temperature tends to rise appreciablydue to the reduction of evaporative cooling. This is thebasis for the modern technique of sensing crop water stressby monitoring canopy temperature with an infrared radiationthermometer (Jackson, 1982). The rise in temperature causesan exponential increase in vapor pressure gradient from theleaf to the atmosphere, partially offseting the increasedstomatal resistance to transpiration. A rise in temperaturealso entails an increase in the rate of tissue respiration,so that while the stressed plant produces less (owing tocurtailed photosynthesis) it actually consumes more of itsown reserves. Hence short periods of stress may reduce netproduction more than transpiration (Huck and Hillel, 1983).

Granted that, for most crops, keeping plant waterpotential high (by maintaining a moist irrigation regime)results in maximum production per unit area, a questionremains as to whether it might reduce production per unitamount of water consumed. The answer appears to be thatstressed plants do not use water more efficiently than well-watered plants (Rawlins and Raats, 1975). From the standpointof dry matter production, either per unit of water used orper unit of land occupied, there seems to be no advantage insubjecting plants to water stress. However, except for someforage crops, dry matter production is seldom equivalent tomarketable yield. For crops that require water stress toinitiate differentiation or maturation of the harvested partof the plant (for example, cotton), programming a period ofwater stress into the growing season may be beneficial.

Results from experiments on the effects of water stresson plant growth (Hsiao, 1973) suggest that periods of reducedleaf water potential during which growth is stopped do notnecessarily reduce net growth if they are not too long.Assimilates can be stored for several hours beforephotosynthesis is decreased, and can then accelerate growthwhen stress is relieved. This is why short daily stressperiods are less damaging overall than a less frequent stressthat lasts for several days.

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Crop Evapotranspiration and Water Requirements

The daily rate of actual evapotranspiration from a cropwill seldom equal the potential rate (PET) as measured from awell-watered turf or as calculated from a Penman-typeformula. Canopy characteristics, stand density, stage ofgrowth and degree of surface cover, and especially themoisture regime (whether wet or relatively dry), all affectactual evapotranspiration.

In the case of a typical annual crop, the seasonal totalevapotranspiration will generally not equal the total PETev,en if the moisture regime is a wet one. Early in theseason, during the germination and seedling-establishmentphase, the rate of transpiration is generally very small and,as the bare soil surface dries between irrigations, thedirect evaporation rate is also lower than PET. Later, asthe crop approaches full cover, the transpiration rateincreases until, at full cover, that rate approximates (andmay even exceed) the calculated or pan-measured rate of PET.Finally, as the crop matures, senesces, and dries, its actualevapotranspiration again falls below the potential rate. Thispattern is illustrated in Figure 3.1.

(d4

0

0z / I I

Evaporation lI

. I

Canopy develop- Full cover Maturation E senes-ment phase phase cence phase

Figure 3.1. Variation of evapotranspirationduring the growing season of an annual crop.

- 22 -

The maximal seasonal evapotranspiration from a well-watered crop stand of optimal density, designated MET, islikely to range between 0.6 and 0.9 of total seasonal PET.To obtain the highest possible yields of many agriculturalcrops, irrigation must generally be provided in an amountsufficient to prevent water from becoming a limiting factor.Knowledge of the MET for the major crops in a given regioncan therefore serve as a basis for planning the irrigationregime. In fact, MET is often taken to represent the crop'swater requirement (CWR). As MET is affected by both theclimate and the characteristics of the crop, it should bestbe measured in the field for each region and major crop.

Knowing PET (whether calculated from a Penman-typeformula, or measured in the field using a reference crop suchas a well-established, well-watered, dense, and extensivestand of grass or alfalfa), it is possible to account for theeffect of specific crop characteristics on crop waterrequirements (CWR) usingian empirical crop coefficient, KC:

CWR = KC.x PET (3.1)

In general, KC is higher in hot, windy and dry climates thanin cool, calm and humid climates. KC values vary among cropsbecause of differences in reflectivity, crop height androughness, degree of ground cover, and canopy resistance totranspiration. In assigning an appropriate KC value to acrop, one should consider the different stages of growth,particularly as they affect wind and turbulence within andabove the canopy. In the case of annual crops, KC typicallyincreases from a low value at seedling emergence to a maximumvalue when the crop reaches full ground cover, continues atthat value during the stage of full activity, then declinesas the crop matures and ends its growth cycle (Figure 3.2).

1.0

z 0.8 _

U / ~~~~~~water X\> 0.6 -

0

C 0.2

Planting Emergence Rapid Full cover Maturationgrowth

Figure 3.2. Variation of crop coefficient KCduring the growing season of an annual crop.

- 23 -

Table 3.1 presents a sample calculation of MET for acrop with a four-month growing season in a warm arid region.

Table 3.1.Water requirements (MET) for a hypothetical crop,calculated from measured monthly PET and the cropcoefficient (KC) as it varies during the season

Month May June July August

KC (monthly mean) 0.5 0.75 1.1 0.7PET (mm/day) 6 8 10 10CWR (mm/day) 3 6 - 11 7CWR (mm/month) 93 180 341 217

Total water requirement for the season: 831mm(About 79% of the total PET of 1,046mm)

For most crops, the KC value for the total growingseason lies between 0.6 and 0.9. The higher values are forsuch crops as bananas, rice, coffee, and cocoa; and the lowervalues for citrus, grapes, sisal, and pineapple. Crops orvarieties with a high yield potential may be highly sensitiveto moisture stress (as to other non-optimal conditions) whilevarieties with a lower yield potential may be less vulnerableand hence preferable under conditions of water shortage.

Actual evapotranspiration from a crop, designated AET,is generally lower than the maximal for that crop (MET), asit is constrained by the availability of soil moisture andthe degree of canopy cover. The drier the soil moistureregime and the thinner the crop stand, the lower will be theactual evapotranspiration. The seasonal total of AET canvary between 0.5 and 1.0 of MET (depending on the waterregime), and the relative yield generally varies accordingly.The demand for water by a crop must be met by the watersupply from the soil via the root system. The actual rate ofwater uptake and transpiration is determined by whether thewater stored in the soil and readily extractable by roots isadequate; otherwide, the crop will suffer stress induced bywater deficit. Actual evapotranspiration (AET) from a densestand tends to equal the maximal rate for that crop (MET) aslong as soil moisture is readily available, but falls belowMET when soil moisture becomes limiting. The "Available soilwater" (ASW) can therefore be defined as the volume of waterwhich can be extracted from a unit volume of soil (or a unitarea of field) without causing AET to become less than MET.

We now know that soil water availability is not anattribute of the soil alone, but of the combined soil-crop-climate system. Crops with extensive and dense roots canutilize soil moisture more effectively, and to a lower

- 24 -

residual soil wetness, than crops with sparse and shallowroots. In an arid climate with high evaporative demand, cropsexperience stress sooner, and at a higher level of soilmoisture, than in a moderate or humid climate. Soilproperties affecting availability are texture, structure, andprofile layering. Available water capacity is of the orderof 150 to 200mm per meter-depth of soil for clayey soils; 100to 150 mm for loamy soils; and 50 to 100mm for sandy soils.These are only rough estimates. Local information is neededfor more exact data, especially for layered (multi-textured)soil profiles. Typical rooting depths for various crops areillustrated in Figure 3.3 and listed in Table 3.2.

aize

SmallPotato grains

# ~beet bean _

0

,u1004JJ

150 ----- -

Figure 8. Typical root distributions of several crops.

Table 3.2.Rooting-zone depths of different crops (meters)

-------------------------------------------------------------

Shallow Medium Deep-------------------------------------------------------------

Beans 0.5-0.7 Barley 1.0-1.5 Alfalfa 1.5-2.5Cabbage 0.4-0.5 Clover 0.6-0.9 Cotton 1.0-1.7Lettuce 0.3-0.5 Wheat 0.9-1.5 Orchards 1.0-2.0Onions 0.3-0.5 Peas 0.6-1.0 Maize 1.0-2.0Potatoes 0.4-0.7 Tomatoes 0.7-1.5 Sorghum 1.0-2.0Spinach 0.3-0.5 W.Melons 1.0-1.5 Sugarcane 1.0-2.0- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -- - - - - - - - - - - - - - - -_ _

- 25 -

Water Use - Yield Relationships

As stated, when the water supply is plentiful throughoutthe growing season, a cropped field can be expected to evapo-transpire at a maximal rate MET and to attain full potentialyield - provided, of course that no additional constrainingfactors such as pest infestations or nutrient deficienciesinterfere. When water is limiting, water use may fall belowMET. Consequently crop yield is related functionally to cropwater use (dictated by the water supply), but this relationmay not be a simple one (Figure 3.4). Interest in thefunctional dependence of crop yields on water supply and usehas grown in recent years because of the increasing scarcityof water for irrigation.

Yielda

Linearapproxi-Imation /

WaterE MET applied

Figure 3.4. Relation of crop yield to water supply.

The first comprehensive analysis of the relation betweentranspiration and yield was offered by de Wit (1958). Hefound that in climates with a large percentage of brightsunshine duration (i.e. arid regions) a relation such as

Y = m(AT/PE) (3.2)

exists between dry matter yield Y and the ratio of actualtranspiration AT and potential (free-water) evaporation PE,m being the proportionality coefficient. In climates with alimited duration of bright (temperate regions) the relation

Y = n(AT) (3.3)

- 26 -

was found; that is to say, dry matter production isproportional to transpiration and hence the ratio of yieldincrement to water increment is constant. The values of theconstants m and n are characteristic for each crop. Theassumption of proportionality between yield and transpirationserves as the basis for recent quantitative models of cropgrowth as related water management (e.g. Hanks, 1974).

The relation of yield to evapotranspiration is morecomplicated than that of yield to transpiration, owing to thevariable component of evaporation from the soil surface. Anempirically based equation to predict yield from known valuesof evapotranspiration was given by Stewart et al. (1977) fordry matter production:

Y/Ym = 1-b(ETD)= (l-b)+b(AET/MET) (3.4)

wherein Y is dry matter yield, Ym is maximum attainable yield(water not limiting), AET actual evapotranspiration, METmaximum evapotranspiration, and b the slope of the relativeyield (Y/Ym) versus the "evapotranspirational deficit"(ETD = 1-AET/MET). To predict yield from this equation, onemust know AET, MET, Ym, and b. As pointed out by Hanks andHill (1980), the ratio AET/MET where.Y/Ym is zero indicatesthe portion of ET due to direct evaporation, E, from the soilsurface (Figure 3.4). In a field study reported by Hilleland Guron (1973), total dry-matter yield of maize per unitquantity of evapotranspiration increased twofold as ET wasincreased 30% in the wetter irrigation regime (in which AETwas nearly equal to MET). The wettest treatment yielded 2.3times as much grain as the driest treatment, while consumingonly 1.3 times as much water. However, when the fraction ofET due to evaporation from the soil was taken into account,the yield appeared to be proportional to transpiration.

Determining the fraction of evapotranspiration due toevaporation from the soil is important since evaporation(unlike transpiration) is not related to plant activity andis therefore a loss. Irrigation applied as spray is in partintercepted by the foliage and then evaporates rapidlywithout entering the transpiration stream, so interceptedwater can also be considered a loss. These facts have abearing on irrigation method and frequency. For systemswhich wet the entire surface, a high irrigation frequencycauses greater evaporation from the repeatedly rewetted soilsurface and from the canopy. On the other hand, too low anirrigation frequency may cause soil desiccation and plantstress. Therefore, the most efficient utilization of watermay result from some intermediate irrigation frequency, evenif it produces less than maximum yields. However, irrigationsystems (like drip) that wet only a small fraction of thesurface, and also avoid wetting the foliage, can reduceevaporation losses even when applied at high frequency.

- 27 -

Doorenbos and Kassam (1979) proposed the followingrelationship between yield and amount of applied water:

l-(Y/Ym) = f(l-(AET/PET)) (3.5)

where again Y is actual yield, Ym is maximum attainable yieldwhen full water requirements are met, f is a yield responsefactor, AET is actual evapotranspiration and PET is potentialevapotranspiration. Empirically obtained values of f havebeen tabulated for different crops and climatic regions.

In many cases, reported relationships between yield andwater use pertain to aboveground dry-matter yield. If theyield of interest is grain, fruit, or fiber, its relation towater use by the crop can be quite different. Although thiscannot be taken for granted in every case, some studies haveshown that grain yield bears a more-or-less constant ratio todry-matter yield (e.g., Hillel and Guron, 1973). The linearrelationships found between yield and water use under limitedwater supply may not hold as potential evapotranspiration isattained and water ceases to be a limiting factor. Beyondthe point where transpiration reaches its climatic limit, thepromise of increasing production lies in identifying and thenobviating any other possible environmental constraints, suchas pests, soil aeration, or nutrient supplies (Figure 3.5).

10 400 kg/ha200

rd 8 _

u)z ~~~~~~~~~100

0 6_

rd 4 50H4

>1 ~~~~~~~~~~~~0z 2_O

. ,I I I I

300 500 700Evapotranspiration (mm)

Figure 3.5. Maize yield vs. seasonal ET at five levelsof N fertilization. Source: Shalhevet et al., 1976.

- 28 -

Finally, we come up against genetic constraints, which cannotbe obviated by any environmental factors. This is where cropbreeders are called upon to develop new varieties with evergreater yield potential. The point to remember is thatimproved varieties may not succeed unless environmentalconditions (chief among them the water regime) are optimized.

The question of what constitutes a "desirable" level ofwater use is a matter of some controversy. Three approachescan be defined (Vaux and Pruitt, 1983): (1) Agronomists arefrequently interested in attaining maximum yields per unitarea of land. (2) An alternative goal is to achieve themaximum degree of "water use efficiency," i.e. to maximizethe yield per unit amount of water applied. (3) Yet anothergoal is advanced by economists who argue that water, to beused effectively, should only be applied up to the pointwhere the revenue derived from the last increment of wateradded still exceeds the price of the incremental application.This issue is most relevant when the amount of irrigable landin a region or project exceeds the area which can beirrigated with the limited amount of water available. Theproblem then becomes how to spread the water over the land soas to achieve the highest returns. Concentrating the waterover a limited area of intensive irrigation (where crop waterrequirements are fully met) is an approach likely to maximizeyields per hectare and minimize the investment of equipment,'energy, and labor, which are often proportional to the areaunder production. The opposite strategy of spreading waterso as to "green up" more land under a reduced per-hectaresupply may produce a greater total yield for the project as awhole but involves additional outlays for delivery, tillage,fertilizer, seeds, et cetera. The problem of determining anoptimal allocation of water has no universal solution, sincethe economic considerations involved (i.e., the relativecosts of the inputs of water, energy, machinery, labor, etc.,versus the income derivable) are specific in each case.

The problem is further complicated where rainfall mayaugment the irrigated water supply. Since the timing andquantity of rainfall are generally uncertain, the conjunctiveuse of rainfall and irrigation becomes an exercise instatistical probability, and the entire management schememust be flexible enough to adjust to changing conditions. Inprinciple, applying the water in small increments so as towet the soil only partially (rather than saturate the entireroot zone) is a preferable tactic, as it allows potentialstorage for possible rainfall during the season. Otherwise,even small rainstroms are likely to cause runoff, erosion,and water-table rise. A flexible irrigation system alsopermits the irrigator to withhold water in the daysimmediately following a drenching rain, so as to save onirrigation costs. A strategy for the conjunctive use ofrainfall and irrigation in semiarid regions has been proposedby Stewart and Musick (1982).

- 29 -

4. IRRIGATION MANAGEMENT

Historical Concepts of Irrigation Management

The concept of soil moisture availability has longserved as a criterion for irrigation management, but itslimitations should be understood. The classical view, firstconceived in the 1920s, was that soil moisture is equallyavailable to most crops in their respective rooting zonesthroughout a definable range of soil wetness, from an upperlimit (called "field capacity") to a lower limit (called"'permanent wilting point"). Both of these values wereconsidered to be characteristic and constant for each soil,and independent of crop or climate. The hypothesis of equalavailability rested on the postulate that plant functionsremain practically unaffected by any decrease in soil wetnessuntil the point of permanent wilting is reached, at whichplant growth is abruptly curtailed (Veihmeyer andHendrickson, 1927). Later investigators (Richards andWadleigh, 1952) produced evidence indicating that soilMoisture availability to plants decreases progressively withdecreasing soil wetness, and that a plant may suffer watersgtress and reduction of growth considerably before thepermanent wilting point is reached. However, the olderconcept, being simpler to understand and apply, remained theaccepted truism for many more years. The alternative viewsos soil moisture availability are illustrated in Figure 4.1.

100% A

44 ~~~~~~~~~~~A0

>1

.eJ -'-4C>

-I-

:>C

4-i-I-)

Q)~I capacity WiltingZ 04 ~~~~~~~~~~point

0 100%Available water depletion

Figure 4.1. Hypotheses regarding soil water availability toplants: (A) equal availability from field capacity to wiltingpoint, (B) equal availability from field capacity to acritical moisture beyond which it decreases, and (C)availability decreases continuously (from Hillel, 1980b).

- 30 -

In practice, the hypothesis of equal availabilityresulted in a regimen of infrequent irrigations designed tofill the soil to its field capacity, followed by intervalsduring which the irrigator awaited depletion to nearly thepermanent wilting point before replenishing soil moisture bymaking up the "deficit" to field capacity. The traditionalirrigation cycle thus consisted of a brief period ofinfiltration, followed by an extended period of soil moistureextraction by the crop. The soil's surface zone wassaturated periodically, with a resulting inhibition of soilaeration, then allowed to desiccate excessively, again to thedetriment of the roots in the upper layer.

