Research Report

The New Eraof Water ResourcesManagement: From “Dry” to “Wet”Water Savings

David Seckler

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INTERNATIONAL IRRIGATION MANAGEMENT INSTITUTEP O Box 2075 Colombo, Sri Lanka

Tel (94-1)867404 Fax (94-1) 866854 E-mail IIMI@cgnet.comInternet Home Page http:/ /www.cgiar.org

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Research Reports

IIMI’s mission is to foster and support sustainable increases in the productivity of irri-gated agriculture within the overall context of the water basin. In serving this mission,IIMI concentrates on the integration of policies, technologies and management systems toachieve workable solutions to real problems—practical, relevant results in the field ofirrigation and water resources.

We expect and hope that publications in this series will cover a wide range of sub-jects—from computer modeling to experience with water users associations—and willvary in content from directly applicable research to more basic studies, on which appliedwork ultimately depends. Some research papers will be narrowly focused, analytical,and detailed empirical studies; others will be wide-ranging and synthetic overviews ofgeneric problems.

While we expect that most of the papers will be published by IIMI staff and theircollaborators, we welcome contributions from others. Each paper is reviewed internally,by IIMI’s own staff, by IIMI’s senior research associates and by other external reviewers.The papers are published and distributed both in hard copy and electronically. They maybe copied freely and cited with due acknowledgement.

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International Irrigation Management InstituteP O Box 2075, Colombo, Sri Lanka

The New Eraof Water Resources Management:From "Dry" to "Wet" Water Savings

Research Report 1

David Seckler

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Seckler, D. 1996. The new era of water resources management. Research Report 1. Colombo,Sri Lanka: International Irrigation Management Institute (IIMI).

/ water resources management / water policy / water use efficiency / water demand / water supply/ irrigated agriculture / irrigation efficiency /

ISBN: 92-9090-325-2ISSN: 1026-0862

© IIMI, 1996. All rights reserved.

This work has also been published in Issues of Agriculture, No. 8, 1996, by the Consulta-tive Group on International Agricultural Research, CGIAR Secretariat, Washington, D.C.

Responsibility for the contents of this publication rests with the author.

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Summary 1

Introduction 5

Part I — The Problem of Water Management 6

Water basins: Sources, sinks, and recycling 6

Open and closed water basins 7

Local and global water use efficiency in water basins 8

Future water demand and supply 10

Future water demands 10

Part II — Increasing the Productivity of Water 12

Evaporation: Eto and Eta 12

Water application 13

Managing water to increase productivity 14

Economic considerations 15

Conclusion 16

Literature Cited 17

Contents

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Summary

This paper addresses recent developments in the fieldof water resources that have substantial practical im-plications for water policies, programs, and projects.But as noted in Keller, Keller, and Seckler 1995, theseconcepts and their practical implications appear to

be difficult for some professionals in the field tograsp—or, at least, to accept. This extended synopsissummarizes the logic of this argument in the hopesthat the larger discussion will become clear.

Indeed, irrigated agriculture consumes over 80percent of the world’s developed water supplies, andthe water use efficiency of a traditional gravity irri-gation system is only about 40 percent. But sprinklerirrigation systems are typically around 70 percentefficient and drip irrigation system efficiency can beas high as 90 percent. Thus, it appears that at leastone-half of the water currently used in irrigated ag-riculture could be saved through increased irrigationefficiency. But in the field of water, “efficiency” is atricky concept. To understand it, we must first un-derstand the basic features of water basins.

supply. This water is not lost or wasted in physicalterms. Third, drainage water becomes polluted. It ab-sorbs, or “picks up,” pollutants as it is used, and thesepollutants are concentrated by evaporation. Thus, aswater cycles and recycles through the system, it even-tually becomes so polluted that it is no longer usableand must be discharged to a sink.

If water is plentiful this is not a problem, which isthe case when water basins are open. In the open state,usable water flows out of the water basin (whichshould include estuaries) to a sink, even in the dryseason. The only problem involved in meeting new

Procuring additional fresh water supplies is highlyproblematical. As a result attention has naturallyturned to “demand management” in the hopes thatincreased efficiency of water use will produce suffi-cient savings to meet future water requirements. Pro-ponents of demand management point to the suc-cesses in the energy sector of developed nationswhere projections of rising energy demand werelargely obviated by increased efficiency of energy use.Thus they contend that physical water use efficiencycan be increased by using less water per unit of out-put. Similarly, economic efficiency can be increasedby reallocating water from lower valued to highervalued uses.

“Efficiency”

Water basins

When a unit of water in a water basin is diverted froma source to a particular use, three basic things happento it. First, a part is lost to the atmosphere because ofevaporation from surface areas, or evapotranspirationfrom plants, or both. Second, the part of the divertedwater that has not evaporated drains to surface or sub-surface areas. It may drain to the sea, a deep canyon,or a similar sink where it cannot be captured and re-used, in which case it is truly lost to the system. Oth-erwise, the drainage water flows back into a streamor to other surface and subsurface areas where it canbe captured and reused as an additional source of

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demands in the open state is to capture and distrib-ute this water for beneficial use. But as populationand economic activity grow in a water basin, it gradu-ally evolves from an open to a closed state, where allthe dry-season flow of usable water is captured anddistributed. Most of it evaporates and whatever re-mains is so polluted that it cannot be used.

This creates massive “head ender-tail ender” prob-lems at the level of the entire water basin. Tail

enders—those users at the bottom of the water ba-sin—receive progressively less water of progressivelylower quality. The Colorado River basin is one suchexample. The water entering Mexico is so pollutedthat a massive desalinization plant has been built inan attempt to satisfy Mexico’s riparian rights. Andduring the dry season even this water vanishes intothe sands of Mexico before the Colorado River reachesthe sea.

Water use efficiency

The fundamental problem with the concept of wateruse efficiency based on supply, that is, diversion to aproject, is that it considers inefficient both the evapo-rative loss of water and the drainage water. This isinvalid for that part of the drainage water which canbe reused. To overcome this confusion in the conceptof water use efficiency, knowledgeable people nowdistinguish between “real” water savings and “pa-per” water savings—or, as they say in California,between “wet” and “dry” water savings—as illus-trated in a simple example:

According to an advertisement now running ontelevision in the United States, if I turn off the waterfaucet when I brush my teeth, I will save 40 gallonsof water each week. Similarly, it is said, water sav-ings of more than 50 percent can be achieved by low-flow toilets and showers.

Let us look at this example more closely. By turn-ing off my faucet, I leave 40 gallons in the source ofwater for use elsewhere in the water basin system.But what happened to the water that I previously hadwasted—before I started turning off the faucet? Itwent “down the drain.” But then where did it go? Ifthe 40 gallons were captured and used by someoneelse downstream, my wastage was not lost to the sys-tem. Turning off my faucet results only in dry watersavings; the supply of water in the water basin as awhole has not changed.

The same distinction between wet and dry sav-ings applies in the slightly more complicated case ofirrigated agriculture. Assume that a group of farm-ers, A, is applying 1,000 units of water to their landat 50 percent efficiency. This means that 500 units ofthe diverted water have evaporated, mainly to meetthe evapotranspiration requirements of the crop,while the other 500 units of the water are lost by sur-face and subsurface drainage. But assume that a sec-ond group of farmers, B, captures all the drainagewater from A and applies it to their fields at 50 per-cent efficiency. Then 250 units of the drainage waterhave evaporated while 250 units are lost to drainage.Now the overall irrigation efficiency of the system,of A and B together, has increased to 750 units of waterdivided by the 1,000 units of initial water supply, or75 percent. Overall efficiency increases further if an-other group of farmers, C, used the drainage waterfrom B—and so on. Without pollution the water ba-sin as a whole eventually would converge to nearly100 percent efficiency. For this reason, if all the farm-ers in the water basin adopted sprinkler irrigation,which has an irrigation efficiency of 70 percent, onlythe distribution of water, not the total water supplyor the irrigated area, would change.

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Conclusion

As water basins become closed, they become, by defi-nition, more efficient. This is why the scope for im-proving water use efficiency is low, and the degreeof water scarcity in the future will be greater thancommonly assumed. This is the central problem ofthe new era of water resources management. Carefulresearch and development work is needed to createwet water savings—and to avoid chasing the red her-ring of dry water savings. The opportunities for cre-ating wet water savings lie in four principal direc-tions:

€ Increasing output per unit of evaporated water

€ Reducing losses of usable water to sinks

€ Reducing water pollution

€ Reallocating water from lower valued to highervalued uses

While considerable progress can be made inachieving wet water savings, it is clear that some ofthe most rapidly growing areas of the world also willrequire additional water development programs. Thisis another challenge in the new era of water manage-ment: to design and implement these projects in amuch better way—from all the important technical,economic, social, and environmental perspectives—than they have been in the past.

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The New Era of Water Resources Management:From "Dry" to "Wet" water savings

David Seckler1

1Director General, Inter-national IrrigationManagement Institute(IIMI). I am grateful toHenrik von Loesch formajor editorial help inwriting the summaryand to the USAID-Winrock EnvironmentalPolicy and TrainingProject, the Ford Foun-dation, the Interna-tional Irrigation Man-agement Institute, andWinrock Internationalfor supporting researchfor this paper. I am alsograteful to the followingpeople for helpful com-ments on the paper:Randolph Barker, An-drew Keller, Jack Keller,Jacob Kijne, Chris Perry,David Purkey, RobertRangeley, DanielRenault, and R.Sakthivadivel.

IntroductionA few months ago I met with IsmailSerageldin, vice president of the WorldBank and chairman of the ConsultativeGroup on International Agricultural Re-search (CGIAR). He jolted me by sayingthat, in his judgment, water would be oneof the major global issues of the twenty-firstcentury. While I always had thought thatwater was important, I had not thought thatit was that important. But considering thatthe population of virtually every countryin Asia (with the notable exception ofChina), Africa, and the Middle East willdouble or triple in the next century, and thatthere are increasingly severe physical, eco-nomic, and environmental constraints ondeveloping additional water supplies inthese countries, I now am persuaded thatSerageldin’s statement is correct.

I believe, for example, that much of thesocial and political instability of sub-Sa-haran Africa is due to the instability of itswater regime and the consequent instabil-ity of food supplies and rural livelihoods.In North Africa, the Government of Egypthas publicly and repeatedly threatened togo to war if necessary to protect its supplyof water in the Nile basin. And as thesewords are being written, an official of theGovernment of Sudan has threatened todisrupt the supply of Nile water to Egyptby unstated means (Washington Post, 15July 1995, p. A 18). Similarly, the conflictover water rights also is exacerbating ten-sions between Palestine and Israel.

In yet another dimension of the problem,India’s future food security depends cru-cially on development of additional irri-

gated area. Indeed, over 70 percent of all ofthe additional food grain production in Asiasince the beginning of the green revolutionin the late 1960s has been on irrigated land.But India’s largest irrigation project, theSardar Sarovar Project in the Narmadawater basin, has encountered so much op-position from the environmental commu-nity that the World Bank has withheld fund-ing for it. While there are valid social andenvironmental problems with this project,I am convinced that they can be managedand that international organizations shouldhelp India and other countries facing simi-lar difficulties to manage them (Seckler1992).