Practical limitations on the frequency of irrigation bythe traditional irrigation methods had made it difficult totest alternative regimens based on the continuous maintenanceof a more nearly optimal level of soil moisture in the rootzone. The traditional mode of irrigation seemed to make goodeconomic sense because many furrow, flood, and sprinklersystems have a fixed cost associated with each application ofwater. With such systems, it is desirable to minimize thenumber of irrigations per season by increasing the intervalof time between successive irrigations. For example, with aportable sprinkler system, the cost of tubing can be aprimary consideration. In this case, it obviously pays tomake maximal use of the equipment by minimizing the amount,oftubes required per unit area irrigated, i.e. by rotating theavailable tubing from section to section so as to cover thegreatest overall area possible before having to return to thesame section for re-irrigation.

In many other cases, traditional irrigation schemes weredesigned at the outset to provide water on a fixed schedulewithout permitting any discretionary change in frequency. Ineffect, the principle of minimizing irrigation frequency hasmeant making the greatest possible use of soil moisturestorage by applying heavy irrigations, and then maximizingthe utilization of the stored water by waiting as long aspossible (so as to deplete the soil "reservoir") prior to re-irrigation.

The classical questions involved in irrigationmanagement have always been: (1) just when to irrigate, and(2) how much water to apply at each irrigation. To the firstquestion, the traditional irrigationist would reply: irrigateonly when the available soil moisture is practicallydepleted. To the second question, the traditional answerwould be: apply an amount of water sufficient to refill thesoil root zone to field capacity. These simple (andsimplistic) criteria have long been the accepted truisms ofirrigation management and are still being taught and followedin many places. They are at variance, however, with modernconcepts and practices.

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Modern Concepts of Irrigation Management

In the last two decades, newer concepts of irrigationmanagement have evolved. A fundamental change has takenplace in our conception of soil-plant-water relations,leading to a more dynamic and holistic approach. The fieldis now perceived to be a unified system in which allprocesses are interdependently linked, as in a chain. Thisunified system has been called the SPAC, for "Soil-Plant-Atmosphere Continuum." Accordingly, the availability of soilmoisture is not a property of the soil alone but indeed acombined function of the plant, the soil, and the climate(Gardner, 1960; Philip, 1966).

In principle, the rate of water uptake depends on theability of the roots to absorb water from the soil with whichthey are in contact, as well as on the ability of the soil tosupply and transmit water toward the roots at a ratesufficient to meet transpiration and growth requirements(Huck and Hillel, 1983). These variables, in turn, dependon: (1) characteristics of the plant (rooting density,rooting depth, rate of root extension, and the physiologicalability of the particular crop plant to maintain its vitalfunctions for some time even while its own water potentialdetcreases); (2) properties of the soil (water retention andconductivity); and (3) weather conditions (which dictate theraLte at which the crop is required to transpire).

When all the controllable variables are optimized so asto avoid any occurrence of moisture stress during the growingseason, many crops show a pronounced increase in yield.Under continuously favorable moisture conditions, improvedvarieties can attain their higher yield potential and canrespond to greater amounts of fertilizer and more intensivemanagement practices.

The desired effect can be produced by optimizing thequantity and increasing the frequency of irrigation, takingcare to avoid wetting the soil excessively (so as not towaste water, impede aeration, leach nutrients, and raise thewater-table). This optimization is difficult to achieve bythe traditional surface irrigation methods still dominant inmany places, and as a result the new approach to irrigationmanagement has not yet been applied very widely, particularlyin developing countries. Although it is gradually gainingground, its progress should be encouraged and acceleratedwherever appropriate.

The advent of newer irrigation systems (includingpermanent or annual installations, called "solid set," oflow-intensity sprinklers, drippers or tricklers, micro-sprayers, bubblers, porous-tube subirrigators, etc.) has madeit possible to establish and maintain soil moistureconditions at a more nearly optimal level than heretofore.

- 32 -

Since these new systems are capable of delivering water tothe soil in controllable small quantities as often and aslong as needed with no significant additional cost for theextra number of irrigations, some of the economic constraintsto high-frequency irrigation have been lifted.

With pressurized irrigation systems that deliver wateron demand to all parts of the field through pipes, thecapital costs depend largely on pipe size, which - in turn -is governed by the maximum required delivery rate. That ratecan be minimized by increasing the duration of eachirrigation. Because it costs little more to use a systemonce it is permanently installed, the best use is almostcontinuous irrigation during the period of peak water demand.

As the frequency of irrigation increases, theinfiltration period becomes a more important part of theirrigation cycle. Changing the irrigation cycle from anextraction-dominated to an infiltration-dominated processbrings into play a different set of relationships governingsoil moisture. For example, with small daily (rather thanmassive weekly or monthly) applications of water, the pulsesof added water are damped down within a few centimeters ordecimeters of the surface, so that flow below this point isessentially steady. By adjusting the rate and quantity ofapplication in accordance with such measurable variables as.PET (or MET), soil hydraulic conductivity, and the soilsolution's salt concentration, it is possible to control themoisture tension prevailing within the root zone as well asthe rate of through-flow and thus also the amount ofleaching. Control of the crop's soil environment thus passesinto the hands of the irrigator more completely than everbefore. Properly managed, the new system can indeed savewater while improving growth and increasing yield (Rawlinsand Raats, 1975; Bucks et al., 1982).

Since a high-frequency irrigation system can be adjustedto supply water at very nearly the exact rate required by thecrop, one no longer need depend on the soil's own ability tostore water during long intervals between irrigations. Theconsequences of this fact are far-reaching. Soil physicalproperties such as "water-holding capacity" or "fieldcapacity," (or "permanent wilting point" or "availablemoisture capacity") formerly considered decisive, are nolonger major criteria for determining which soils areirrigable. New lands, until recently considered totallyunsuited for irrigation, can now be brought into production.One outstanding example is the case of coarse sands andgravels, where moisture storage capacity is minimal and wherethe conveyance and spreading of water by surface floodingwould involve inordinate losses by excess and non-uniformseepage. Such soils can now be irrigated quite readily, evenon sloping ground without expensive leveling, by means ofdrip or micro-sprayer systems.

- 33 -

With high-frequency irrigation, the farmer need nolonger worry about when available soil moisture is depletedor when plants begin to suffer stress. Such situations canbe avoided entirely. "Field capacity" and "permanent wiltingpoint" indeed lose their relevance. To the old question"when to irrigate?" the modern irrigator answers: "asfrequently as possible, even daily." To the second question"how much water to apply?" he replies: "enough to meetcurrent evaporative demand and to prevent salinization of theroot zone."

Evaporative demand can be determined by monitoring theweather (as described in our section on irrigationscheduling), while salinity can be monitored by sampling thesoil solution.

By supplying water at a controlled rate that exceedsevapotranspiration by a measured amount, the irrigator can infact maintain a nearly steady rate of drainage out of theroot zone. This is in marked contrast with the extremevariability of internal drainage under low-frequency high-volume irrigation management, where drainage is rapid rightafter an irrigation but later declines to a very slow rate.With low-frequency irrigation, one attempts to compensate forthe periodically high drainage rate by causing the crop todry the soil profile between irrigations. Only with high-frequency low-volume irrigation does it become possible tomaintain a wet root zone while keeping the drainage rate low.

High-frequency systems that wet only a fraction of thesoil volume (e.g, drip-trickle, or microsprayer systems) havetheir own shortcomings, however. With less soil moisturestorage, the crop depends almost entirely on the continuousoperation of the system. This is particularly true in thecase of coarse textured (sandy or gravelly) soils, which holdpractically no moisture reserves). Any short-terminterruption in the system's operation, caused by mechanicalfailure or temporary water shortage, can result in severedeprivation, stress, and possibly even total crop failure.The stringent requirement of maintaining perfect operationcontinuously is especially difficult to meet where major andancillary equipment (particularly spare parts) and expertiseare lacking. Hence it is necessary to simplify the modernsystems so as to adapt them to the needs and circumstances ofdeveloping countries. Fortunately, these systems can indeedbe simplified, as described in our section on irrigationmethods.

The modern concepts of irrigation management would haveremained theoretical only were it not for the fortuitous andtimely advent of durable, low cost plastic materials capableof being molded into variously sized and configured tubes andfittings. Progress in irrigation has come about because thenew ideas became practical and the need for them undeniable.

- 34 -

Irrigation Scheduling

Irrigation scheduling is the term commonly used todescribe the procedure by which an irrigator determines thetiming and quantity of water application. In principle,it is possible to schedule an irrigation program on the basisof monitoring the soil, the plant, and/or the microclimate.We shall review each of these alternative or complementaryapproaches in turn.

Monitoring the Soil

This is the traditional method of determining when andhow much to irrigate. The idea is to observe the moisturereserve of the root zone as it gradually diminishes followingeach irrigation, so as to know when that reserve has beendepleted to some level predetermined to serve as the minimumallowable level (i.e. the "time to refill" criterion). Atthat point the irrigator is to apply the volume of watercalculated to replenish the soil reservoir of the root zoneto its "full" level.

A precondition to effective management of root zone soilmoisture is to establish the rooting depth of the specificcrop as it varies during the growing season. Typically, theseedling of an annual crop will begin to develop and extendits roots both vertically and laterally, until its lateralextent impinges upon a region that is already occupied andtapped by the roots of neighboring plants. Thereafter, themajor direction of root extension tends to be downward. Asit deepens, the root system also proliferates within themoist layers it has invaded, and continues thus to ramify andutilize a growing volume of soil until it is constrained fromextending any further by environmental, physiological, orgenetic factors. Among the environmental factors involvedare moisture, salinity, acidity or alkalinity, aeration, andmechanical impedance as they vary in space within the soilprofile.

The volume of soil included within the rooting zone of aplant is the first determinant of the size of the soilmoisture reservoir potentially available to it. That volumecan be assessed by considering the areal extent and densityof the crop stand and the depth of root penetration. Thelatter can be observed by sampling the soil with an auger andexamining the extracted samples for the presence of liveroots. The phenology of root system development (i.e. thedepth of root penetration over time during the growingseason) should be determined for the major crops grown in agiven region. This determination should be repeated severaltimes and in several fields, as necessary to assess the rangeof variation and to gain confidence in the data. Onceobtained, the data on root development can be useful not only

- 35 -

for the season in which it was measured but for futureseasons as well, provided that planting dates andenvironmental conditions remain more or less the same fromseason to season.

Once the rooting volume is known, the potential andactual water contents of the root zone can be determined,with a view to establishing the root zone's water deficit atany time. Concurrent measurements can be made of the soilmoisture tension, in order to assess (and forestall) thehazard of subjecting the plants to excessive moisture stress.The relationship between the water content and the tension isemnbodied in the soil moisture characteristic curve, alsocalled the moisture release curve, illustrated in Figure 4.2.

0.6

0.5

ClayaI 0.4

a)

0.3

0.2-

0

> 0.1 _

I I _ Il

0.1 1 10 100Tension (bars)

Figure 4.2. Moisture release curves for various soils.

In addition to measuring soil water content and tension, thesalt concentration of the soil solution can be monitored todetermine the adequacy, or inadequacy, of internal drainageand the possible need for leaching.

The potential ("full reservoir") water content of theroot zone is generally taken as equal to the field capacity,defined in practice as the water content of the specifiedvolume of soil measurable two days after a thoroughirrigation. In the past, that value was usually expressed interms of weight percentage, but the more convenientexpression is in terms of fractional volume, or equivalentdepth units.

- 36 -

For example, suppose that the root zone is 1.2 meter(1,200mm) deep, and that its field capacity in volumetricterms is 22%. Then the equivalent depth of water containedin the root zone is 1,200x22%=264mm. For a hectare of land(10,000 square meters), the volume of the root zone isl0,OOOxl.2=12,000 cubic meters, and the total water contentis 12,000x228=2,640 cubic meters. Now suppose that withinten days following the last irrigation the volumetric wetnesshas decreased from 22% to, say, 13%. The deficit to fieldcapacity is therefore 9% of 1.2m, namely 0.108m or 108mm. Sothe amount of water to apply is: 0.108mx10,000sq.m.=1,080cubic meters per hectare. Note that in this example wecalculated the amount of irrigation needed to make up for theoverall loss of water from the root zone, while ignoring theissue of assessing how much of that loss may have been due todirect evaporation from the soil, to transpiration from thecrop, and to internal drainage below the root zone.

The limitations of the field capacity concept havealready been mentioned. Despite its limitations, manyirrigationists still consider it a useful criterion, orindex, of the limit of soil moisture storable for subsequentcrop use in a specifiable depth of soil. Attempts toestimate the field capacity from indirect measurements in thelaboratory (e.g. the soil wetness at a tension of 1/3 or1/10 bar) can be misleading, as the process of internaldrainage and the dynamics of soil moisture retention in thefield are influenced by the composition of the entire profilerather than of any particular layer in the profile. Hencethe field capacity must be measured in the field itself.

The lower limit of soil moisture presumably "available"to crops was originally taken to be the "permanent wiltingpoint," obtainable by growing indicator plants in containersfilled with samples of the relevant soil and by observing theextraction of soil moisture by the plants until they wilt"permanently" (i.e. without being able to recover if placedovernight in a humid atmosphere). The more prevalentpractice in the field is to never allow the crop to depleteall the "available" water in the root zone, so as not to risksubjecting the plants to a level of stress that might causean appreciable reduction in yield. Hence the allowabledepletion normally recommended is about 50% of the rangebetween field capacity (FC) and the permanent wilting point(PWP). In the example given above, if the wilting point is,say, 8% by volume, then the hypothetical available watercontent for a root zone depth (RZD) of 1,200mm is:

(FC-PWP)xRZD=(22%-8%)xl,200mm=14%xl,200mm=168mm.

Since, however, the allowable depletion is only 50% of theavailable range, the irrigator should apply water when soilmoisture has fallen to 15% (midway between 22% and 8%), atwhich point the deficit to field capacity is 7% of 1,200mm,

- 37 -

or 84mm. The required irrigation is 840 cubic meters perhectare (assuming no extra water is provided for leaching).If one wishes to add, say, 10% of this required irrigationfor the purpose of ensuring the removal of salts from theroot zone, the irrigation might total 920-930mm/ha.

Soil moisture content can be measured by using an augerto extract representative samples and then determining theirweight loss upon drying in an oven. That is an intrusive,laborious, and time consuming method, involving trampling andaugering. Moreover, it yields values of wetness by weightrather than by volume, so the results must be multiplied bythe soil's bulk density (mass of dry soil per unit bulkvolume). This introduces yet another complication, sincebulk density, which must be measured separately, varies fromlayer to layer and from location to location in the field.

Much preferable to gravimetric sampling is the use of aneutron moisture meter (Figure 4.3), which senses volumetricwetness with minimum disturbance (Hillel, 1980a). Themeasurements can be made repeatedly in the same locations andthe results are available immediately in the field.

SCALERCABLE

SHIELD AND FACESTANDARD

RADIOACTIVE SOURCEPROBE

.- ACTLIVE ZONE

ACCESS TUBE

Figure 4.3. Components of a portable neutron soil-moisturemeter. The instrument includes a probe (with a source offast neutrons and a detector of slow neutrons) lowered from ashield containing hydrogeous material into the soil throughan access tube. A scaler-ratemeter is shown alongside theprobe. Recent models incorporate the scaler into the shieldbody, and the integrated unit weighs no more than 8 kg.

- 38 -

The difficulty here is the relatively high cost of theinstrument (about $3,000 to $4,000) and of the access tubing,and the possible health hazard which might result frominappropriate use of the radioactive probe. A new instrumentto monitor soil moisture, called "time-domain reflectoXetry"(TDR), is now available,.but at this stage is still somewhatexperimental (Topp and Davis, 1985). Numerous other methodshave been proposed, but require calibration or are difficultto use or fail to provide reliable data over time.

Of fundamental interest is the measurement of soilmoisture tension, in addition to moisture content. This canbe done in the field by means of tensiometers (Figure 4.4).

Opening SVacuum t to fill with | WEgauge water E

C

> ~~~~Air|. a

Soil surface

Connect- Depth -ding tube

Porousceramiccup

Figure 4.4. Schematic illustration of a tensiometer.

After an irrigation, as soil moisture is depleted byevaporation and root extraction, the tensiometers register anincrease in tension and, if properly interpreted, can providea forecast of when the plants might begin to suffer stress.Commercial tensiometers are available in various lengths,allowing the monitoring of soil moisture tension at variousdepths so as to characterize the root zone as a whole.However, tensiometers are far from being trouble-freeinstruments. They must be supervised constantly and servicedperiodically. They cost about $30 per unit, so instrumentinga field with a sufficient number to characterize the soilmoisture regime properly can be fairly expensive, in terms ofthe initial investment, the subsequent cost of trained laborneeded for operation and maintenance, and the possible

- 39 -

obstruction of normal field operations. The variation ofsoil moisture tension (suction) during successive cycles ofirrigation is illustrated in Figure 4.5.

(b)

(a)

Tlme

Figure 4.5. Variation of soil suction (moisture tension) inthe root zone during successive irrigation cycles.Note: (a) Average suction (as measured by a tensiometer), and(b) suction of the soil in direct contact with the roots.

monitoring the Crop

In addition or as an alternative to monitoring the soil,it is possible to monitor the water status of the plants.Trhis can be done visually, as well as instrumentally, todetect early signs of thirst (incipient stress), in time toirrigate and thus prevent any significant reduction of yield.