Globally, I am concerned that what maybe called the “reserve food production ca-pacity” of the world is decreasing, just asactual world food reserves are at historiclows. At the beginning of the green revolu-tion, the gap between potential food pro-duction and actual food production in-creased to a historic high, largely becauseof the unrealized potential of the high-yield-ing varieties (HYVs) and inorganic fertil-izer, and the rapid expansion of irrigatedarea. Now, however, the gap is closing. Thepractical yield potential of the HYVs is be-ing reached in most countries due to highrates of fertilizer use, and the net growth ofirrigated area in the world is now probablynegative. As investments in irrigation devel-opment decrease, as urban and industrialsprawl spreads over irrigated land, and asincreasingly large amounts of water are di-verted out of agriculture to these sectorsand to serve environmental needs, both the

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area of irrigated land and the quality of ir-rigation necessarily decrease. All of thesefactors reduce the supply elasticity and theresponsiveness of food production to ran-dom conjunctions of global events, mainlyweather-related, that could create severefood shortages. With weather problems inthe United States, Russia, and China, “ana-lysts expect total world grain supplies toslip to 208 million metric tons next year—the smallest reservoir measured as a per-centage of total use since the governmentbegan tracking it in the 1960s” (Wall StreetJournal, 11 July 1995, p. A2).

Hence, part of what I mean by “the newera of water management” refers to the in-creasingly difficult problems facing watermanagement all around the world. But in

basin from past and present precipitationis constant. Thus, unless there are techni-cally and economically feasible opportuni-ties for trans-basin diversions or desaltingseawater, any growth of population andeconomic activity within water basinsmeans that water inevitably becomes morescarce relative to demand.

This problem becomes even more acutein the light of the fact that the supply of anddemand for water vary dramatically by sea-son. In the wet season the demand is lowand the supply is plentiful. The marginalvalue of water is zero or negative, as mostof the water floods out to salt sinks. In thedry season the situation is reversed. Esti-mates and projections of average per capitawater demand and supply conditions bycountry, such as those of the World Re-sources Institute (1994), should be made interms of the minimum dry season supply—not, as is usually the case, in terms of an-nual averages.

The water sinks are: (a) water evaporatedto the atmosphere from surfaces and the

this phrase I also want to emphasize theneed to develop new and creative conceptsin water management to adequately man-age these problems. I believe that good so-lutions to problems are the result of defin-ing as precisely as possible what a problemis, as well as what it is not. In Part I of thispaper, I will attempt to define the genericproblem of water management, as I see it,and show that it is a much more severeproblem than is commonly realized. Andonce we understand this problem clearly,we can avoid pursuing red herrings, ratherfocusing our thinking on the kinds of cre-ative and innovative devices that will leadto real solutions of the problem. That is thesubject of Part II.

Water basins: Sources, sinks, andrecycling

To fully understand the generic problem ofwater management, it is necessary to thinkin terms of water basins as whole units.There are several well-known facts aboutwater basins that, considered together, leadto several rather surprising andcounterintuitive conclusions about waterresources management. The ecological con-cepts of sources, sinks, and recycling pro-vide a useful means of understanding wa-ter basins (see Keller, Keller, and Seckler1995).

The sources of water in a basin are: (a)present precipitation, past precipitation (inthe form of melting snow and ice), and sur-face and subsurface storage in reservoirs,lakes, the soil profile, and aquifers; (b) trans-basin diversions from water-surplus towater-scarce basins; and (c) desalinizationof seawater.

Excepting long-term climatic change, theaverage annual supply of water in a water

Part I — The Problem of Water Management

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evapotranspiration of plants; (b) surfaceand subsurface flows of usable water to saltsinks—oceans, inland seas, or saline aqui-fers;2 and (c) pollution of surface and sub-surface water by salts and toxic elementsto the point that the water becomes unus-able.

One of the most important yet least ap-preciated facts about water basins is that asubstantial amount of water is recycled be-tween the sources and the sinks. Becauseof recycling, it is helpful to think of watersupply in terms of two distinct components.The primary water supply is from past andpresent precipitation, interbasin transfers,and seawater desalting. The secondarywater supply derives from recycling the pri-mary water supply.

When a unit of the primary water sup-ply is diverted to a beneficial use, four im-portant things happen to it:

€ Part is evaporated and lost to the atmo-sphere.

€ The remainder is drained from the pointof use to some other surface or subsur-face place in the system.

€ Some amount of salt or other pollutantsis picked up, or absorbed, in the use of thewater and carried in the drainage water.

€ The concentration of pollution in thedrainage water increases both becauseof absorption of additional pollutantsand evaporation losses from the divertedwater.

As drainage water flows from a particu-lar use, it may flow directly into a sink, suchas a sea. More commonly, it flows back intothe surface or subsurface water systemwhere it becomes a secondary source ofsupply.

The quality of the secondary supply ofdrainage water is always less than that ofthe primary water supply because waterpicks up pollutants as it is used, and be-cause less water runs off than was initiallyprovided. This consumptive use of water

concentrates the pollutants that were in theinput water. Thus as water is repeatedlyrecycled in the water basin, the amount andconcentration of pollutants it carries in-crease substantially.