As in the case of soil moisture, numerous methods havebeen proposed over the years to monitor the physiologicalstate of water in the plant. Included among these aretechniques to estimate transpiration using excised leaves,determinations of leaf tissue hydration with punched disks orintact leaves, observations of stomatal aperture, monitoringistem diameter, pressure-cell and psychrometric measurementsof leaf water potential, and more. Perhaps the mostcomprehensive measurements are of total plant transpirationand photosynthesis, using portable tents with transparent(plastic) walls. Nearly all of these techniques requireispecialized instrumentation and trained personnel, and aredifficult to carry out on a routine basis. Hence they aregenerally impractical in ordinary irrigation management.

One method that may become practical in the future isthe monitoring of crop canopy temperature by remote sensing

- 40 -

with an infrared radiation thermometer (Jackson, 1982). Thedevice can be hand-held or mounted on a stationary or movingmast, and it can be used to scan the temperature of thecanopy over an area large enough to represent the field.This method is based on the fact that the transpirationprocess has a cooling effect, so the foliage of well-wateredplants, transpiring freely at the maximal (potential) rate,are generally cooler than the ambient air. However, as soonas plants sense a deficit of water and begin to close theirstomates, the leaves begin to warm up and may rise above thetemperature of the surrounding air. The temperaturedifference between the leaves and the ambient air increasesas the state of stress intensifies, particularly in aridregions. Infrared thermometers are available commerciallyand cost from $1,000 to $3,000.

Still the most common way to monitor the crop is by thetried and true method of direct visual inspection. Anexperienced agronomist or farmer who knows his crop candetect early signs of thirst by the appearance of thefoliage, especially during the period of peak transpirationaldemand (generally at midday). Young leaves are the mostsensitive: they begin to curl or become flaccid. When thathappens, an irrigation is indeed overdue, so it is good to beable to discern plant thirst before such signs are tooevident, that is to say before the stress becomes severeenough to reduce the yield potential.

Monitoring the Weather

The idea here is to follow the meteorologically imposedevapotranspirational demand as it varies over time, and toset the quantity of irrigation accordingly. The estimationof potential evapotranspiration, as defined and calculated bythe Penman formula or its various derivatives, has alreadybeen discussed in a preceding section. While it is of greatfundamental interest, and is indeed practical as anirrigation scheduling criterion in some situations (withwell-equipped and well-staffed central advisory stations),this approach is generally too difficult to apply in thecontext of small-scale farming in arid regions, particularlyin the developing countries. Other agro-climatologicalmethods that seem equally difficult to apply routinely forguiding irrigated farming are radiation balance andlysimetry.

It appears that the most practical of the variousmicroclimatological methods is the use of a pan evaporimeterto characterize the evaporative demand. Experience has shownthat, despite its fundamental shortcomings, this method (asdescribed in a preceding section) can indeed provide a fairlyreliable basis for assessing crop water requirements if it isused with calibrated crop coefficients (Bander, 1984).

- 41 -

General Considerations

Flexibility in irrigation scheduling is essential forefficient irrigation management. In some places, water isavailable most of the time but may become unavailable part ofthe time, often unpredictably. In such cases, a safetyfactor or buffer of several days is needed to protect thecrops. This can be done by lowering the "refill" criterion,i.e. by increasing the range of allowable depletion duringperiods of restricted water supply.

The whole issue of irrigation scheduling becomes moot,however, if water is never available on demand or if it isprovided on an arbitrary, rigid schedule which is not subjectto control and to adjustment in accordance with varying cropneeds. Such is the case in some (perhaps too many) of theolder irrigation districts, where water is diverted from acanal to each farmer for a limited number of hours onspecified dates during the growing season. In such a regime,the only choice left to the irrigator is to "take it or leaveit," so most irrigators obviously "take it," and as aninsurance against possible future disruptions of deliverythey tend to take as much as they can, even much beyondreasonable needs. This tendency, as we have already pointedout, may do more harm than good and can indeed be self-defeating.

Often, an irrigator needs to manage not just one cropbut a mix or array of crops in a centrally managed farm. Theirrigator's concern is then to develop an overall irrigationschedule which optimizes the allocation of water throughoutthe season and ensures an adequate supply during the peakwater use periods. This may mean that one or another of thecrops will not receive water on its own optimal schedule. Inview of the fact that some crops require more water thanothers at certain critical periods and less at other periods,it is necessary to plan the crop mix on the farm so as tobalance the demand and the supply. This involves not onlythe selection of complementary crops but also the allocationof the appropriate fractional land areas, and waterapplication schedules, among them.

A crucial task is to determine how much land can beirrigated with a given amount of water and a given scheduleof water delivery. Two approaches can be taken (Cassel,1984): (1) plant only the area for which water is availableto meet peak demand, or (2) plant a calculated excessive areaand store water in the root zone for a planned deficitirrigation during the period of peak demand.

There are, of course, numerous other considerationswhich might warrant a modification of the irrigationschedule. It is vital, in any case, that irrigationists befully aware of the major factors involved.

- 42 -

Efficiency of Water Use and Water Conservation

The term efficiency is generally understood to be ameasure of the output obtainable from a given input.Irrigation and water-use efficiency can be defined in variousways, depending on the nature of the inputs and outputsconsidered. For example, one may attempt to define as aneconomic criterion of efficiency the financial return inrelation to the investment in the water supply. One problemis that costs and prices fluctuate from year to year and varywidely from place to place. Another problem is that some ofthe costs of irrigation, and certainly some of the benefits,cannot easily be quantified in tangible economic or financialterms, especially in places where a market economy is not yetfully developed. often, only the short-term costs andimmediate benefits are discernible, whereas the long-termadvantages or disadvantages are unknown a priori. How can weassign monetary value, for instance, to the possibility thatan irrigation project might save the population of a regionfrom the dire effects of a drought if the frequency orprobability of droughts of varying degrees of severity cannotbe determined?

Quite different from the strictly economic criterion ofefficiency is the physiological one, i.e. the plant water-useefficiency. The criterion here is the amount of dry matterproduced per unit volume of water taken up by the plant fromthe soil. As most of the water taken up by plants in thefield is transpired (in arid regions - 99% or more!) whilegenerally only a small fraction is retained, the plant water-use efficiency is in effect the reciprocal of what has longbeen known as the "transpiration ratio," defined as the ratioof the amount of water transpired to the amount of dry matterproduced (tons per ton). That ratio can run as high as 500or even 1,000 in regions and seasons of high evaporativity.

What we shall refer to as the technical efficiency iswhat irrigation engineers call "irrigation efficiency." Itis generally defined as the net amount of water added to theroot zone divided by the amount of water taken from somesource. As such, this criterion of efficiency can be appliedto complex regional projects, or to individual farms, or tospecific fields. In each case, the difference between thenet amount of water added to the root zone and the amountwithdrawn from the source represents the seepage andevaporative losses incurred in conveyance to the crop, aswell as the losses due to deep percolation below the rootzone within the field and to runoff from the field.

From the point of view of water use, some large-scaleirrigation projects operate in an inherently inefficient way.In many of the surface irrigation schemes, one or a few farmsmay be allocated large flows representing the entiredischarge of a lateral canal for a specified period of time.

- 43 -

Where water is delivered to the consumer on a fixed scheduleand charges are imposed per delivery regardless of the actualamount used, customers tend to take as much water as they canwhile they can. This often results in overirrigation, whichnot only wastes water but also causes project-wide problemsconnected with the disposal of return flow, waterlogging ofsoils, leaching of nutrients, and elevation of the water-table requiring expensive drainage. Although it is difficultto arrive at reliable statistics, it has been estimated thatthe average irrigation efficiency in such schemes is probablywell below 50% (and may be as low as 30%). Since it is aproven fact that, with proper managment, it is possible toachieve irrigation efficiencies as high as 85% or even 90%,there is obviously much room for improvement.

Particularly difficult to change are managementpractices which lead to deliberate waste not necessarilybecause of insurmountable technical problems or lack ofknowledge but simply because it appears more convenient, oreven more economical in the short run, to waste water ratherthan to apply proper management practices of strict waterconservation. Such situations typically occur when the priceof irrigation water is lower than the cost of labor or of theequipment needed to avoid overirrigation. Very often theprice of water does not reflect its true cost but is keptdeliberately low by direct or indirect government subsidy,which can be self-defeating.

Incidentally, the cost of water may be distorted evenwithout government subsidy. For example, consider the caseof an operator drawing water from an aquifer in excess of therate of natural recharge: the cost of pumping may be only asmall fraction of the cost of replenishing the aquifer afterit has been depleted. So the ultimate cost of using "cheap"water may be high indeed!

Where open and unlined distribution ditches are used,uncontrolled seepage and evaporation, as well astranspiration by riparian phreatophytes, can cause majorlosses of water. Even pipeline distribution systems do notalways prevent loss. Leaky joints resulting from poorworkmanship, corrosion, ill-maintained valves, or mechanicaldamage by farm machinery may cause large losses. Sometimesthe damage is not immediately apparent, as when a buried pipeunder pressure fails at night, with no one in attendance.

Surface runoff resulting from the excessive applicationof water ideally should not occur. Sprinkler irrigationsystems should be designed to apply water at rates whichnever exceed soil infiltrability. In the case of gravityirrigation systems, however, it is often virtually impossibleto achieve uniform water distribution over the field withoutincurring some runoff ("tail water"). Only when provision ismade to collect irrigation and rainwater surpluses at the

- 44 -

lower end of the field and to guide them as controlled returnflow (i.e., reuse for irrigation) can this runoff water beconsidered anything but a loss.

Evaporative losses associated with water applicationinclude any evaporation from open water surfaces or borderchecks or furrows, evaporation of water droplets during theirflight from sprinkler to ground surface, wind drift ofdroplets away from the target area, and evaporation fromwetted crop canopies (foliage) or from the wet soil surfaceimmediately after irrigation. While some of these waterlosses cannot be totally eliminated, most can be greatlyreduced. Transpiration by weeds is also a largelypreventable loss.

In the open field, little can be done to decreasetranspiration by the crop if the conditions required for highyields are to be maintained. Attempts to use chemical spraysknown as "antitranspirants" have failed, and the use ofwindbreaks to control wind movement above and through a cropstand does not always produce the desired effecteconomically.

It appears at present that the greatest promise forincreasing water use efficiency lies in allowing the crop totranspire freely by alleviating any water shortages while atthe same time controlling all other processes of water lossand obviating the other environmental constraints toattainment of the full productive potential of the crop.This is particularly important in the case of the new andsuperior varieties which can attain their full potentialyields only if water stress is eliminated and such otherfactors as soil fertility, aeration, salinity, and soil tilthare optimized. Plant diseases and pests may depress yieldswithout a proportionate decrease in transpiration and wateruse. All management practices can thus influence theefficiency of water use in irrigation, so the practice ofirrigation should not be regarded merely as the provision ofwater to thirsty crops, but more comprehensively as anintegrated production system designed to maximize theefficiency of land, water, manpower, machinery, and energyutilization.

In many parts of the world, far greater returns can beobtained from intensification of production in existingirrigation systems, i.e. by improving methods of water, soil,and crop management, than by building ever new irrigationprojects on the basis of the same antiquated and inflexibledesign. Since it is difficult to convert traditional systemsto modern irrigation scheduling, it is important to makedecisions affecting irrigation frequencies and quantities inthe early stages of planning new projects, before thedistribution system is designed and installed and all futureirrigators are thereby locked into an inefficient pattern.

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Measurement of Irrigation Water

An essential condition for the efficient management ofwater in irrigation is that the water delivered and used bemeasured. Methods for measuring water in channels and pipesare described in numerous handbooks of hydraulics. Simplemethods applicable in irrigation practice have been publishedby Hagan et al. (1968), Schwab et al. (1966), USDA Handbook224 (1979), Replogle and Bos (1982) and Turner et al. (1984).Most measurements of flow rate are based on the relationship:

Q = A x Vm (4.1)

wherein Q is the volume flow rate, called "discharge" (cubicmeters per second); A the cross-sectional area of flow; andVm the mean velocity of flow through the cross-section. Theproblem is to assign realistic values to A and Vm. In thecase of uniform, smooth-sided channels, determining A isrelatively easy, but in the case of uneven streams, A must beaveraged for several points along the streambed (Figure 4.6).

Directionoff low

Figure 4.6. Measuring the cross-sectional area of a stream.

Obtaining good estimates of Vm is even more difficult, as thevelocity distribution within the stream is highly variable,being maximal near the center and minimal near the bottom andsides of the channel (as illustrated in Figure 4.7).Moreover, the mean velocity is influenced by the channel'sroughness. If flow is steady (i.e., mean velocity isconstant), the total volume delivered during a specifiedperiod of time is simply the discharge multiplied by theduration. If the discharge is variable, however, one mustintegrate the flow rate over the time period (e.g., bymeasuring the area under the discharge versus time curve).

- 46 -

water surface

0O2d

0.6d

d

vav

Figure 4.7. Velocity profile in a channel. Note: Vav = 0.8 VsSource: Turner et a. (1984).

Perhaps the simplest way to estimate the discharge in anopen channel of known cross-section is to use a light floatto measure the velocity at the surface. For best results,one should locate a straight section of the stream, about 50meters long, and measure the time needed for a light floatingbody carried by the stream to traverse the marked distance.The average velocity of the stream is often taken to be about0.8 the surface velocity as obtained with the float, but thatfactor should be reduced wherever the stream is particularlynarrow, irregular, or rough. Although this is a crude methodof measurement, it is much better than no measurement at all.More accurate methods for measuring the mean velocity ofwater are based on the use of a current meter. Such a meterusually consists of a set of cups that rotate about avertical axis when placed in flowing water. The rate ofrotation of the cups can be indicated and recordedelectrically. This measurement must be made systematicallyover the cross-section of the stream so as to obtain thevelocity distribution and average it appropriately. Becauseof the delicate and specialized equipment on which thismethod depends, it cannot be considered a routine measurementfor on-farm use in developing countries.

An empirical formula often used to estimate channel flowis based on the well-known Manning equation:

Vm = (R ) (i )/N (4.2)

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wherein Vm, as before, is the mean velocity; R is the"hydraulic radius" (=A/P, where A is the cross-sectional areaand P is the perimeter of the cross-section); i is thedownstream slope of the water surface (or of the stream bed);and N is a coefficient characterizing the smoothness orroughness of the stream bed and banks. Values of N varyfrom 0.013 for smooth concrete to 0.04 for earth channels andas much as 0.08 for small natural streams.

Sluice gates are commonly used in irrigation systems asflow control structures within channels and as outlets fromchannels to the land surface. They can be used to estimatef:Low rates, but must be calibrated for that purpose. Moreaccurate measurements of discharge in channels are obtainableby means of weirs, various types of which are shown in Figure4.8. The basic equation for sharp-crested weirs is

Q = (C)(L)(hn) (4.3)

where L is the length of the crest, h is the head or depth ofwater on the upstream side above the weir crest, and C and nare empirical constants depending on the type of weir used.Exponent n is usually assigned the value of 1.5 for openingsof rectangular or trapezoidal shapes, 2.5 for triangularopenings, and 3.5 for weirs with parabolic-shaped openings.

/

Rectangular and trapezoidal weirs are used when the flowis fairly uniform (i.e., there is not much difference betweenhigh and low flows), whereas triangular and parabolic weirsare used when the flow is highly variable and greateraccuracy is desired in the measurement of low flows. Acommon problem encountered with weirs is the tendency of theelevated plate (the crest) to trap silt and other debris.Devices that can allow more accurate measurement of flow withsilt-laden water are called "flumes." These are speciallyshaped sections of channels that are set into the bed of astream. By increasing the velocity of flow, they induce thesilt and debris to sweep past the measuring point withoutcausing an obstruction. Water depth is measured as forweirs. Two commonly used flumes are shown in Figure 4.9.

The methods described above pertain to open channelflow. Devices used in closed conduit (piped) irrigationsystems are generally usually flow meters which are insertedinto the pipeline, typically at the head of the farm or thefield unit. They are generally equipped with gauges showingcumulative discharge in cubic meters. Among the moresophisticated modern devices are metering valves, capable ofautomatically shutting off after delivering a predeterminedvolume of water. Such devices (each controlling a particularfield or orchard) can be arranged to operate in sequence, ina programmable pattern. In addition to helping economizewrater use, such valves are important as labor saving devices.Where nutrients are applied through the irrigation system,

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Flow over a sharp hcrested weir withfully ventilated i,,.,>nappe. I

Il/fl,T T Il/ 7

Sharp crested rectangular [ F r h I

weir - full width ~I hrih

Q - 1.84 bh 3/

frnt Elevation Pbn

b

Sharp crested rectangular _-iweir - with end hcontractions

Q - 1.84 (b - 0.2h) h 3/2 FFront Elevation Plan

Sharp crested trapeziodalweir - special design I Ii_- a Cipoletti Weir

Q - 1.84 bh 3i2

Sharp crested V-notchweir

Q ' d15 /2g tan 8 h 5/2 _

Broad crested weir

Q - 1.5 bh 3/2 /

Side Elevation

Figure 4.8. Various weirs for flow measurement.Source: Turner et al. (1984).

- 49 -

these valves can help control the rate of fertilization aswell as of irrigation. Such devices are most useful in largescale drip and micro-sprayer systems, but may be tooexpensive for small scale irrigation in developing countries.