On the other hand, if the polluted drain-age water is blended with less pollutedwater, the concentration of pollutants in thetotal water supply decreases, and the wa-ter can become more usable even thoughthe amount of pollutants in the two blendedstreams has not changed. This blending ef-fect is not valid for highly toxic, nondegrad-able pollutants such as heavy metals. But,for example, saline drainage water from ir-rigated lands is often purposefully blendedwith less salty water so that it can be re-used in irrigation. Similarly, treated drain-age water from municipalities is blendedback into the municipal supply stream forrecycling. Many cities in the United Statespurposefully recycle a high percentage oftheir drainage (or treated sewage) water,including a deliberately vague amount indrinking water.

Open and closed water basinsAs population and economic activity in-crease in water basins, they evolve from an“open” to a “closed” state (Seckler 1992).In the beginning—in the open state—thereis sufficient water to satisfy demands evenin the dry season, and primary water sup-plies of fresh water flow out of the basininto salt sinks. But as growth continues inthe basin, water supplies progressivelytighten. Most of the primary supply is di-verted to meet demands, and an increas-ingly large percentage of the drainage wa-ter is captured and reused. A progressivelysmaller quantity of water, of diminishingquality, flows into the sinks in the dry sea-son. Eventually, either all of the water isevaporated upstream leaving no dry-sea-son flow into sinks, or the flow is so pol-luted that the water is not usable. At thispoint, the water basin becomes completely

2Estuaries could be in-cluded as part of thewater basin, andestuarian benefits couldbe counted as a benefi-cial use of water. Butthis complication is ig-nored here.

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“closed”—i.e., there is no usable water leav-ing the water basin.

A closed water basin can be reopened.In terms of annual supplies of water, it canbe reopened by trans-basin diversions andseawater desalinization. In terms of sea-sonal supplies, it can be reopened by inter-temporal allocations of water from the wetseason to the dry season through storage inreservoirs, aquifers, and the soil profile. Butthese traditional “water development”techniques eventually reach the limits ofeconomic and environmental viability andthe water basins become permanentlyclosed for all practical purposes. The Nilewater basin, and many other water basinsin the Middle East are or soon will be per-manently closed. The same is true of majorriver basins in Asia.

As water basins approach closure, mas-sive “head ender-tail ender” problems de-velop, with the tail enders at the bottom ofthe water basin receiving progressively lesswater of progressively worse quality. Over20 percent of the world’s population livesin urban conglomerations in coastal areas(World Resources Institute 1994), and a highpercentage of the rural population and bestagricultural lands are at the tail end of thewater basins. This can cause major prob-lems. For example, studies indicate thataround Lake Manzalla near the mouth ofthe Nile villagers’ life expectancy is only 38years because of water pollution.

Local and global water useefficiency in water basinsA well-known facet of the optimizationtheory is that it is possible to obtain a “lo-cal optimum” position in a suboptimal por-tion of the whole system. This can easilyhappen in water basins, especially in closedwater basins. Since this is a complex andrather counterintuitive subject, it is best tobegin with a simple example, or mental ex-periment.

According to an advertisement now run-ning on television in the United States, if Iturn off the water faucet when I brush myteeth, I will save 40 gallons of water eachweek. Similar water savings can beachieved by low-flow toilets and showers.The implication is clear—through suchsimple devices enormous quantities of wa-ter can be saved to meet future needs,thereby reducing or altogether eliminatingthe need for future water developmentprojects. This position, combined with wa-ter pricing and other incentives to inducewater efficiency, represents a school ofthought that advocates “demand manage-ment” in the field of water resources man-agement, in opposition to the “supply man-agement” approach of those who advocatewater development projects.

Certainly, the position of demand man-agement is valid in terms of local efficiency.In the case of tooth brushing, the same func-tion (brushing teeth) is achieved with sub-stantially (on the order of 90 percent) lesswater. This gain in efficiency requires sub-stantially less water to be diverted for tooth-brushing and can be used to serve otherneeds. Or as the number of tooth brushersincreases, their needs can be met by thespread of increased efficiency among exist-ing tooth brushers, without increasing thesupply of water for this purpose.

But is this position valid at the globallevel, in terms of higher water efficiency inthe water basin as a whole? When waterflows out of a faucet, it “goes down thedrain.” Since drains typically are pipe sys-tems, there is little evaporation of drainagewater. The water disappears from view butdoes not disappear from the system. Be-cause all of the efficiency gains in this tooth-brushing example are local efficiency gainsdue to reducing drainage water, the gain inglobal efficiency achieved by this water conser-vation technique depends crucially on whathappened to the drainage water before thechange.

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If, as is too often the case in sea resorts,for example, the drainage water from toothbrushing flows directly into the sea, thenthe practice of leaving the faucet on createsa “real” loss of water, and turning the fau-cet off creates a correspondingly “real” gainin water efficiency.3 But if, as is more oftenthe case, the drainage water flows back intothe water supply and is captured and re-used by downstream users, there is only anapparent, or “paper” gain in water effi-ciency. While diversions of water to tooth-brushing decrease, and water is saved inthis dimension, the flow of drainage waterback into the water supply decreases by thesame amount. Thus, the total water supplyin the water basin remains the same.

This mental experiment provides ameans of understanding the concept ofwater efficiency in greater depth. First, itshows the effect of “composition problems”in water resources management: what istrue of all the parts is not necessarily trueof the whole. There is nothing mysteriousabout this part-whole paradox (as propo-nents of “holistic” philosophy seem tothink); it is simply due to interrelationsamong the parts, which create new phe-nomena (also called “scale effects” or“emergent properties”) at the level of thewhole (Seckler 1992; Keller, Keller, andSeckler 1995).

These effects may be briefly illustratedin the case of irrigated agriculture. Assumethat a certain group of farmers, A, is apply-ing 1,000 units of water to their land at 50percent efficiency. This means that 500 unitsof the diverted water are used beneficiallyto meet the evapotranspiration require-ments of the crop, while the other 500 unitsof the water are lost to these farmers’ fieldsby surface and subsurface drainage. Butassume that a second group of farmers, B,captures all 500 units of drainage waterfrom A and applies it to their fields at 50percent efficiency. They use 250 units of thedrainage water beneficially to meet evapo-transpiration requirements, but again, 250

units are lost to drainage. The overall, glo-bal irrigation efficiency of the system, thatis, of A and B together, has increased to 750units of water used beneficially divided bythe 1,000 units of initial supply, or 75 per-cent. Global efficiency would increase fur-ther if another group of farmers, C, usedthe drainage water from B—and so on.

Second, this example shows that in thenew era of water management we mustconcentrate on achieving “real” not “paper”water savings—or, as they say in Califor-nia, achieving “wet,” not “dry,” water sav-ings. If a water conservation technique sim-ply reduces the amount of drainage waterfrom a particular use and this drainagewater was beneficially used downstream,this would be only a “dry” water saving.But if the drainage water flowed directlyinto a salt sink, “wet” water would besaved. By definition, all of the usable drain-age water in closed water basins is alreadybeing beneficially used, and thus water ef-ficiency measures that only reduce drain-age water create only “dry” water savings.In open systems, on the other hand, usabledrainage water is being lost to salt sinks.Reducing this loss by reducing drainagewater will result in “wet” water savings, areal gain in efficiency.

Keller and Keller (1995) have created animportant new definition of “effective” ir-rigation efficiency that incorporates theserecycling effects along with pollution ef-fects. Willardson, Allen, and Frederiksen(1994) have recommended doing away withthe term “irrigation efficiency” altogetherin favor of an interesting approach basedon various “fractions” of water. Frederiksenand Perry (1995) have applied the conceptof “basin efficiency” to many cases aroundthe world with important results to waterresources analysis.

In sum, real global gains in water effi-ciency achieved by reducing drainagelosses depend on whether the water basinis open or closed. But this is only one sourceof efficiency gain. Whether in closed or open

3Direct drainage to thesea accounts for a largepercentage of the realwater losses by urbanand industrial sectors.Since more than 20 per-cent of the world’spopulation lives incoastal regions, it isvery important from awater efficiency pointof view.

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water basins, real efficiency gains also canbe achieved by

€ Increasing output per unit of evaporatedwater

€ Reducing water losses to sinks

€ Reducing the pollution of water

€ Reallocating water from lower valued tohigher valued uses

These four areas contain the set of op-portunities for increasing the productivityof water in the new era of irrigation man-agement.

Future water demand and supplyOne of the many important consequencesof thinking about water resources in thisnew way—that is, in terms of the total wa-ter basin—is that conventional estimates ofwater demand and supply—past, present,and future—become highly ambiguous.Most of the data are based on the amountsdiverted to the various sectors, with the sumof all diversions taken as the aggregate de-mand for water. But this tells us nothingabout water demand in relation to primarywater supply. Since much of the water di-verted is recycled, from previous diver-sions, it is very difficult to know what sup-ply and demand figures actually mean insuch publications as World Resources In-stitute 1994. Clearly, we need a concept ofnet diversions. We need a portmanteau termthat distinguishes between “wet” and“dry” water in our conversation, writing,and most importantly, thinking.

For this reason, with some trepidation, Ipropose to redefine “consumptive use” ofwater to mean water that is lost to humanuse by every cause. Consumptive use by thisdefinition includes: (a) evaporative lossesof water (its original meaning), (b) waterlost to sinks, and (c) water rendered unus-able because of pollution.

Of these three, it is most difficult to mea-sure water losses due to pollution. If it is

absolutely polluted, in the sense that it can-not be used at all, it is discharged to sinksand can be estimated as an addition to theusable water lost under (b). But if, as in thecase of salt pollution in concentrations be-low the threshold levels of crops, it onlyreduces the productivity of water, there isno physical, only an economic, measure ofthe amount of water involved. However, inthe case where pollution losses are due toconcentration levels, as in the case of salt inirrigation water, one can follow the inge-nious method of Keller and Keller (1995),and measure the physical amount of waterlost to pollution from a particular use bythe amount of fresh water that would berequired to dilute it back down to its origi-nal concentration of pollutants. This couldbe the basis of a pollution tax on water, forexample, the rate of tax being set at themarginal value of fresh water times theamount required to restore the drainagewater to the quality of the diverted water.This would not work, of course, in the caseof heavy metals or other toxic elements thatmust simply be prohibited from enteringthe water stream. But on the whole, thisprovides a reasonable, if rough, measure ofthe damage to water by ordinary forms ofpollution.

With this definition it is possible to dis-cuss the demand for the consumptive use ofvarious water sectors with conceptual clar-ity and then to measure the actual amountsof consumptive use. This provides a mea-sure of how much real, “wet,” water needsto be supplied to meet real, “wet,” waterdemands by sectors.

Future water demandsI would guess that the global demand forconsumptive use of water has historicallyincreased at a rate of about 2.0 percent peryear, doubling every 35 years, and that over80 percent of the total developed water inthe world is consumptively used in irri-gated agriculture. Thus, the demand for

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water is largely a function of the demandfor food and, since most of the favorablerainfed areas have already been developed,of the demand for irrigated agriculture.Since population growth will be substan-tially lower in the future than it has been inthe past, the growth in demand for foodand, therefore, for water for irrigated agri-culture also will be lower (Seckler 1993,1994).