Small syphon tubes made of metal or plastic are oftenused in furrow irrigation to deliver water from a head ditchover the side bank into the furrows. The discharge througheach tube is determined by the head difference between theinlet and outlet, and by the hydraulic resistance of thetube. Such tubes can be calibrated easily during operationby means of direct volumetric measurement.

\ \ *N 1 \ / .n I v , zz ecoder

~~~~~~~~~~~~~~~~~la wellS~

H-type flume

Parshall flume

Figure 4.9. Flumes for measuring irrigation water. In usinga Parshall flume, water depth is measured at 2 points in theflume. Then flow rate is read from a table or graph preparedfor the particular flume. Source: Turner and Anderson (1985).

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5. IRRIGATION METHODS

The aim of modern irrigation is to make the best use ofwater in conjunction with all other essential inputs (energy,machinery, labor, fertilizers, pest control) so as to enhanceand sustain crop production. Widely varying methods ofirrigation are employed toward this end, under different setsof circumstances.

The selection of an appropriate irrigation technologyfor any given combination of physical and socio-economicconditions involves numerous complex, and often conflicting,considerations. Where water shortage is acute, the obviousoverriding need is to raise the efficiency of waterutilization. Where capital is short, the major requirementmight be for an irrigation technology with minimal dependenceon expensive equipment. In other cases, the deciding factormight be energy requirements, labor input, or maintenancecosts. Since the economic considerations, along with thephysical conditions and cropping patterns, are necessarilyspecific to each location, an irrigation system that isoptimal in one country or region may not be so in another.In particular, it is a mistake to assume that a modern systemwhich operates efficiently in a highly industrializedcommercial economy will necessarily, or even probably,succeed in the context of an emerging economy.

There are, in principle, three main ways to apply waterto plants: (1) run the water over the surface of the soiland allow it to infiltrate, a method known as surfaceirrigation; (2) spray the water into the air and allow it tofall onto plants and soil as simulated rainfall, a methodcalled sprinkle irrigation; and (3) apply the water directlyto the root zone, a method known as drip or sub-irrigation.

In the following sections, we shall review thesealternatives, and their variations, with respect to theirpossible applicability in developing countries. Physicalfactors involved in system selection include soils, crops,climate, topography, water quality and availability, water-table depth, field size, system performance, maintenance andrepair, et cetera. Human and economic factors involved arelabor and management skills, availability, and cost; as wellas capital and energy costs in relation to expectablereturns. Not all of the relevant factors can be defined orweighed quantitatively in each case, so often the decision asto which system to select rests in part on personal judgmentand subjective preference rather than on completely objectiveanalysis. There is, altogether, no "best" system for allcrops, soils and farm unit sizes. What we should seek is notthe "best system" but an optimal one for the circumstances,and the process of searching for an optimum always involvesjudicious compromise, born of both analysis and trial anderror experience.

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Surface Irrigation

General Description

Surface irrigation is the most ancient of irrigationtechnologies. Irrigation was first developed in level rivervalleys (flood plains) that could be easily ponded bycontainment of the natural river overflows. Later, dams andcanals were built and fields were leveled to extend the areaunder irrigation, but in principle the basic method forapplying water over land has changed little. It has beenestimated that this method still serves more than 95 percentof the irrigated land worldwide.

Surface irrigation, also called gravity irrigation, canbe defined as the process of introducing a stream of water atthe head of a field and allowing gravity and hydrostaticpressure to spread the flow over the surface throughout thefield. To move forward, the surface of the flowing watermust have a downward slope in the direction of flow. This isgenerally provided by running water over a sloping landsurface, but in the irrigation of level land the water mustbuild its own slope (from where the water is deepest at thepoint of entry to where the depth is near zero at theadvancing front). The soil surface thus serves the dual roleof water conveyance and distribution, and, as it conveys thewater, it controls the spreading pattern and hence theopportunity time for water to infiltrate. Spatial andtemporal variability in the soil's infiltrability (that is,the downward flow rate which the soil can admit while itssurface is covered with a thin sheet of water) translatesinto non-uniformity of water distribution to the root zone.Consequently, in order to irrigate the entire fieldadequately, one must necessarily overirrigate some parts ofit. The main task of the surface irrigation designer is todevise a system that will provide the greatest possibledegree of water distribution uniformity attainableeconomically, i.e. with minimal initial and operating costs(Bishop et al., 1967).

Typically, the section of the field nearest to the waterinlet receives the greatest opportunity time and hence thegreatest depth of infiltration, whereas the downfieldsections farthest from the inlet receive the least. Thisnon-uniformity is most pronounced in coarse textured (sandy)soils, in which the infiltration rate is so high that much ofthe water entering the field infiltrates near the inlet andrelatively little water remains for the farther reaches ofthe field. On the other hand, fine textured soils such asclays generally exhibit low infiltrability, so a significantportion of the water applied might tend to flow overland tothe lower sections of the field while the higher sections(near the inlet) remain insufficiently watered. Thedistribution of water is obviously affected by the slope and

- 52 -

length of run. The major components of a typical surfaceirrigation system are illustrated in Figure 5.1, and theidealized distribution of water in an almost level border isillustrated in Figure 5.2.

Water application efficiency is usually higher on finetextured (clayey) soils than on coarse textured (sandy) soilsbecause, along with their lower infiltration rates, they alsoexhibit lower internal drainage rates and thus retain morewater per unit depth within the root zone. On the otherhand, clayey soils are more prone to excessive wetness,compaction, and impeded aeration. Internal drainage, waterretention and aeration all depend on soil layering,particularly if the profile contains flow-impeding "pans",and on depth of the water-table. In general, the applicationefficiency values attained with surface irrigation arerelatively low, while the dangers of water-table rise,waterlogging, and salinization are relatively high incomparison with alternative irrigation systems which allowbetter control over the application and the distribution ofsmaller volumes of water.

Land leveling and smoothing are essential operations inpreparation for successful surface irrigation. The cost ofearthwork to form level basins or uniformly graded borderscan be excessive if the slope is steep and irregular. Inmany alluvial river valleys, the slopes are less than 1t, butthe soils may be highly variable and the water-table tends torise, so while basin irrigation may seem advantageous in theshort run, it may become problematic in the longer run. Soilvariability may actually increase as a result of the cut andfill operation of land leveling.

On regularly sloping lands, graded long furrows andborders can significantly reduce leveling costs. Systemperformance can be improved because the slope can help tospread water downslope more rapidly. Rapid water spreadingreduces the difference in intake opportunity time between theinlet point and locations downslope. However, runoff can bea problem if the system allows excess surface water tocollect at the lower reaches of the field. If runoff is notprevented or removed, temporary waterlogging can reduceproduction. Excess water accumulation at the downstream endof the field can be reduced by adjusting the flow to matchinfiltration along the furrow or border, but this requires agreat deal of experience and control. An alternative is torecycle the runoff by returning it to the water supply.

Surface irrigation is generally more labor intensivethan sprinkle or trickle irrigation. Since the soil conveys,distributes, and infiltrates the water, and because soilconditions vary in time and space, it is necessary to adjustthe water supply to compensate for these variations. Theirrigator must control the supply for the purpose of ensuring

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Flume

.______________________________________ -LFromHead ditch -* water

=1 1 = = t = 7 ,> - ~source

Border Border Borderstrip strip strip

reuse -- Collecti n ditch V

reservoir

Figure 5.1. Components of typical surface irrigation systems.

Supplyditch

Ponded water

L l

Infiltration during advance

IIn iltration during recessic n

Figure 5.2. Ideal water distribution in a nearly level border

- 54 -

adequate irrigation throughout the field. The level of skillrequired increases as we go from dead-level systems (wherethe volume applied is the primary decision) to graded systemswhere the optimum flow rate may vary with time.

Initial capital investment for a surface irrigationsystem is usually lower than for sprinkle or trickle systems.Inflow control devices such as siphons, flumes, spills orgated pipe inlets are less expensive than a complete networkof pipes which is needed to deliver water to sprinklers ortricklers. Although less expensive at the field level,surface irrigation may involve greater costs for theconveyance system and pumping plant because the instantaneousflow rates required to quickly and efficiently spread waterover a basin or border are often greater than for a sprinkleror trickle system, especially on sandy soils.

Surface irrigation systems have relatively low energyrequirements in routine operation. Since gravity distributesthe water over the surface, the required hydraulic head isonly that which is sufficient to deliver water to the top ofthe border, basin, or furrow. However, since the efficiencyof water application is generally low for surface irrigationsystems, more water must be lifted per unit of land areairrigated and energy costs are increased accordingly, so partof the energy savings is lost thereby. Nevertheless, theoverall energy costs for surface irrigation systems aregenerally considerably less than those for pressurizedsystems (especially large scale sprinkle systems).

certain crops are sensitive to flooding and must beplanted on raised beds. On the other hand, certain fruitsand vegetables (such as strawberries and tomatoes) which canbe damaged by sprinkling do well with surface irrigation, asthe danger of leaf-scorch from the salt residue of sprinkledwater is obviated when water is applied below the foliage. Afurther advantage of surface irrigation is its avoidance ofthe wind-drift and canopy interception losses common undersprinkle irrigation.

Possibly the most important advantage of surfaceirrigation is its mechanical simplicity and easy adaptationto small land holdings. It requires little in the way ofmachinery. Such devices as gated pipes and siphons areeasy to operate and maintain, and require little priortraining or spare parts. Since the system operates at verylow pressures, the water loss and the associated damageby the occasional occurrence of leaks will be much less thanin the case of higher pressure sprinkle or trickle systems.

The principal disadvantage of surface irrigation remainsits generally low application efficiency, waste of water, andthe attendant dangers of water-table rise, waterlogging, andsalinization.

- 55 -

We shall now proceed to describe in brief several of themain variants of surface irrigation.

Wild Flooding

This is the least controlled of all surface irrigationtechniques. Water is delivered in a ditch to the upper partof an unlevelled plot and allowed to spread over the land ina manner dictated by the natural microtopography. Since thedegree of control over the flowing water is minimal, theresulting distribution of water is usually highly uneven.Consequently, some parts of the area become waterlogged whileother sections are salinized or remain completely dry. Thisunevenness is reflected in the eventual pattern of cropgrowth. Despite its obvious disadvantages, wild flooding isstill prevalent in many areas, used primarily for irrigatingpastures, hay or forage crops, and small grains.

Basin Irrigation

Basins are small level plots surrounded by low earthdikes (also called berms, or checks), within which water canbe impounded to irrigate a single tree or a few trees, aswell as vegetables or other crops grown in patches. Wherethe natural land surface is nearly level (as in wide rivervalleys), the minor surface undulations can be levelled byhand or by means of simple mechanical devices. Under theseconditions, the basins can be fairly large (a hectare ormore) and can be cultivated with machinery (Dedrick et al.,1982).

Where the land surface slopes or is irregular, thebasins are necessarily smaller. With basins as small as 100square meters, or even less, the use of tractors and tillagemachinery becomes uneconomic and even the use of draftanimals can be problematic, so nearly all of the work must bedone by hand.

Water is generally delivered to basins from small earthditches, but can also be supplied from a pipeline either onthe surface or underground. Risers from underground pipescan be placed at the corners of basins, so one riser mayserve four basins.

On sloping land, basin irrigation can be carried out inconjunction with terracing (Figure 5.3). Containment of thewater is likely to be a problem where the soil is either toosandy (and hence highly permeable), or too clayey (owing tothe tendency of the banks to crack upon drying). Terracesrequire much labor to construct and maintain, and where theslope is steep the basins are necessarily very narrow.Nevertheless, terracing is a widespread practice.

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Siphon or gote

Head ditch Orain

(a) A basin

(b) Terraccs

Figure 5.3. Basin irrigation on level and sloping land.

Border Irrigation

Here the land is divided into elongated plots confined/between low earth banks and configured to slope uniformlyfrom,the point of supply (being generally an outlet from aconveyance channel). The land surface should slope gently(at no more than 0.5%) in the direction of flow, and it isgenerally levelled laterally, along all cross sectionsperpendicular to that direction. The entering water movesdown the slope as an advancing wave, and infiltrates the soilas it moves (Figure 5.4). During the advancing phase, theupslope section of the strip, being closer to the source andhaving the longer period of ponding, naturally infiltratesmore water than the downslope section. At the termination ofthe supply, however, the water moving over the surfacerecedes downslope and augments the supply to the lowersection, thus tending to equalize the initially non-uniforminfiltration.

Properly designed and maintained, border strips canindeed be irrigated fairly uniformly (Turner et al., 1984).The important field characteristics to be considered in thedesign are the soil's infiltrability and the slope. Thedesign variables are the width and length of the strips andthe input discharge (inflow rate). The difficulty ofoptimizing the design and the variability of the fieldparameters can create problems in practice. Border checksare suitable for a variety of crops, including tree crops,close growing crops, and row crops. Where the latter cropsrequire ridges or beds, furrow irrigation can be practicedwithin the border checks.

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Siphons Field bunds

.; _A ~~** Advancing

a ! o' - ~~~su foc@ flow p / -~ front of I

ChO surfosurfcefllo

rSupply channel Advancing front of\ ~~~~~~~~~~~~~surface flow,,

\3Il Siphon /

' - Wett~~ir -n _ _ _- _

Figure 5.4. Advance of water into a border strip.Top: Movement of water over the surface.Bottom: Movement of water into the soil.

Border strips, also called border checks, are generally,though not necessarily, rectangular, the length rangingbetween 50 and 400 meters and being typically 10 to 30 timesthe width. Large border strips obviously require largeinflow streams to attain satisfactory water distribution.Typically, the required flow rates vary between 10 and 50cubic meters per hour, per meter width of plot. Commonly usedtotal discharges vary between 50 and 500 cubic meters perhour. The optimal inflow rate depends on the plot size, soilinfiltrability, and surface roughness, and must generally bedetermined by field trials. Border irrigation does notpermit the application of small quantities of irrigationsuch as can be applied by sprinkler or drip systems. Thesmallest practical application is of the order of 1000 cubicmeters per hectare, and larger quantities are often given inpractice. Assuming that the water applied can be stored inthe soil, a typical irrigation should suffice for 10-20 days.Border irrigation requires a high initial investment in landshaping, but offers low maintenance and operating costs.

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Furrow Irrigation

Here the soil surface is shaped into a series of furrowsseparated by ridges (Figure 5.5). At each irrigation, wateris conveyed into the furrows, which can be perceived asnarrow basins or borders. The ridges between the furrowsserve as planting beds for row crops, and they absorb waterfrom the adjacent furrows by capillary suction (Figure 5.6).The flow rate needed to achieve adequate water distributionin a furrow depends on the length and cross-section of thefurrow, as well as on the infiltrability and retentivity ofthe soil. Compared with other surface irrigation variants,furrow irrigation exposes a smaller area of open water, so itresults in less evaporation overall. Moreover, since theridges remain relatively dry in a properly irrigated furrowsystem, the movement and operation of workers and lightmachinery can take place sooner after each water application.

Furrow irrigation is most suited for the irrigation ofridge-grown crops such as groundnuts, potatoes, cotton,maize, and various vegetables. It is also used for irrigatingyoung orchards and vineyards. In young orchards, one furrowrunning alongside each tree row may suffice to supply thelimited zone of rooting, but as the trees develop they willrequire several furrows between the rows in order to wet thegreater volume of soil occupied by the extensive root system.

Furrow lengths vary from about 20 m to 300 m or more.Effective furrow widths range from 20 to 60 cm with between-furrow ridge spacings of 60-120 cm (crest to crest). Ridgesshould rise about 30-60 cm high above furrow bottoms. Poorlybuilt or maintained ridges are prone to overtopping, whichresults in loss of control, waste of water, erosion of soil,waterlogging and scouring of the upper portion of the rootzone, and disruption of normal field operations and traffic.

The dangers of overtopping, scouring, and loss ofcontrol combine to limit furrow inflow rates to the range of2 to 15 cubic meters per hour per furrow. Slopes along thefurrow may range from 0.2% to 2%, but on occasion may be ashigh as 5%. Various soils differ greatly in erodibility,structure, natural or tillage-induced roughness, as well asinfiltrability, so the optimal inflow stream should betailored to local conditions in each case. Water is usuallysupplied to furrows from head ditches, and less frequentlyfrom pipelines. To transfer water from a ditch, use is oftenmade of light-weight portable siphons, or of short tubes(called "spiles") inserted into the bank of the ditch. Thewater level in the head ditch is generally raised during eachirrigation by means of temporary dams (called "flags"), builtby hand. Better control can be achieved over the watersupply by the use of a gated pipe, and better still by theuse of a new technique called "cablegation" (Kemper et al.,1987).

- 59 -

Figure 5.5. Furrow irrigation.

A - to surfaceB - to rootsc - drainage

Figure 5.6. Infiltration of water from a furrow.

- 60 -

Since furrow shapes, surface roughness, and soilproperties vary spatially in the field, the rate of advanceof water in the furrows is never uniform (Shuval et al.,1986). Consequently, individual streams reach the downstreamedge of the field at different times. Because the upstreamend of each furrow normally has a longer opportunity time forinfiltration, the irrigator must compensate by running waterinto the furrow, albeit at a reduced rate, in order toprovide sufficient water for adequate infiltration along themiddle and lower reaches of the furrows. The effort toprovide an adequate irrigation throughout the length of eachfurrow results, practically inavoidably, in overirrigation ofsome furrows. The excess water, called "tail water,"generally accumulates at the bottom of the field or runs offthe field, creating a hazard of erosion or waterlogging, andmaking field roads impassable. Whenever possible, tail watershould be collected in a drainage ditch at the lower fieldboundary and led to where it can be reused advantageously.

An important difference between border (or basin)irrigation and furrow irrigation is that in the former theentire cropped area is ponded during each irrigation, henceinfiltration proceeds only downwards. In furrow irrigation,on the other hand, only about half the surface is ponded, somovement of water from the furrows is partly downward (underthe furrows themselves), partly sideward into the adjacentridges, and partly upward within the ridges. (See Figure24). As mentioned above, all irrigation waters contain somesalts, and these tend to move with the water. Since some ofwater sorbed into the ridges evaporates directly from theridge tops, leaving the salts behind, a fraction of thesurface zone may become saline. This may hinder germination,seedling establishment, and subsequent crop growth. Whenplanning a furrow irrigation system, the designer mustconsider the likelihood that the subsequent rainy season willleach the root zone of accumulated salts, the possibility ofalternating furrow irrigation with border irrigation of thenext crop in the rotation so as to facilitate leaching, andthe danger that salt accumulation even in the course of asingle growing season might cause an intolerable degree ofsalinization.

The rate and distance of lateral and upward sorption ofwater into the ridges depend on soil properties and on soilmanagement practices (tillage, compaction, organic residuecontent, etc.). Fine grained soils and densely compactedridges increase the distance of lateral sorption but decreaseits rate. Irrigators who believe they must accomplish thecomplete wetting of the ridges (as observable from the top)often waste water by overirrigation. Coarse grained soilsexhibit a very high rate of downward infiltration beneath thefurrow but very little lateral sorption into the ridges,hence they may require very closely spaced furrows or may becompletely unsuitable for furrow irrigation.

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Sprinkle Irrigation

Sprinkle irrigation, also called overhead irrigation, isthe application and distribution of water over the field intlhe form of a spray, or a jet which breaks into drops ordroplets, created by expelling water under pressure from anorifice (or nozzle). In effect, it is a simulated series ofrainfalls of controllable frequency, duration, intensity, andrange of drop sizes. A system of sprinkle irrigationconsists of a water source, a pumping unit, a pressurizedconveyance system (pipe network), and a set of nozzles toeject the water into the air and spray it over the field.

In contrast to surface irrigatioa, sprinkle systems aredesigned to deliver water to the field without depending onthe soil surface for water conveyance or distribution (Figure5.7). To prevent ponding and surface runoff (which may causeerosion as well as water loss), sprinklers are designed andarranged to apply water at a rate that does not exceed thesoil's infiltrability. Application rates lower than soilinfiltrability are preferable as they promote soil aerationand minimize structural damage to the soil surface. Althoughsprinkler systems are not so well adapted to soils with verylow values of infiltrability, they do permit irrigation ofshallow and sloping soils which cannot be irrigated bysurface methods without detrimental, or unduly expensive,land forming operations.

Flood Part flood Sprinkle Drip

~~~~~~~~~- A - ' 's~ .

I______ 1 /~ X 1' < '

Figure 5.7. The pattern of infiltrationunder alternative irrigation methods.

Water application uniformity under sprinkling depends onuniformity of the sprinklers and not on soil properties, solong as the application rate does not exceed infiltration.

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Hence high application uniformity (greater than 85% asdefined by the commonly used Christiansen Uniformity Index)and high application efficiency can be achieved with aproperly designed and operated system (Seginer, 1987).Excess application, so difficult to avoid with surfaceirrigation, particularly on sandy soils, can be minimized,and therefore the hazard of water-table buildup (and thecosts of groundwater drainage) can be greatly reduced.Sprinkling systems allow the application of frequent, lightand uniform irrigations, as required for sandy soils in aridregions because of the low retentivity of the soils and thehigh evaporativity of the climate. Uniform water applicationalso reduces the amount of water needed for leaching salts,thus, again, making drainage easier.

Water application efficiency under sprinkling irrigationis strongly affected by wind (Figure 5.8), especially duringdaytime when the air is warm and dry, and if the droplets aresmall and the application rate is low. Application uniformityand efficiency are not appreciably reduced as long as windspeed is below about 10 km/hr, but deteriorate progressivelyas windspeed increases beyond 15 km/hr. High-pressuresprinklers having long trajectories are the most affected.To avoid excessive losses due to wind drift and evaporation,nighttime irrigation may be preferable, though it is lessconvenient for the operator if he must be present. Automatedsystems are now available, but they are expensive. High windand evaporative conditions can also present problems when theirrigation water contains an appreciable concentration ofsoluble salts. Some crops are particularly sensitive and maysuffer leaf scorch because of the salts deposited on theleaves as the intercepted irrigation water evaporates.

WINDDIRECTION

I N G

Figure 5.8. Wind distorts the distributionof water under sprinkling irrigation.

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Sprinkle irrigation is an effective water applicationsystem for many but not all crops. Some crops are especiallysensitive to fungal diseases, leaf scorch, or fruit damage,and tall crops may obstruct hand-move or side-roll portablesystems. Another possible problem is the impact of fallingdrops on bare soil, causing slaking and surface sealing(crusting) which can be severe when the sodium ionpredominates in the water affecting the soil's clay fraction.

Sprinkle irrigation systems include four types of units:pump, mainline pipe, lateral pipe and sprinkler (Figure 5.9).

Fiur 5.9 tyia srinl iriain_ytm

- <s - ~~~~~~~~~~~~S PRIN KLE R

W s/ H 0 < --- sosssLAT~~~~~~LAERAL

-<Z//y,/> ~~~~~~~~~~~~MAINLINEg , ~~~~~~PUMPING UNIT PIPE UNIT,

Figure 5.9. A typical sprinkle irrigation system.

Numerous types of sprinklers are available commercially,the most common being the rotating hammer head and spraynozzle types, illustrated in Figure 5.10.

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Rotatingsprinkler

Spraynozzle

Figure 5.10. Common types of sprinklers.

Sprinkling systems can be either completely portable,or installed seasonally, or set permanently in place. Themost technologically advanced systems are the self-propelled,programmable irrigation machines, of the linear or rotarytypes. Such systems are usually designed for largescaleoperations and cannot easily be scaled down to the size ofthe individual farming units prevalent in developingcountries. Hence they are more suitable for corporate orgovernment managed irrigation projects.

Among the principal disadvantages of sprinkling systemsare their high initial capital costs, their high maintenancerequirements, and their high operating pressures (involvingenergy costs). Constant and meticulous maintenance ofsprinkle irrigation systems is crucial if these systems areto justify their costs. The savings in labor costs resultingfrom mechanization and automation can quickly be offset andeven exceeded by losses due to yield decline caused byfailure of the system during critical periods in the growingseason. The danger of system failure increases withtechnological complexity and requirements of expertise and

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quick availability of spare parts. For example, center-pivotsystems (Figure 5.11) are beautifully efficient as long asthey operate perfectly, but without expert maintenance, andspare parts, they are prone to break down. A malfunction inany one of numerous parts can soon transform a working marvelof technology into a standing monument of inefficiency.

Figure 5.11. A center-pivot sprinkle system.

Sprinkle irrigation does not require surface shaping orleveling and can be applied to areas of variable topography.To avoid runoff in sloping fields, the system designer mustensure that the application rate not exceed the expectableinfiltrability of the soil during any irrigation event. Theapplication rate of a sprinkle assembly depends on spacing ofthe sprinklers, on their nozzle radii, and on the operatingpressure. Precise tailoring of application rate to soilproperties and crop water requirements is difficult toachieve and generally involves trial and error experienceunder local conditions.

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The pressure or energy requirement of a sprinkleirrigation system is an important component of its runningcosts. Part of the pressure loss is due to the conveyance ofwater in pipes (dependent on pipe diameter and length) andpart to the particular sprinklers or sprayers used. Widerpipes have lower hydraulic resistances and hence reduce thepressure loss in conveyance, but wider pipes are also moreexpensive than narrow pipes. Sprinkler units requiring lesspressure have smaller radii of throw and hence supply waterto smaller areas, so more of them are necessary per unit ofland. Again, the designer is called upon to find the optimalcombination of variables, and must therefore consider therelative costs of equipment, energy, and labor against theexpectable benefits of alternative system designs.

Most commercially available sprinklers spread the waterover a circular area at a rate that diminishes with radialdistance from the sprinkler. If uniform distribution overthe field is desired, the sprinkled circles must overlap(Figure 5.12). Alternative geometric patterns (square,rectangular, or triangular) can be used to obtain the optimaloverlap among adjacent units.

Q)

'-I

0.-I

C)

04

Radial distance from sprinkler

Cumulative application rate

Figure 5.12. Distribution patterns for singleand overlapping sprinklers.

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RectangularTriangular

Square

* 0

Figure 5.13. Alternative arrangements of sprinkler grids.

For maximal efficiency of water utilization, the rate ofwater application should be equal to the water requirementsof the crop, not just on the average but everywhere in thefield. If the application rate is not uniform spatially,some parts of the field may receive insufficient water andwill provide low yields even while other parts of the fieldmay receive excessive water - again leading to reduction ofyield as well as to waste of water and leaching of nutrients.Part of the nonuniformity of application rate may be due tothe variation of operating pressure caused by friction lossesalong the pipelines constituting the conveyance system.These friction losses are affected by the lengths, diameters,and traightness (or "crookedness") of the conveyance lines,as well as by the number and type of sprinkler outlets enroute. Flow rate (discharge) through an individual sprinklernozzle is generally proportional to the squate root of thepressure at that point in the line:

Q = kAJ2gH (5.1)

wherein Q is discharge, k a specific friction or dischargecoefficient for the particular nozzle-of cross-sectional area.k, g is acceleration of gravity, and H the pressure head ofwater in the pipe discharging through the nozzle.

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A commonly accepted standard of irrigation uniformity isthat the difference between the sprinkler with the highestand that with the lowest discharge in a set of simultaneouslyoperating sprinklers not exceed 10%. From Equation (5.1) wecan deduce that the ratio of discharges between any twosimilar sprinklers, a and b, should be proportional to thesquare root of the ratio of their respective pressures:

Qa/Qb = I¶Ha/Hb (5.2)

If the tolerable ratio between the sprinkler with the highestdischarge to the one with the lowest discharge in the set is1.1 (corresponding to a 10% difference), then (since 1.1 isnearly equal to the square-root of 1.2) the maximal tolerablepressure difference between points in the line should be 20%.This is the basis or the so-called "20% rule of thumb" oftenrecommended in sprinkler irrigation.

Uniformity of pressure in the pipelines is harder tomaintain where the land is not level, as differences inelevation (in addition to friction losses) cause differencesin pressure head. Pressure regulators can help to ensure thedesired degree of pressure uniformity, but such devices addto the expense of the installation and themselves involvepressure losses.

The areal uniformity of sprinkling can be determined inpractice by arranging a set of open containers on the groundat equal spacing intervals and then measuring the amount ofwater intercepted in each of them during an irrigation. Theclassical and still standard method of characterizing theresults is to calculate the so-called Christiansen UniformityIndex, Uc (Christiansen, 1942):

Uc = 100 [1.d - (LIDI/MN)) (5.3)

wherein|Ii is the absolute (always taken to be positive)deviation of each of N measured amounts from their mean M. Avalue of 100% (i.e., D equals zero) would of course indicateperfect uniformity, but in actual practice Uc valuesexceeding 90% are very rare and a value of 85% is oftenconsidered satisfactory (Christiansen and Davis, 1967).

The operating pressure affects not only discharge butalso the array of drop sizes emitted by the sprinklers. Lowpressures result in large drops and poor distribution aroundeach sprinkler. High pressures, on the other hand, result ina fine spray (or mist) and consequently in a smaller radiusof throw and a greater vulnerability to wind drift andevaporation. Each type of sprinkler has an optimal operatingpressure, generally specified by the manufacturer, and acharacteristic functional dependence of discharge and of dropsize distribution on hydraulic pressure. The designer mustset his operating pressure and chose sprinklers accordingly.

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Drip Irrigation

Drip irrigation is the slow localized application ofwater, drop by drop, at a point or grid of points on, orjust below, the soil surface. Water is delivered to theplants via a set of plastic (e.g., weathering resistantopaque polyethylene) lateral tubes laid along the ground orburied at a depth of 15-30 cm and supplied from a field main.These tubes are left in place throughout the irrigationseason. They are commonly 10-25 mm (0.5-1 inch) in diameter,and are either perforated or fitted with special emittersdesigned to drip water onto the soil at a controlled rate asclose as possible to the mean current rate of waterconsumption by the crop. The trickling rate, generally inthe range of 1-10 liters/hr per emitter, should not exceedithe soil's infiltrability if runoff is to be avoided.

The operating water pressure is usually in the range of1 to 3 atmospheres (15 to 45 psi). This pressure isdissipated by friction in flow through the narrow passages ororifices of the emitters, so the water emerges at atmosphericlpressure in the form of drops. Commercial emitters areeither of the in-line type, spliced into the tubes; or of theon-line "button" type, plugged onto the tubes through a hole:jpunched into the tubing wall. These are shown in Figure 5.14.

Figure 5.14. Section of in-line emitter with capillary spiralflow path, and of on-line (plug-in) narrow-orifice emitter.

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Button emitters are easier to install than in-lineemitters but are also more vulnerable to accidental breakageor dislodging. Emitters are precalibrated to discharge at aconstant rate of 2, 4, or 8 liters per hour (0.5 to 2 gal/hour). Discharge is always affected by changes in pressure,but less so in the case of "pressure compensated emitters"now offered commercially.

The frequency and duration of each irrigation iscontrolled by means of a manual valve or of a programmableautomatic valve assembly. Water tends to spread sideways anddownwards in the soil from the point of introduction, underboth suction and gravity forces. Generally, however, onlypart of the soil is wetted by drip irrigation (Figure 5.15),that part being affected by the configuration and density ofthe drip points as well as by the internal water-spreadingproperties of the soil. In any case, the active rootingvolume is usually confined to a fraction (often less than50%) of what would be the normal root zone of a uniformlywetted soil.

,-=

Figure 5.15. The restricted wetting patternunder drip irrigation.

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It is a fact, however, that even very large andproductive fruit trees, and certainly smaller crop plants,can be sustained from a fractional soil volume, provided thatthis 'active rooting volume is supplied with the requisiteamounts of water and nutrients. The supply must be sofrequent and abundant as to make these resources readilyavailable to the crop on an essentially continuous basis.'The frequent irrigation regime, generally at intervals of oneday to no more than one week, relegates soil moisture storageto a less important role in drip irrigation than it plays inirrigation systems that are based on the infrequent "filling"(at 2-4 week intervals) and subsequent depletion of the soilmoisture reservoir.

Under drip, the wetted portion of the soil is maintainedin a continuously moist (though unsaturated, and hence wellaerated) state and that soil volume is never allowed to bedepleted or to approach the so-called wilting point. Thiscreates a uniquely favorable soil moisture regime and givesdrip irrigation a distinct advantage over surface andsprinkle irrigation, most especially for sandy soils of lowmoisture storage capacity and in arid climates of highevaporativity. In contrast with sprinkle irrigation, dripirrigation is practically unaffected by wind conditions, andunlike surface irrigation, it is little affected by slope,topography, or surface roughness.

The pattern of infiltration under drip irrigation issuch that water flows radially away from the point sourceunder the influence of capillary suction forces, as well asdownward under the influence of gravity. The wetted zone ishemispherical at first, but in time (as the tendency towardlateral spread lessens) it becomes increasingly elongated inthe downward direction. If irrigation is applied in excessof crop requirements, the initially onion-shaped wetted zonegradually becomes carrot-shaped, and eventually may form a"chimney" leading the excess water downward toward the water-table. The salts carried by the water tend to concentrate atthe surface periphery of the wetted circle, forming, as itwere, the "skin of the onion." This ring of salt, thoughoften visible from the top, is not generally as harmful as itseems, unless allowed to accumulate from year to year.

With drip irrigation, it is possible to use somewhatbrackish water (of the order of, say, 1,000 mg/liter ofsalts) for the irrigation of crops that are not too sensitiveto salt. Unlike in sprinkle irrigation, the brackishirrigation water does not come in direct contact with thefoliage, which is therefore not as prone to saline scorching.As the soil in the wetted zone is kept constantly wet (ratherthan being allowed to dry periodically), the salts areprevented from concentrating and the osmotic pressure of thesoil solution in the rooting zone is prevented fromincreasing to the point where it might affect the crop

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significantly. With brackish water, there is an increasedtendency for salts to accumulate at the peripheries of thewetted circles (Hoffman, et al., 1978), and these salts, ifnot leached by subsequent rainfall, should be leachedperiodically by means of a portable sprinkler system.

Drip irrigation can save water by reducing the portionof the soil surface that is wetted, thus decreasing theamount of direct evaporation. The magnitude of this effectdepends on the density of the grid of drippers, on thelateral spread of water beneath each dripper, and on thedegree of plant cover. In addition to saving water, thereduction in wetted area also discourages weed growth in theinter-row strips of an orchard or row crop, thus reducing theneed for frequent cultivation and the associated hazard ofsoil compaction by machinery. Under optimal managementconditions, yield increases of 20% or more have been reportedper unit area, and of 40% or more per unit volume of water.The potential increase in water use efficiency in dripirrigation (as compared to conventional irrigation) is dueboth to the possible water savings and the possible increasein yield resulting from the favorable soil moisture, air,salt, and nutrient regimes. However, under nonoptimalmanagement, excess water can be applied and wasted under dripirrigation no less than under alternative irrigation systems.