Urban and industrial demand for water,however, is largely a function of the rate ofeconomic growth—which is now muchhigher in developing countries, especiallyin Asia, than it has been in the past. Largeamounts of water already are being reallo-cated from the agricultural to the urban andindustrial sectors, thereby lowering foodproduction capacity, especially in develop-ing countries. Fortunately, the consumptiveuse of water in the urban and industrialsectors is a much lower percentage of thewater diverted to these sectors than it is inagriculture. Thus, with proper treatmentand management, most of the drainagewater from these sectors can be capturedand reused. The greatest opportunity forreal water savings occurs in coastal urbanareas where drainage water now is simplydumped into sinks, causing the consump-tive use of water to approach 100 percentof the water diverted to these areas.

But the most rapidly growing and, incertain places, even the largest demand forwater is from a sector that was not evenexplicitly recognized as such until a fewyears ago. This is the environmental sector.This sector demands water for preservationin its natural state, for maintenance of wild-life habitats, for aesthetic and recreationalpurposes, and similar uses. In California,for example, large amounts of water have

been reallocated from agricultural uses toenvironmental uses, as well as to urban andindustrial uses. Indeed, in terms of diver-sions of water, the environmental sector isnow the single largest user of water in Cali-fornia—using 45 percent of the total waterdemand of the state, compared to 42 per-cent for agriculture (Department of WaterResources 1994), which leaves only 8 per-cent for the other sectors.

Unfortunately, the environmental sectoralso can be a highly consumptive user ofwater because of streams that discharge intosinks and large shallow surfaces of waterexposed to evaporation in rivers, lakes, andwetlands. It is estimated, for example, thatfully 50 percent of the water in the NigerRiver is lost to evaporation in the vast wet-lands below Timbuktu in Mali. These wet-lands provide a critically important sanc-tuary for migratory birds and other wild-life. But it is questionable if this parchedregion of the world will be able to sustainsuch a highly consumptive use of water forenvironmental purposes in the future.

In terms of the political economy of wa-ter, it may be noted that while the demandfor water from the other sectors generallyexpressed itself in terms of increasing thesupply of water through water develop-ment projects, environmental demands aregenerally expressed in terms of preservingwater in its natural state, thus opposingwater projects. The political power of theenvironmental sector assures that develop-ing additional supplies of water throughadditional projects to meet increasing de-mands (even environmental demands) willbecome more difficult in the future. It isrightly said that “water runs uphill: towardpower.”

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This part of the paper focuses on specifictechniques for increasing the productivityof water in irrigated agriculture. It is bestto begin the discussion with a brief reviewof the basic principles of irrigation.

Evaporation: Eto and EtaThe evaporative use of water in irrigatedagriculture is partly due to evaporationfrom exposed surface areas of water in theirrigation and drainage canal systems andon the surfaces of fields, but it is mainly dueto the evaporative requirements (or evapo-

transpiration) of plants. The rate of evapo-ration is determined mainly by the “poten-tial evapotranspiration” (Eto), which is afunction of the climatic conditions of a re-gion at a point of time—mainly heat, wind,and humidity. Eto can be approximated bythe rate of evaporation from an open panof water. But the actual evapotranspirationof crops (Eta) varies somewhat among cropsat various stages of growth. The specificcrop coefficients are multiplied by Eto to

This mixture of good and bad in Eta cre-ates several problems in trying to improvethe productivity of irrigation by reducingconsumptive use. For example, it is com-monly thought that the consumptive use ofwater can be reduced by substituting highEta crops with low Eta crops. There are twoproblems with this view. First, as shown intable 1, there is little difference in Eta amongmajor crops under the same Eto conditions.Second, crop yields and Eta are highly cor-

Part II — Increasing the Productivity of Water

TABLE 1.Seasonal Crop Coefficients.

Crop Condition Crop ConditionMoista Dryb Moista Dryb

Olive 0.40 0.60 Sugar beet 0.80 0.90Safflower 0.65 0.70 Citrus (weeds) 0.85 0.90Grape 0.55 0.75 Cotton 0.80 0.90Citrus (no weeds) 0.65 0.75 Green bean 0.85 0.90Fresh pepper 0.70 0.80 Wheat 0.80 0.90Groundnut 0.75 0.80 Dry onion 0.80 0.90Green onion 0.65 0.80 Grain maize 0.75 0.90Cabbage 0.70 0.80 Tobacco 0.85 0.95Dry bean 0.70 0.80 Potato 0.70 0.95Tropical banana 0.70 0.80 Fresh pea 0.80 0.95Sunflower 0.75 0.85 Sweet maize 0.80 0.95Watermelon 0.75 0.85 Sugarcane 0.85 1.05Sorghum 0.75 0.85 Alfalfa 0.85 1.05Tomato 0.75 0.90 Rice 1.05 1.20Soybean 0.75 0.90

aHigh humidity (RHmin > 70%) and low wind (µ < 5 m/s).bLow humidity (RHmin < 20%) and strong wind (µ > 5 m/s).Source: Hargreaves and Samani 1986.

obtain Eta. Table 1 shows the seasonal cropcoefficients of some major crops under thesame Eto conditions.

One of the curious things about irriga-tion is that, while Eta is “bad” in the sensethat water vapor is lost to the atmosphere,it is “good” because that is exactly whatcrops need water for. Less than one percentof the water consumed by crops is used forfluids in the plant: the rest is used to con-trol the heat of the plant. Plants transpirefor the same reason that people and someanimals perspire: to dissipate heat throughevaporation.

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related: the same factor, radiant energy,drives both yield and, through heat, Eta(under favorable conditions of water, fer-tilizer, and other inputs). This is a classiccase of statistical multicolinearity (althoughthe evaporation and radiant energy corre-lation may differ by climatic factors suchas clouds and wind).

While it is thus generally true that wheatconsumes substantially less water per unitof yield than does rice, and sugar beet lessthan sugarcane, the reason is not Eta, butEto. Wheat and sugar beet are cool-weathercrops, while rice is largely grown in the hotseason (when Eto is high), and sugarcane,with a 12 to 18 month growing season,grows through the hot season. The intersea-sonal and interregional variation in Eto is muchlarger than the intercrop variation in Eta.

Thus, in regions where water is scarce inthe hot season, large savings in the con-sumptive use of water can be achieved bysubstituting crops grown in the hot seasonby crops grown in the cool season (so longas radiant energy and yield remain roughlythe same).4 Large savings also can beachieved by moving crop production fromhigh Eto regions to low Eto regions—forexample, out of windy regions to more tran-quil regions. In addition, it should be notedthat most trees are heavy evaporative con-sumers of water because of the large sur-face area of their leaves and their height,which place them (like wind energy de-vices) up where wind speeds can be sev-eral times that at ground level.

Studies of the crop systems of the Nilebasin below the High Aswan Dam, for ex-ample, show that about 10 percent of thetotal consumptive use of the water in thesystem could be saved if crops were notgrown in the upper Nile around Luxor dur-ing the hot, windy season, but were grownlower in the Nile where it is cooler andwinds are less severe. The farmers could bepaid not to grow crops during that period,just as they are paid not to grow crops inthe United States and Europe under land-

retirement plans. This means that theywould be paid not to grow sugarcane at all.

This presents a major challenge to agri-cultural research and plant breeders to de-velop more cool-season varieties of crops—like wheat, barley and sugar beet. Bettercool-weather maize varieties and a nine-month variety of sugarcane, for example,would be very helpful. Also, if possible, itwould be valuable to find economical plantspecies and varieties that have lower Eta inhot, windy regimes—like olive. Are therevaluable plants that shut down, like cactus,when the heat (and wind) is on?

Water applicationA substantial loss in water productivity isdue to the lack of reliability of irrigationwater in surface irrigation systems. Wateris applied and consumptively used to startthe crop, but then one or two irrigationturns are missed (especially in the tails ofthe system), sometimes at a critical growthstage of the crop, causing substantially re-duced yields. Part of this problem is due tomismanagement and part to “surge” effectsin the supply of water to the farmers’ fields.This problem can be solved by standby tubewells along the distribution channels to pro-vide supplementary irrigation in times oftemporary shortage.

The problems of water distribution andunreliability of supply are particularly acutein the use of drainage water. Most of thedrainage water enters the irrigation man-agement system as secondary surface andsubsurface supply. But a substantial amountof drainage water is simply discharged tolocal sinks in an unmanaged way. If thequality of the drainage water is good andthese are not salt sinks, this water can beused for irrigation. Much of the irrigatedarea of rice and hemp is accidentally irri-gated by this means. But if these are saltsinks, the drainage water creates waterlog-ging and salinity problems. Similarly, goodquality drainage water is often dumped to

4In many of the tropics,however, the hot seasoncorresponds with highprecipitation. Becauseof the ability to captureprecipitation in ricefields, rice can be ahighly water-efficientcrop in the hot-wet sea-son.

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the sea for lack of proper attention andmanagement. One of the major tasks of thenew era is to actively manage drainagewater as secondary supply because in manywater basins this is virtually the only sur-plus “wet” water there is.

On this subject an intriguing conjecturemay be noted. The Eta requirements ofcrops increase with yields, although theexact nature of this relationship is not alto-gether clear. Since yields in most irrigatedareas have increased substantially over thepast few decades, evapotranspirationshould also have increased. If this is true,then irrigated areas are becoming relativelymore stressed for water. This may accountfor part of the widely held view that irriga-tion systems are now performing morepoorly—e.g., with more tail-ender prob-lems—than they have in the past. They may,in fact, need more water inputs.

Managing water to increaseproductivityThese considerations deserve seriousthought about policy and management is-sues. One issue is to decrease the variabil-ity of the water supply through better con-junctive use of water (with deliberate over-irrigation in times of surplus to rechargeaquifers) and pumping into the canal sys-tems, as well as from private tube wells.Another way to increase the water supplyis to reduce evaporative losses in the wa-tersheds by replacing some trees withgrasses, which would also reduce soil ero-sion. Barring additional water inputs to ir-rigation systems, water productivity maybe increased by consolidating the area, withmore reliable water supplies to less irri-gated area. But this would seriously disturbthe distribution of benefits of irrigation.Clearly, such alternatives need to be care-fully studied under specific conditions oftime and place before decisions are made.

Another source of real water savings isbetter management of fallow land (C. Perry,

personal communication, 1995). Even bar-ren land will evaporate water through cap-illary action down to a depth of two meters.The draw on shallow water tables and re-plenishing soil moisture in the soil profilecan amount to a substantial loss. Perry es-timates that in the Nile basin below theHigh Aswan Dam, as much as 3 billion cu-bic meters of water (7 percent of the totalsupply to irrigation) are evaporated in thismanner. Also, in most developing countriesweeds are permitted to grow on fallow land.This not only assures a supply of weedseeds for the next crop, but the weeds pumpout subsurface moisture and mine highwater tables. But if fallow lands are keptbarren and a “dust mulch” of loose soil onthe surface is maintained, soil moisture isretained.