The fact that the rooting volume is restricted underdrip irrigation, so that the reservoir of soil moistureavailable to the crop is limited, makes the crop vulnerableto even short-term interruptions of the irrigation regime.Hence the constant maintenance of the system in perfectworking order is essential, and frequent irrigation is notmerely desirable but indeed mandatory. Since the supply ofnutrients might also be limited in the restricted rootingzone, the injection of supplementary fertilizers into thewater supply is also warranted. The advantage here is thatthe injection of fertilizers (and possibly also ofpesticides) into the irrigation water avoids the laborgenerally expended in ground application. Higher fertilizeruse efficiency might also result from the more preciseapplication of nutrients directly into the zone of maximalroot activity and in a form (dissolved in water) which isreadily available to the plants.

The capital investment costs of drip irrigation systemsare relatively high because large quantities of pipes, tubes,emitters and ancillary devices are required to deliver waterto specific sites in the field via closed conduits, ratherthan via the air or over the soil surface as in sprinkle andsurface irrigation. In addition, since the drip emitterorifices are narrow, expensive filtration equipment isnecessary to prevent clogging. Labor saving automation addsto the capital costs, as does fertilizer injection into thewater supply. Hence drip systems tend to be more expensive

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initially than surface irrigation systems (except wherecostly land forming is needed for the latter), and often moreexpensive even than sprinkle system. The savings of waterrelative to surface irrigation, and the savings of energyrelative to sprinkle irrigation, can reduce the longtermcomparative operating costs of drip systems. A typical dripiLrrigation system is illustrated in Figure 5.16.

KMTCJ; MITTER§R~

LATERALS; 1SUBMAIN LINE

MANIFOLD OR HEADER-i

BACKFLOW IMAIN LINEPREVENTOR 1 FIELD SYSTEMAUTOMATION,

\ / 1/7 CONTINUES

P \ |N|SPRESSURE /P U M P| \ GAUGE IL

> C JATER 17 1l1PESURECHEMICAL METER REGULATOR

TANK _ YATSPONINJECTOR PUMP

CONTROL STATION

Figure 5.16. A basic trickle irrigation system (schematic).

Drip irrigation can greatly reduce labor costs, butits successful operation demands continuous maintenance byskilled technicians with ready access to spare parts. It iscertainly not the sort of system that can be installed onceand for all, and that can continue to-operate trouble free byitself. Rather, it is a delicate system that needs constantattention and fine-tuning. The reliability of the system iscritical, as the available soil moisture reservoir is very

- 74 -

limited. If the system fails to deliver water even for a fewdays, the plants may suffer severe stress under the incessanttranspirational demand. Drip emitters must be inspectedregularly and cleaned or replaced whenever any fail byclogging or mechanical damage. Though the plastic tubingused in drip irrigation has many advantages (i.e., longevity,flexibility, smoothness, moldability, etc.), it is vulnerableto kinking when bent repeatedly, and to damage by rodents andby traffic. Lateral tubes are placed on the ground surfacefor annual crops, but sometimes buried (with risers to bringthe emitters to the surface) for perennial crops (e.g., fruittrees. Tubes on the surface are more vulnerable to traffic,but are easier to inspect and repair than buried tubes.

The most important aspect of maintenance is preventionof clogging by suspended particles (silt), by biologicalagents (e.g., algae), and by chemical precipitation of salts.Algae and other biological "slimes" can be controlled bychlorination. Special care is needed where the irrigationwater is drawn from eutrophic open reservoirs filled withsilt-laden runoff water. Such salts as calcium carbonate canbe prevented from precipitating by acidifying the water.Particles of various sorts can be removed from the irrigationwater by means of screens, media filters (filled with gravel,sand or diatomaceous earth), and/or centrifugal separators.Filters, in fact, are integral components of drip irrigationsystems. Gravel and sand filters are effective in removingsuspended solids from water, and are less expensive thanscreen filters. Their drawbacks are their large size andweight, and appreciable pressure loss. As the pores of thegravel or sand medium become clogged with retained solids,pressure loss increases and flow rate diminishes. Hencethese filters require frequent flushing by back-flow. Thisaction may not remove all of the fine material trapped in thefilter, so the filtering medium itself must be replacedperiodically. Screen filters are still more delicate andrequire even more rigorous inspection and servicing.

The spacing between lateral tubes is determined by thespacing of the crop rows, as these tubes are generally laidalongside each row. In crops with closely spaced rows, it isoften possible to economize in tubage requirements by using askip-row arrangement or by placing a single lateral between apair of close rows grown on a bed. This is not possible, ofcourse, in the case of widely spaced tree crops. Because ofthe pressure losses associated with narrow tubes, laterallengths (typically 30-150 meters) are generally shorter indrip irrigation than in sprinkle irrigation. In principle,drip irrigation is most suited to orchard crops and to fieldand garden crops grown in rows and beds, as well as toornamental plants, and least suited to close-growing fieldcrops requiring uniform wetting of the entire soil. Bookson drip irrigation have recently been published by Dasbergand Bresler (1985) and by Bucks and Nakayama (1986).

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Micro-Sprayer Irrigation

In recent years, several new methods of irrigation havebeen developed in the continuing effort to improve water useefficiency. Some of these methods are adaptable to the needsof small-scale farming in developing countries. One of themost promising and fast growing of these methods is micro-sprayer irrigation.

Micro-sprayers, also called mini-sprinklers or spitters,are similar in principle to drip systems in that water isapplied only to a fraction of the ground surface. However,instead of dripping water from emitters with capillaryorifices, micro-sprayer systems eject fine jets that fan outfrom a series of nozzles. Each nozzle can cover an area ofthe order of a square meter or more, which tends to be muchlarger than the individual areas wetted by drip emitters.Micro-sprayers thus can help to enlarge the volume of soilavailable for water and nutrient uptake by roots, a factorwhich can be particularly important for tree crops growing insandy soils.

Another important advantage of micro-sprayers over dripsystems is that, thanks to the larger nozzle orifices, thehazard of clogging of emitters is minimized and the stringentfiltration requirements of drip irrigation are relaxed. Forthis reason the installation costs are somewhat lower thanthose of drip irrigation. The pressure requirements ofmicro-sprayers are similar to those of drip systems (namely,of the order of 2 atmospheres) - much lower than those ofregular sprinklers. In other respects, as well, micro-sprayer irrigation retains the potential advantages of dripirrigation: it permits high frequency irrigation and theinjection of nutrients (fertilizers) into the water supply,and it lends itself to automation. Moreover, micro-sprayersystems can be scaled down readily to accommodate smallirrigation units such as are prevalent in many developingcountries.

The disadvantages of micro-sprayer irrigation relativeto drip irrigation are minor. The evaporation component ofthe water balance is increased somewhat (owing to the largerwetted area of ground, the spraying of water into the dryair, and the wetting of the lower foliage of the crop),though evaporation is still much less than in regular (fullcoverage) sprinkle irrigation. Because of the wetting ofleaves, the use of brackish water can be more problematicwith micro-sprayer than with drip irrigation. The pressurerequirements of the two systems are comparable.

Micro-sprayer systems are served by the same tubingnetwork as drip systems. A wide variety of emitter units(risers with spray nozzles), generally of durable plasticmaterials, is now available commercially.

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Low-Head Bubbler Irrigation

Closed-conduit (piped) irrigation distribution systemsare generally capable of saving water by increasingapplication uniformity and avoiding conveyance losses byseepage and evaporation. But, because most of them requirepumping to pressurize water for distribution, the water issaved often at the expense of increased energy consumption.Bubbler irrigation is a relatively new system designed toreduce the energy requirement by using inexpensive, thin-walled, corrugated plastic pipe of sufficient diameter thateven the low pressure head available from a surface ditchmight suffice.

Bubbler irrigation is another modification of dripirrigation, designed to simplify the system and make it lessdependent on components which are likely to deteriorate ormust be imported and are difficult to maintain. Here, nomanufactured emitters of any kind (neither drippers normicro-sprayers) are used, and the water is simply allowed to"bubble out" of open vertical tubes. These tubes, orstandpipes, roughly 1-3 cm in diameter, rise from buriedlateral irrigation tubes. The vertical risers are anchoredto stakes or posts, and their heights are adjusted up ordown, by calculation or by trial-and-error, so as to deliverwater at the exact rate desired.

Bubbler systems are particularly suited to theirrigation of widely spaced crops, like fruit trees orgrapevines, in which a standpipe bubbler can be installedalongside each tree or group of vines. The irrigation waterdelivered by each bubbler is distributed uniformly by fillingsmall level basins (surrounded by low ridges) with equalquantities of water. Such basins can be constructed by handand may be either circular or rectangular.

By the simple means described, the principle of closed-conduit delivery and of controlled low-volume high-frequency(and even partial-area) irrigation can be applied as in thecase of trickle or micro-sprayer irrigation, while not onlyminimizing pressure requirements but also (and perhapsequally importantly) obviating the need for filtrationaltogether.

Because of its simplicity and the absence ofmanufactured components such as nozzles, fittings, pressureregulators, and filters, bubbler irrigation has not beenpromoted as a commercial product by equipment salesmen, somany potential users are unaware of its advantages and of itsease of installation and operation.

A procedure for installing and calibrating bubblersystems was described in detail by Rawlins (1977). The totalcost of installing the proposed system was estimated at about

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$1600 per hectare. This cost is comparable with or lowerthan most drip or microsprayer irrigation systems (includingpumps and filters).

The advantages of the longer life of a completely buriedsystem, as well as the lower energy requirements, may makethis closed-conduit, gravity system an attractive alternativefor tree crops, particularly on relatively level lands thatcan be converted from surface irrigation methods. Bubblerirrigation is illustrated in Figure 5.17.

Figure 35. Bubbler irrigation.

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Considerations and comparative Costs

As mentioned above, the selection of an appropriateirrigation method for any combination of physical, agronomic,and socio-economic conditions involves complex, and sometimesconflicting, considerations. Among the physical factorsinvolved in system selection are topography, soils, crops,climatic conditions, water quality and availability, water-table depth, and size of irrigation unit. Among the socio-economic factors involved are the relative costs of land,water, energy and labor in relation to expectable returns;availability of capital, services and expertise; and thegeneral level of labor and management skills. Not all therelevant factors are known or can be weighed quantitatively.Therefore, the decision of what system to adopt is still, inpart, a matter of judgment, based on one's evaluation of therelative importance of the factors involved. Some of thesefactors are listed in Table 5.1.

A quantitative economic analysis of the actual costsand benefits of the alternative irrigation systems isdifficult to carry out because of the extreme variability ofcost factors, and of yields and produce prices, among variouscountires. Differential roduction costs not directlydetermined by irrigation methods but indirectly affected bythem (e.g., tillage and pest control) are even more difficultto assess. Cost estimates are available from California(Reed et al., 1976) and from Israel (Shani et al., 1979).These costs include local factors such as the price of water,machinery, energy, labor, depreciation, interest on capital,and even taxes - all of which differ greatly from place toplace.

A summary of these cost estimates is given in Table 5.2.The data is very tentative, and should be treated with greatcaution. We certainly do not wish to imply that the costfigures from California or Israel can be applied directly toany other country, or even necessarily to the same countriesunder changed conditions of crop and economy. At best, thedata provide a rough estimation of the relative costs ofinstallation and operation of alternative irrigation systemsin two particular settings that happen to exhibit the mostintensive type of commercial agriculture which justifies highcosts of the latest irrigation technology. Such estimationsshould not be applied blindly in other locations.

Above all, we wish to point out and emphasize theimportance of collecting data from additional locations,particularly in developing countries, and of learning as muchas possible from their experiences. We are still not surejust what constrains the modernization of irrigation aroundthe world, whether tangible and quantifiable economic andphysical factors, or perhaps intangible social factors thatmay still be unrecognized.

- 79 -

Table 5.1. Factors affecting the selectionof an appropriate irrigation methods.

Factors affecting selectionIrrig. * _method Land Soil Crop Climate Plusses j MinusesSurface Level or Suited For most For most Low cost Prone tc

graded for med. crops, climates Simple. over-to cntrl to fine except Only Low irrigateslope & textures those slightly pressure & risingsurface but not sensi- affected require. water-smooth- for in- tive to by wind tableness filtra- standing

bility water or15mm/hr poorlmm/hr aeration

Sprink. For all For most For most Affected Control Initiallands soils crops, by wind of rate costs &

except (drift, & freq. pressuresensit. evap. & Allows require-to fung. poor irrig. mentsdisease distri- slopingI& leaf bution) & sandyscorch soilsby salts

Drip For all For all For row Not af- High- Initialslopes, soils & crops & fected freq. & & annualregular intake orchards by wind. precise costs.& ir- rates but not Adapted irrig. Requiresregular close- to all Can use expert

growing climates saline managmntcrops water & Prone to

rough cloggingland. RequiresReduced filtra-evapor. tion

icro- For all For all For row May be High- Initialsprayer lands intake crops & affected freq. & costs &

rates orchards by wind precise mainte-irrig. nanceLessproneto clog

Bubbler Flat For all For tree Not High- Not alands & intake crops affected freq. commer-gentle rates by wind irrig. cialslopes No clog- product.

ging.-l_____ l_______ ___Simple.

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Table 5.2. Costs (U.S.$/hectare) of alternative irrigationsystems in California and Israel. (California data fromReed et al., 1976; Israel data from Shani et al., 1979).

Initial costs ($/ha) Annual costs ($/ha)Irrigation method

California Israel California Israel

Surface:

Border checks 1,400 300Furrows 1,000 480

Sprinkle:

Wheel line 1,620 350Center pivot 2,400 390Hand move 1,150 410

Field crops 1,220 170-350Truck crops 2,700 500-850Citrus trees 1,600 350-650

Tow move 1,500 1,400 510 250-550Permanent set 3,340 550Truck crops 4,120 700-1,200Citrus trees 2,820 510-1,040

Drip: 1,700 420

Truck crops 5,180 1,350-2,000Citrus trees 1,900 500-750

Micro-Spray: 2,370 470-700

Note:

The lack of data pertaining to surface irrigation inIsrael is due to the fact that practically no surfaceirrigation is practiced there. Agriculture in both Israeland California is highly intensive and commercial, withhigh-value cash crops justifying the costs of expensiveinstallations. In both locations, the ready availability ofexpert technical services and of spare parts facilitiates theadoption of technological innovations. Unfortunately, thesecircumstances are very far from being universal, so extremecaution must be exercised in any attempt to transfer theabove data to different locations. -

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6. DRAINAGE AND SALINITY CONTROL

Drainage Needs and Criteria

The word "drainage" has multiple meanings. It is usedin a general sense to denote water outflow from a section ofland. More specifically, it can serve to describe theartificial removal of excess water from cropped fields.Removal of free water tending to accumulate over the soilsurface is called "surface drainage." It is generallycarried out by shaping the land so as to direct and disposeof overland flow. The term "groundwater drainage," on theother hand, refers to the natural outflow or to theartificial disposal of excess water within the soil orsubsoil, and it generally involves lowering the water-tableor preventing its rise.

The presence of a high water-table in the soil profile,provided it is not too high, can in certain circumstances bebeneficial. Where precipitation or irrigation water isscarce, the availability of groundwater within reach of theroots can supplement the water requirements of crops. Soilsaturation per se is not necessarily harmful to plants. Theroots of many plants can, in fact, thrive in water, providedit is free of toxic substances and contains sufficient oxygento allow normal respiration. However, prolonged saturationeventually tends to cause anaerobiosis (oxygen deprivation).

As is well known, plant roots must respire constantly.Since most terrestrial plants are unable to transfer oxygeninternally from their canopies to their roots at the rateneeded to sustain normal root respiration, they require acontinuous supply of oxygen within the soil. The problem isthat a saturated soil can seldom provide sufficient oxygen tosatisfy the respiration requirements of the roots of anactively growing crop, since the excess water in the soiltends to block soil pores and thus retard the entry andtransmission of oxygen from the atmosphere. In a waterlogged soil, gas exchange between the soil and the atmosphereis in effect restricted to the surface zone, so the deeperparts of the profile become anaerobic (deficient in oxygen).'When anaerobic conditions persist, various substances arereduced from their normally oxidized states, and toxicconcentrations of ferrous, sulfide, and manganous ions candevelop, along with products of the anaerobic decompositionof organic matter (e.g., methane). At the same time,nitrification is prevented and denitrification may occur,causing loss of a vital nutrient. Various root pathogens,especially fungi, tend to proliferate in wet soils.

The development of a high water-table condition is notalways clearly evident at the surface, which may remaindeceptively dry even while the soil below is completely

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water logged. Where the effective rooting depth is thusrestricted, plants may suffer not only from lack of oxygen(and excess of carbon dioxide) in the soil, but also fromlack of nutrients. If the water-table drops periodically,plants growing in a water logged soil and having very shallowroots may even, paradoxically, suffer from occasional lack ofwater, especially when the transpirational demand is high.

Moist conditions near the surface cause the soil to besusceptible to compaction by animal and machinery traffic.Necessary operations (e.g., tillage, planting, spraying, andharvesting) are thwarted by poor trafficability. Tractorsare bogged down and cultivation tools are clogged by thesoft, sticky, wet soil. Furthermore, the surface zone of awet soil does not warm up readily at springtime, owing togreater thermal inertia and downward heat conduction, and toloss of latent heat by the higher evaporation rate.Consequently germination and early seedling growth areretarded in a water logged soil.

Plant sensitivity to restricted drainage is itselfaffected by temperature. A rise in temperature is associatedwith a decrease in the solubility of oxygen in water and withan increase in the respiration rate of both plant roots andsoil microorganisms. The damage caused by excessive soilmoisture is therefore likely to be greater in a warm climatethan in a cold one.

In addition to the restriction of aeration, the presenceof a high water-table condition can result in salinization ofthe soil, particularly in arid regions. The process ofevaporation inevitably results in the precipitation of saltsat or near the soil surface, and these salts can be removedand prevented from accumulating only if the water-tableremains deep enough to permit leaching without subsequentresalinization through capillary rise of the groundwater.

For all these reasons, irrigated lands typically requiredrainage. In fact, irrigation without drainage, especiallyin river valleys prone to high water-table conditions, can bedisastrous. Once-thriving civilizations based on irrigatedagriculture in river valleys (as in Mesopotamia, forinstance) have been destroyed through the insidious, and fora time invisible, process of groundwater rise and consequentwater-logging and soil salinization. The problem is stillprevalent, and getting worse, in many irrigated districts inthe irrigated valleys of the Indus, Nile, Tigris-Euphrates,Jordan, Murray, Rio Grande, Colorado, and San Joaquin Rivers,to name just a few. Because groundwater drainage is acomplex, exacting, and expensive operation, it is altogethertoo tempting to start new irrigation projects while delayingthe installation of drainage "until needed." By the time theneed for drainage becomes inescapably evident, however,drainage might also become prohibitively expensive.

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Artificial drainage of groundwater is generally carriedout by means of subsoil drains, which may be ditches,perforated pipes, or machine-formed "mole channels," intowhich groundwater flows as a result of the hydraulicgraidients existing within the soil or subsoil. The drainsthemselves are made to flow, by gravity or pumping, to thedratinage outlet, which may be a stream, a lake, anevaporation pond, or the sea. In some places, drainage watermay be recycled, or reused, for agricultural, industrial, orresidential purposes. Because drainage water may containpotentially harmful concentrations of salts, fertilizernultrients, and pesticide residues, it is not enough toprovide means to "get rid" of it; one must be concerned withthe quality of the water to be disposed of, and with thelong-term consequences of its disposal. Especially carelessis the practice of dumping the drainage back into a river,where it might salinize the water supply for other usersdownstream.

The flow rate from soil to drains depends on thefollowing main factors: (1) hydraulic conductivity (orpermeability) of the soil; (2) configuration of the water-table; (3) depth of the drain; (4) horizontal spacing betweenthe drains; (5) character of the drains, whether open ditchesor tubes; (6) nature of the drain-enveloping materials(generally gravel) used to increase the seepage surface andto prevent scouring of the soil and clogging of the drains;(7) diameters and slopes of the drains; and (8) the rate ofseepage ("recharge") to the groundwater resulting from theexcess of infiltration over evapotranspiration.

Various equations, empirically or theoretically derived,have been proposed for the purpose of determining thedesirable depths and spacings of drain pipes or ditches indifferent soil and groundwater conditions. Limitations ofspace preclude a comprehensive review, so we will confineourselves to a few general observations. Since fieldconditions are often complex and highly variable, theproposed equations are generally based upon simplifyingassumptions which idealize the system. Hence these equationsshould be treated as approximations which must be examined inthe light of all information obtainable in actual practice.

The classical approach, still widely used, is that ofHooghoudt (1937), designed to predict the height of the watertable which will prevail under a given rainfall or irrigationregime when the conductivity of the soil and the depth andspacing of the drains are known. That height above thedrains, H, is a function of the horizontal distance to thenearest drain, X:

H=QX(S-X)/2KD (6.1)

where Q is the percolation flux (the excess of infiltration

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over evapotranspiration), S the spacing between adjacentdrains, K the saturated soil's hydraulic conductivity, and Dthe height of the drains above the impervious layer that ispresumed to exist at some depth in the subsoil. At themidpoint between the drains, where X=S/2, we get the maximumheight Hmax of the water-table mound:

Hmax=QS2 /8KD (6.2)

This equation shows that the height of rise of the water-table between drains is related directly to the rechargingflux Q and to the square of the distance S between thedrains, and inversely related to the soil's hydraulicconductivity K (Figure 6.1).

Alternative equations have been offered by, amongothers, Kirkham (1958), Ernst (1962), and the U.S. Bureau ofReclamation (Luthin, 1966; van Schilfgaarde, 1974). Theranges of depth and spacing generally used for the placementof drains in field practice are shown in Table 6.1, based onmean values of hydraulic conductivity for different Soiltypes.

Table 6.1. Typical depths and spacings of tile drainsfor different soil types

Soil type: lHydr. conduct.lSpacing of drainslDepth of drainsiI cm/day I meters I meters I

-------------------------------------------------------------

Clay I 0.15 10-20 I 1-1.5Clay loam 1 0.15-0.5 i 15-25 1-1.5Loam 0.5-2.0 I 20-35 J 1-1.5Sandy loam I 6.5-12.5 30-70 I 1-2Peat 12.5-25 I 30-100 I 1-2

It is of interest to note that in Holland, the countrywith the most experience in drainage, a common criterion fordrainage is to provide for the removal of about 7 mm/day andto prevent a water-table rise above 50 cm from the soilsurface. In more arid regions, because of the greater rateof evaporation and salinity of the groundwater, the water-table must generally be kept considerably deeper. In theImperial Valley of California, where the soils are coarse tomedium textured, drain depths range from about 150 to 300 cm,and the water-table depth midway between drains is about 120cm. For medium and fine-textured (clayey) soils the depthshould be still greater where the salinity risk is high.Since there is a practical limit to the depth of drainplacement, it is the density of drain spacing which must beincreased under such conditions. -

A typical drainage layout for a small farm unit isillustrated in Figure 6.2.

- 85 -

Steady inf,lrotion

Surface I I I I I I I I i

Constant water table > (a)

Drain Rad oa Drain

Imnpervious loyer

Soil surface's ///// // /// /// /7/7/

Drain j Drain

Impervious layer

Figure 6.1. Groundwater drainage under steady flowconditions (water table at constant depth) andunsteady conditions (falling water table).

500m

Sur~facek1contours

(5m intervals)

91SE~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.

Lateral drainv

Surfaceslopedirection

Outflow

Figure 6.2. Drainage layout for a 25 hectare farm unit.

- 86 -

Soil Salinity and Leaching Requirements

Soil salinity is the term used to designate a conditionin which the soluble salt content of the soil reaches a levelharmful to crops. Soil salinity affects plants directly byreducing the osmotic potential of the soil solution and bythe toxicity of specific ions such as boron, chloride, andsodium. High relative concentrations of the sodium ion,moreover, can have a deleterious effect on soil properties,causing a deterioration of the desirable aggregate structureand a reduction of porosity and permeability.

Soil salinity is an ever present hazard in irrigatedfarming, particularly in arid regions. Proper irrigationis aimed at maintaining a supply of good-quality moistureneeded by plants to answer the climatically imposed demandfor transpiration, while at the same time ensuring adequateaeration, temperature, nutrient supply, and salt balancethroughout the root zone. Efficient irrigation, furthermore,avoids wasting water through runoff or excessive drainage,except insofar as some drainage is necessary to flush outpotentially harmful salts which would otherwise accumulate inthe root zone. Crop plants extract water from the soil whileleaving most of the salt behind. Unless leached away, eithercontinuously or at least periodically, such salts will sooneror later begin to hinder crop growth. However, unlesscoupled with effective drainage, the application ofirrigation in excess of plant water requirements (in aneffort to leach out salts) can in itself cause soilsalinization by raising the water-table and allowing thesalts to return via capillary action.

In the same manner that we presented the water balancefor the root zone, it is instructive to formulate the saltbalance, as follows:

Salt content change = [Input of salt] - [Output of salt]S=f(Vi)(Ci)+(Vr)(Cr)+(Vc)(Cc)]-j(Vd)(Cd)-(Vp)(Cp)] (6.3)

where S is the change in the salt content of the root zone;Vi, Vr, Vc, Vd, Vp are - respectively - the volumes of waterentering as irrigation (i), rainfall (r), and capillary riseof groundwater (c), or leaving the root zone as drainage (d)or plant uptake (p). Ci, Cr, Cc, Cd, Cp are the correspondingaverage concentrations of salt in the same water volumes.Since the concentration of salts taken up by the crops isusually very low, as is the concentration of salts inrainfall (except in areas where saline sea-spray is waftedonto the land), the salt balance can be simplified asfollows:

S=(Vi)(Ci)-(Vd-Vc)(Cd) (6.4)

If the water-table is kept deep enough so that no substantial

- 87 -

capillary rise of groundwater into the root zone takes place,the equation further simplifies to:

S=(Vi) (Ci)-(Vd) (Cd) (6.5)

Assuming no salt accumulation, we obtain:

(Vi) (Ci)=(Vd) (Cd) (6.6)

The importance of leaching becomes apparent if we ponderthe startling fact that a 1 meter depth of irrigation (anamount normally applied in a single season) of evenreasonable quality water contains sufficient salt to salinizean initially salt-free soil (i.e., about 5000 kg/hectare).However, any attempt to leach without provision of adequatedrainage is not merely doomed to failure but can indeedexacerbate the problem.

The question is how to determine the optimal quantity ofwater which must be applied to effect leaching. Clearly, theapplication of too much water can be as harmful as theapplication of too little. Exaggerated leaching not onlywastes water but also tends to remove essential nutrients andto impede aeration by waterlogging the soil.

The "leaching requirement" concept developed by the U.SSalinity Laboratory (Richards, 1954) has been defined as thefraction of the irrigation water that must be leached out ofthe bottom of the root zone in order to prevent average soilsalinity from rising above some permissible limit. (Thatlimit was estimated to be 4 mmhos/cm - expressed in terms ofthe electrical conductivity of the soil solution - forsensitive crops like clover, celery, strawberry, and radish;8 mmhos/cm for tolerant crops like beets, alfalfa, andcotton; and 12 mmhos/cm for very tolerant crops like barley).Thus, the leaching requirement depends upon irrigation waterquality, the amount and rate of soil moisture extraction bythe crop (evapotranspiration), and the specific salttolerance of the crop itself.

Assuming steady-state conditions of through-flow (thusdisregarding short-term changes in soil moisture content,flux, and salinity), and furthermore assuming no appreciabledissolution or precipitation of salts in the soil and noremoval of salts by the crop or by the capillary rise ofsalt-bearing water from below, we obtain from equation(6.6):

Vd/Vi=Ci/Cd (6.7)

Since the volume of water drained, Vd, is the differencebetween the volumes of irrigation and-of evapotranspiration(V'i-Vet), we have

(Vi-Vet)/Vi=Ci/Cd (6.8)

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Hence

Vi=[Cd/(Cd-Ci))Vet (6.9)

The water volumes are usually expressed per unit area ofland as equivalent depths of water, D (10 cubic meters perhectare being 1 mm in terms of depth). Accordingly, we obtainthe formulation first given by Richards (1954):

Di=[Cd/(Cd-Ci)]Det (6.10)

where Di is the depth of irrigation and Det is the equivalentdepth of "consumptive use" by the crop (evapotranspiration).

The leaching requirement concept implies that it ispossible to control the salinity of the drainage water and tomaintain the desirable level of salt concentration in themain part of the root zone at some intermediate level betweenCi and Cd by varying the fraction of applied water which ispercolated through the root zone. A distinct disadvantage ofthe leaching requirement concept is its disregard of short-term fluctuations in the salt concentration of the soil'supper zone which take place during individual irrigationcycles, as affected by the frequency, as well as quantity, ofirrigation. In particular, the spatial and temporal variationof root zone salinity is affected by the degree to which soilmoisture is depleted between irrigations. The less frequentthe irrigation regime, the greater the build-up of saltconcentration between successive irrigations and the greaterthe fluctuations in osmotic pressure of the soil solutionduring the irrigation season.

With the recent trend toward high-frequency irrigation(Rawlins and Raats, 1975), it has become possible to maintaina zone of soil near the surface with a soil solutionconcentration nearly equal to that of the irrigation water.This depth can be increased by increasing the volume (orrate) of water application. Beyond this depth, the saltconcentration of the soil solution increases with depth to asalinity level which depends on the leaching fraction,defined as Vd/Vi. It becomes especially important tomaintain a higher soil moisture condition by frequent andsufficient irrigation whenever brackish water is used forirrigation.

In determining the optimal leaching fraction, a possibleconstraint must be taken into account, namely the limitedhydraulic conductivity of the subsoil at the lower boundaryof the root zone. If the soil is relatively impermeable,then it might restrict the attainable leaching fraction(Rhoades, 1974). Unless this limitation is recognized anddefined, a blind attempt to follow the leaching requirementconcept may lead to waterlogging the soil and thus toaggravating, rather than alleviating, the salinity problem.

- 89 -

Quality of Irrigation Water

Thus far, we have dealt with irrigation only in terms ofthe quantity and frequency of the water applications.However, it should be obvious from our discussion of soilsalinity and drainage that the quality of irrigation watercan be just as important a determinant of crop growth as thequantity. The application of saline or brackish water mayhiinder crop growth directly, and may also cause soildcegradation (both salinization and alkalinization). Moreover,beyond its effects on the crop and the soil, irrigation waterof low quality can also affect the larger environment byintroducing potentially harmful substances into the surfacewraters or the groundwater toward which it drains. Quality ofirrigation water is especially important in arid climates,where the high rate of evapotranspiration further contributesto the increase of salt concentration in the soil anddlrainage waters.

irrigation water is usually drawn from surface orgjroundwater sources and typically contains salts in the rangeof 200 to 2,000 parts per million (ppm), i.e. 200 to 2,000grams of total dissolved solids (TDS) per cubic meter.Irrigation water thus contains 10 to 100 times more salt thanrain water (except where the rain is affected by sea-spray,which is prevalent in certain coastal areas). In a singleseason, the application of 1,000mm of medium quality waterintroduces around 5 tons of salt to a hectare of land. Ifallowed to accumulate, such an annual increment can salinizethe root zone to the point where continued cropping becomesimpossible and the soil is rendered completely sterile withinjust a few seasons. Obviously, the need to include properleaching and drainage as an integral part of the irrigationprogram becomes all the more imperative when the irrigationwater is even slightly brackish.

As the requirements for water increase, for agricultureas well as for industry and domestic use, the available highquality water resources are becoming ever more limiting.Hence there is a growing tendency to seek ways to utilizewater resources of lower quality formerly consideredunusable. With proper management, however, the use ofbrackish water for irrigation has been proven to be feasible,particularly for tolerant crops and with high-frequencyirrigation methods.

The quality of irrigation water is determined by thefollowing criteria (Shainberg and Oster, 1978): 1. Totalsalinity, i.e. the total concentration of all salts in thewater supply; 2. Sodicity, i.e. the concentration of sodiumrelative to other cations; 3. Anion composition of thesoLution, especially the concentration of bicarbonate andcarbonate anions; and 4. Concentration of toxic elements,chiefly boron.

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Total salinity affects crop-water relations mainly bylowering the osmotic potential of the soil solution. Toabsorb water from the soil, plant roots must overcome thisosmotic potential, which acts in concert with the soil'smatric potential (called "tension") to' oppose uptake. Ineffect, increasing the salinity of the soil solution istantamount to increasing the dryness of the soil. Henceplants growing in the presence of brackish soil moistureexperience stress sooner, and therefore wilt at a highervalue of soil wetness than plants grown with non-saline water(Figure 6.3).

100

80 \ SalinitiesUnsuitable

for Crop'a \ \ \ Production

20 3

L)6

40

> 40_ \ \ \ \_

20

Sensitive Moderately Moderately TolerantSensitive Tolerant

0 5 10 15 20 25 30 35Electrical Conductivity of Saturation Extract (ECa, millimhos/cml

Figure 6.3. Effect of soil solution salinity on crop yield.

The salinity of irrigation water, as well as of the soilsolution, is most conveniently measured and characterized interms of the electrical conductivity. The measurement is asimple one, usually done by dipping a pair of electrodes inthe solution and determining the resistance to theconductance of an alternating current between them, using aWheatstone bridge.

Though the electical conductivity is affected by thenature of the specific salts present, as shown in Figure 6.4,it is roughly proportional to the total concentration ofelectrolytes (salts) in solution. The units commonly used toexpress the electrical conductivity are mhos/cm ormillimhos/cm (the mho being the reciprocal of the unit ofresistance, Ohm), or, in SI units, Siemens/m. Since theelectrical resistance is also affected by temperature, it isimportant to reference all measurements to standardtemperature (25 C).

- 91 -

4 I 1111 I l 11111 I . I4

:3-

2 -

Na2 SO4

6 000 HC //XE 4000

0

2000 NaHCO,

800

400 NaCI

200

100.2 A .6 .8 1 2 4 6 8 10 20 40

Conductivity (millimhos/cm (EC x 10't at 250 C)

Figure 6.4. Relation of electrical conductivityto the concentrations of various salts.

The principal solutes generally present in irrigationwaters are the cations calcium, magn'esium, sodium, andoccasionally potassium; along with the anions chloride,sulfate, nitrate, and bicarbonate. In addition to these,some trace elements (such as boron, lithium, selenium, andseveral heavy metals) may be present and may have aninhibitory effect on plant growth. Of these, boron deservesspecial attention. Although it is essential for plantgrowth, it becomes highly toxic at concentrations onlyslightly above optimum (Shainberg and Oster, 1978). Amongthe crops that are particularly sensitive to boronconcentrations as low as 0.3-1 ppm are citrus, avocado,apricot, peach, cherry, plum, grape, apple, pear, and navybean. The relatively tolerant crops,-which are able towithstand boron concentrations exceeding 2 ppm, includecarrot, lettuce, cabbage, turnip, onion, alfalfa, beet, date,and asparagus.

- 92 -

One of the greatest hazards associated with the use ofbrackish irrigation water is the possible increase of theexchangeable sodium percentage (ESP) of the soil's clayfraction. This increase depends on the ratio of the sodiumion concentration to the square root of the meanconcentration of the divalent cations (calcium and magnesium)also present in the solution. That ratio is called the"sodium adsorption ratio" (SAR), defined as (Richards, 1954):

SAR = (Na]/ [([Ca]+[Mg])/2 (6.11)

The higher the sodium adsorption ratio, the greater thehazard that the soil will be sodified (Figure ). Thetendency toward sodification further increases whenbicarbonate ions are present. Sodification can result in thesevere degradation of soil structure, including the slakingof aggregates, dispersion of clay, clogging of pores by claymigration, reduction of soil permeability to water and air,surface sealing and crusting, impairment of infiltration andaeration, and inhibition of germination and subsequent cropgrowth. An exchangeable sodium percentage in the soil as lowas 10% may initiate some or all of these effects, and thedamage exacerbates rapidly as ESP increases still further.

1.00

r 75

50~~~~~~~~~~~~~~

o

0~~~~~a*

0 10 20 30 40 50 60

Sodiumn Adsorption Ratio WSARS

Figure 6.5. Relation of exchangeable sodium ratio ofsoils to sodium adsorption ratio of the soilsolution. Source: Shainberg and Oster, 1978.

- 93 -

As the concentration of the soil solution is increasedby root water uptake and by direct evaporation from the soilsurface (Figure 6.6), it can exceed the concentration of theapplied irrigation water by' severalfold, especially under aregime of infrequent irrigation. Since roots extract waterfrom the soil while largely excluding the salts, the soilsolution tends to increase in concentration in time and indLepth as it moves downward into the soil profile. As SARincreases in proportion to the square root of the totalconcentration, the gradual increase in concentration of thesoil solution results in an increase of SAR with time betweenirrigations and with depth in the profile even though theremight be no change in the relative composition of thesolution. This further emphasizes the importance of a high-frequency irrigation regime, which prevents the concentrationof the soil solution from rising unduly in the principal rootzone.

t EvaporotiOnleaving salt

in ridge

Saline water

ScitII

Leaching effect

Root zoneleoached

Figure 6.6. Effect of evaporation on salt concentrationat the soil surface (under furrow irrigation).

- 94 -

The soils which are most vulnerable to sodicity arethose with an appreciable content of montmorillonite clay,which is prevalent in soils of arid regions. On the otherhand, arid-zone soils are often rich in calcium carbonate(and sometimes contain calcium sulfate as well), which tendsto buffer sodicity to some extent. A high salt concentrationprevents the clay-dispersive effect of adsorbed sodium.Hence the danger of structural degradation is greatest,paradoxically, whenever an initially saline soil is leachedwith water of low salinity. Thus, "high-quality" water isnot always the most desirable for irrigation. When highsodium water is used for irrigation, difficulties may notappear during the irrigation season, but when the rainyseason comes and the salts leach from the upper soil layer,that layer disperses, the surface forms a seal (crust), andthe permeability is drastically reduced. A classification ofirrigation water quality, with respect to both the salinityand sodicity hazards, is shown in Figure 6.7.

_ - l IF 11T 11TTl

030 2

I

24

22-

20-

N I-

14

0

0 0

00

00 250 750225CONOUCTIVITY- MICROMNOS/Am. (ECWtOl AT 25' C.

L.ow mEDiUMA XiH VERY "416

SAUNITY HAZARD

Figure 6.7. Classification of irrigation waters accordingto salinity and sodicity hazards. Source: Richards, 1954.

- 95 -

7. ISSUES AND IMPLICATIONS

Human Aspects of Irrigation Development

Irrigation is not simply an exercise in mechanics ortechnology. It is a human activity and a social undertaking.No publication on irrigation methods or economics in relationto the environment and to crop production should fail toemphasize that, ultimately, the efficiency of an irrigationproject depends on the quality of the human effort investedin it. Moreover, an irrigation project is not a systemdesigned exclusively for producing crops but also - andperhaps even primarily - a place for a community of people towork cooperatively and gainfully while leading healthy andharmonious lives.

As in other human activities, particularly cooperativeones, the first requirement for success is that the workersinvolved be well coordinated, and that each of them bestrongly motivated and personally committed to the task. Thesecond requirement is that they be properly informed, notmterely trained in the performance of routine operations butindeed able to understand the fundamental processes and toakpply the basic principles involved in proper irrigationrmanagement. The necessary cadre of skilled and motivatedworkers will not come of itself, but must be initiated andnurtured. An investment in personnel is even more vital inthis respect than an investment in pipes or pumps.

One of the worst mistakes this writer has seen is thetendency of some irrigation project managers to operate in anauthoritarian manner, and to require that their workers actunthinkingly and obey the instructions handed down to themwithout question. Depriving intelligent human beings of anypersonal stake in their own work, and of the opportunity andincentive to generate and apply their own creative ingenuity,is a waste of a resource even more precious than soil andwater. People who are given a sense of participation, andallowed to reap rewards commensurate with their initiativeand contribution to the enterprise, care much more for theirwork and devote much more of themselves to the task ofhelping it to succeed. The incentives offered can be social,administrative, or economic; or, best of all, a combinationof the three.

Perhaps the greatest incentive is to allow, indeed toencourage, people and families to work for themselves, ontheir own plots of land and with access to assured suppliesof water and all other essential means of production.However, the complex problems involved in land tenure, landreform, water rights, and the coordination of resourceallocation and utilization, range far beyond the limitedscope of this work.

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In addition to providing workers with incentives, anirrigation scheme must also contribute to human welfare in alarger sense. Many, perhaps most, irrigation systems in thedeveloping world are used for non-agricultural purposes aswell: for domestic water needs, livestock water requirements,waste disposal, power generation, transportation (as well asfishing and possibly even recreation). Some of theseundeniable needs may indeed interfere or conflict with thebasic functic7ning of the irrigation project, particularly ifthey are not recognized at the outset and included in theinitial planning stage.

One of the most serious problems in irrigation projectsis the potential health hazard resulting from the use of openwater channels for drinking, bathing, washing of clothes, andthe disposal of human and animal wastes. Such activitiestend to pollute the water supply for users downstream. Ithas often been said that "wherever water goes, diseasefollows." Unfortunately, water storage and conveyancestructures present very favorable breeding grounds forvectors (such as mosquitoes and snails) of some of the mostdebilitating diseases rampant in the developing world. Thecooperation of public health specialists should therefore beincluded in the design and operation of all new irrigationschemes, as well as in the rehabilitation or modernization ofexisting schemes.

Among the factors which may contribute to the control ofwater-borne diseases are the following: (1) concrete liningand shaping of the conveyance and drainage channels in orderto increase the water flow velocity and thus to preventstagnation along the banks (as well as, incidentally, toreduce seepage); (2) control of riparian vegetation withinthe channels, to prevent clogging, stagnation, and harboringof diseases; (3) protection of the channels from wading byanimals, which may breach the banks and pollute the water;(4) control of waste disposal by humans, who must of coursebe provided with alternative and more sanitary means of wastedisposal that are environmentally safe and sustainable; and,finally, (5) treatment of the water used directly for humanuse (drinking and bathing), possibly including the selective- and very careful - application of chemicals to controlparasites.

The proper design and management of an irrigationproject is thus seen to be a complex and comprehensiveundertaking, involving much more than hydraulic engineeringand agronomy. Such a design is necessarily site specific notonly because of the variable physical and agronomicconditions but also because of the special combination ofhuman and economic factors which exists in each case andwhich must be recognized explicitly and taken into account inany attempt to improve the practice of irrigation and thelives of people dependent on it.

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A Look Back

There was a time when experts in the industrializedcountries believed that they had ready-made solutions to theproblems of underdevelopment. All that was needed, theybelieved, was to transfer already existing know-how andequipment, and then development and modernization wouldiimmediately ensue. Unfortunately, that was a costly fallacy,resulting all too many times in the hasty introduction, oreven imposition, of systems that were at variance or inconflict with the existing environmental, cultural, or socio-economic conditions. False starts and faltering initiativeshave plagued attempts at technology transfer. Huge investmentof resources often led to disappointment and disillusionment.

Most capital expenditures for irrigation in developingcountries have been spent on large scale projects. Typically,a well meaning national or international agency wouldconceive and finance a showcase project, based on elaborateengineering. Experts would be hired from abroad to designthe system, then a contracting and supplying firm would beengaged to implement the design. Soon, the marvel of moderntechnology would be assembled and demonstrated, with greatpride and fanfare. The gap of centuries had been bridged, soit seemed, in a single master stroke. Then the foreigncontractors, having done their job and reaped their profit,would disappear. Soon afterwords, the elaborate system wouldcease functioning, owing to the failure of a single cog or toinexpert operation. Paucity of local resources and thedifficulty of obtaining replacements or expertise fromabroad, exacerbated by an indifferent work force deprived ofincentives, would combine to delay the necessary repairs andto perpetuate the failure. The entire expensive system wouldthen stand idle, a mute monument to inappropriate technologytransfer.

A case in point are the large scale center-pivotsprinkler systems, prefabricated in the U.S.A., for instance,and assembled in various developing countries where thetraditional scale of farming, the cost of energy, and theavailability of equipment and of technical services, contrastsharply with those in America. In too many places, theseimposing machines have become white elephants.

Most organizations responsible for designing irrigationprojects have had a strong civil engineering orientation(Power, 1986), and thus have tended to emphasize design andconstruction over operation and maintenance aspects. Therehas generally been insufficient input from personnel trainedin irrigation agronomy and agricultural hydrology (includingthe physical and physiological aspects of soil-crop-waterrelations). In some countries, there is still a dichotomybetween the agency responsible for developing water resourcesand for allocating and delivering water via canals to fields,

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and the separate agency responsible for utilizing the wateron the farm. Often, the water resources agency is endowedwith greater power and prestige than the on-farm managementagency, and hence the first is unlikely to accept guidelinesor criteria for water allocation from the second.

This disjointed approach has in some cases resulted inan inconsistent policy and an inability to solve the problemsinvolved in the task of modernization and the achievement ofgreater efficiency. These problems may include: (1) poortechnical design, failing to consider and optimize soil-crop-water relations; (2) omission of drainage in the initialplanning and funding of the project; (3) neglect ofdeleterious environmental impacts such as salinization,alkalinization, waterlogging, water-table rise andgroundwater contamination; (4) failure to train the staff andto provide them with a sense of involvement and personalincentives to improve the system; (5) disregard of the needto keep a spare parts inventory and to provide adequately forsteady maintenance and repair of the irrigation system as itscomponents inevitable deteriorate with age and usage; and(6) lack of sensitivity to deep-seated cultural factors andsanitation requirements.

Key decision makers have tended to favor high-visibilitycapital projects with impressive machinery, while neglectingissues of training and maintenance, which are of interest tolower level personnel without decision-making power (Power,1986). Top-level decision-makers have also tended to harborunrealistic expectations as to the time required foragricultural development, and have tended to be impatientwith technical or human constraints. Some have beeninsufficiently aware that the technology they were trying totransfer from the industrialized countries was designed for acapital-intensive market economy based on the readyavailability of technical services and a complex economicinfrastructure. In addition, corporations serving ascontractors have tended to prefer large projects involvingthe sale of expensive hardware, whereas in reality it isoften small pilot projects with major emphasis on technicalsupport and training, rather than on complex equipment, thatare more likely to be successful and to initiate progress.

Needed in the area of irrigation development is not somuch technology transfer per se, but the growth of anawareness of the need for, and the principles of, improvingthe efficiency of land and water use. The inefficient use ofland and water is not an exclusive preserve of any particulargroup of countries. The malady is indeed universal, and sois the increasingly urgent need to cure it. No technologywill succeed automatically, though some have a greaterpotential than others. Ultimately, the job must be done bypeople working in separate locations, endowed with thenecessary incentives, the knowledge, and the means.

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A Look Ahead

The major aim and thrust of this publication is toinduce greater awareness and understanding of the conceptsanld practices governing the efficient use of water inirrigation. Just how to apply these principles in variouslocations depends on specific circumstances which affect theeconomics of land, water, labor, machinery, and energy, aswell as the crops to be raised. It further depends on socialfactors and on long-term environmental effects. No simpleuniversal prescription or panacea can be offered on how todesign irrigation systems to provide high efficiency, socialbenefits, and resource conservation all at once. None of theavailable technologies guarantees success, as indeed each oftlnem can be used with greater or lesser efficiency.

If efficiency of land and water use is to be more thanjust an abstract concept, it must become the explicit goal ofsystem designers and of workers in the field. Irrigationsystems should be tailored from the outset as to provide theoperators with knowledge, responsibility, motivation, andtools to improve the practice of irrigation continuously.

Irrigation units in developing countries vary greatly inscale and organization. On the one hand are huge government-sponsored or commercial projects, ranging in size from a fewthousands to many tens of thousands of hectares. Incontrast, there are numerous small family units on the scaleof 0.1 to 10 hectares. Quite obviously, irrigation methodssuitable for the one scale of operations can be quiteunsuitable for the other. High pressure sprinkle systems,for example, may be too expensive for small-scale farming inmkost developing countries; whereas drip, micro-sprayer, orbubbler systems may be more suitable. The technology ofchoice should be such that can be scaled down flexibly to thesize of the operation, and that can be adapted to theparticular local combination of soil, water, climate, crop,and people. Moreover, the machinery must be of a kind whichcan be readily obtained, maintained, and repaired with theskills and materials available locally. Finally, the systemselected should be compatible with the integrated managementof all agricultural inputs, including improved varieties,fertilizers, tillage, pest control, et cetera.

Overall, the best chance for improving the efficiency ofwater delivery is embodied in a system which conveys water inin closed conduits and that provides measured amounts ofwater on demand, at a rate calibrated to meet continuous cropneeds while preventing waste, salinity, and water-table rise.Likewise, the most promising strategy- for improving theefficiency of water utilization appears to be a regime oflow-volume, low-pressure, high-frequency, partial-areairrigation applied to suitable crops of high potential yield.

t

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WORLD BANK TECHNICAL PAPERS (continued)

No. 33. Guidelines for Calculating Financial and Economic Rates of Return for DFC Projects

(also in French, 33F, and Spanish, 33S)

No. 34. Energy Efficiency in the Pulp and Paper Industry with Emphasis on Developing Countries

No. 35. P'otential for Energy Efficiency in the Fertilizer Industry

No. 36. Aguaculture: A Component of Low Cost Sanitation Technology

No. 37. Municipal Waste Processing in Europe: A Status Report on Selected Materials

and Energy Recovery Projects

No. 38. Bulk Shipping and Terminal Logistics

No. 39. (Cocoa Production: Present Constraints and Priorities for Research

No. 40. Irrigation Design and Management: Experience in Thailand

No. 41. Fuel Peat in Developing Countries

No. 42. Administrative and Operational Procedures for Programs for Sites and Services

and Area Upgrading

No. 43. Farming Systems Research: A Review

No. 44. Animal Health Services in Sub-Saharan Africa: Alternative Approaches

No. 45. The International Road Roughness Experiment: Establishing Correlation and a Calibration

Standard for Measurements

No. 46. Guidelines for Conducting and Calibrating Road Roughness Measurements

No. 47. Guidelines for Evaluating the Management Information Systems of Industrial Enterprises

No. 48. Handpumps Testing and Development: Proceedings of a Workshop in China

No. 49. Anaerobic Digestion: Principals and Practices for Biogas Systems

No. 50. Investment and Finance in Agricultural Service Cooperatives

No. 51. Wastewater Irrigation: Health Effects and Technical Solutions

No. 52. Urban Transit Systems: Guidelines for Examining Options

No. 53. Monitoring and Evaluating Urban Development Programs: A Handbook for Program Managers

and Researchers

No. 54. A Manager's Guide to "Monitoring and Evaluating Urban Development Programs"

No. 55. Techniques for Assessing Industrial Hazards: A Manual

No. 56. Action-Planning Workshops for Development Managment: Guidelines

No. 57. The Co-composing of Domestic Solid and Human Wastes

No. 58. Credit Guarantee Schemes for Small and Medium Enterprises

No. 59. World Nitrogen Survey

No. 60. Community Piped Water Supply Systems in Developing Countries: A Planning Manual

No. 61. Desertification in the Sahelian and Sudanian Zones of West Africa

No. 62, The Management of Cultural Property in World Bank-Assisted Projects: Archaeological,

Historical, Religious, and Natural Unique Sites

No. 63,, Financial Information for Management of a Development Finance Institution: A Guideline

The World Bank

Headquarters European Office Tokyo Office U1818 H Street, N.W 66, avenue d'lena Kokusai BuildingWashington, D.C. 20433, U.S.A. 75 116 Paris, France 1-1 Marunouchi 3-chomeTelephone: (202) 477-1234 Telephone: (I) 47.23.54.21 Chiyoda-ku, Tokyo 100, JapanTelex: WUI 64145 WORLDBANK Telex: 842-620628 Telephone: (03) 214-5001

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ISSN 0253-7494ISBN 0-8213-0914-5