In thinking about reducing evapotrans-piration in irrigated agriculture, the“evapo” part should be separated from the“transpiration” part. While it may be pos-sible to develop more heat-resistant and,therefore, less-transpiring plants, thiswould appear to be an exceptionally diffi-cult task. But the “evapo” part, which is dueto the evaporation of moisture in fields, iseasier to control. As shown in table 1, mostof the difference in Eta between rice andother crops occurs in the planting seasonbecause of high evaporation losses beforethe crop cover is established. An Interna-tional Irrigation Management Institute(IIMI) study of dry seeding rice in the MudaIrrigation Project of Malaysia showed wa-ter savings of 25 percent by eliminating pre-transplanting flooding of rice fields. Someof this was probably “paper” water savingsof drainage water, but some of it was un-doubtedly real water savings of evapora-tion losses. Studies of planting sprouted riceseeds by the International Rice ResearchInstitute (IRRI) have shown similar results(Bhuiyan, Sattar, and Khan 1995). Interest-ingly, farmers are adapting these water-sav-ing techniques not to save water, but to savethe high labor costs of transplanting rice.

15

Field evaporation losses can also be re-duced by drip and trickle irrigation sys-tems, which apply water directly to the rootzone of the crop in correspondence with Eta.Sprinkler irrigation systems, however, arenot so efficient. In fact, throwing fine par-ticles of water through hot air is about thebest way to maximize evaporation losses.The common belief that sprinkler systemsare water efficient is due to their high uni-formity of water application—which low-ers drainage water losses, which may beonly “paper” savings. However, modern,downward sprinkling systems substantiallyreduce evaporation losses.

In areas that have good, salt-free waterand soils, subirrigation can be a highly pro-ductive form of irrigation. By putting bar-rages in rivers, water tables can be raisedto the root zone of plants. This provides ir-rigation with less evaporation and a con-siderable amount of subsurface water stor-age. A substantial, although unknown, partof the Eta of crops in Egypt is met throughsubirrigation. Similarly, in Indonesia streambarrages lower drainage losses from ricefields by creating high water tables.

In areas that do have water salinity prob-lems, the productivity of water can be sub-stantially increased by carefully controllingthe application of irrigation water throughsprinklers and other forms of pressurized(pipe-based) water application systems.Combined with tube wells, these systemscan lower water tables and be used to drivesalts below the root zone of plants, where itcan be permanently and harmlessly stored.This may be the only real solution to thesalinity problems of Pakistan and other sa-line areas of the world that do not havegood drainage to the sea.

There has been promising research indeveloping commercially valuable halo-phytes, that is salt-loving plants (I am grate-ful to Jack Keller for this information). InCalifornia, for example, salty drainage wa-ter from a normal crop is captured and usedto irrigate cotton, which is highly tolerant

to salt. Then the drainage water from thecotton, which now is highly salted, is usedto irrigate halophytes. Then the drainagewater from the halophytes, which may havea higher concentration of salts than seawa-ter, is pumped into evaporation ponds. Af-ter evaporation, the salt residue is scrapedup and transported by truck or train out ofthe system. Indeed, the salt may be sold tocommercial users. Here is another tech-nique for salt control that should be thor-oughly investigated.

Economic considerationsTurning to the economic dimensions of theproblem, it is clear that the productivity ofwater can be increased by substituting cropswith high economic value per unit of wa-ter consumptively used for crops with lowvalue. While this is valid in principle, it maynot be easy in practice. Since the consump-tive use of water by crops is largely a func-tion of Eto, not Eta, there is not much dif-ference in the consumptive use of crops inthe same season, and crop substitutionsmust occur in the same season of the cropcalendar. Otherwise, the land and other fac-tors of production would be idle. But if thenet value of a crop is higher than that ofanother crop in that same season, it is likelythat the farmers would already have madethe substitution.

In closed water systems, the quality ofwater is as important as the quantity ofwater in determining ultimately usable sup-ply. There is no question that excessiveamounts of fertilizer (whether organic or in-organic) are used in some of the major riverbasins and that the salts from these fertiliz-ers substantially reduce the quality of wa-ter. Reducing fertilizer use by such meansas a tax on fertilizers may then be appro-priate.

Last, at the global level, it is clear that aswater becomes progressively more scarcein the major crop producing nations, inter-national trade in agricultural commodities

16

will increasingly be determined by theamount of water required to produce crops,their “water content,” if you will, in rela-tion to the relative water supplies of trad-ing nations. This will give even greater com-parative advantages to the favorable rain-fed areas of Europe, North America, andparts of South America. Production of hot-season crops like sugarcane, summer rice,and maize will concentrate in areas of highwater availability. Carruthers (1993) con-tends that in the future Asian nations willbecome the greatest exporters of industrialproducts while the western nations willspecialize in food exports. The economiclogic of water lends support to that hypoth-esis. Recent food demand and supply stud-ies (Agcaoili and Rosegrant 1995), for ex-

ample, project that international trade incereals will roughly double by 2010 and thatvirtually all of the increased trade will bein the form of exports from North Americaand Europe to Asia.

However, these international water trad-ing ideas depend crucially on the ability ofcountries to finance food imports, oninfrastructural investments in irrigation,transport, and other facilities, and on theglobal supply and distribution of water. Ifall of the agriculturally productive waterbasins in the world are encountering waterscarcities, then, obviously, the scope of in-ternational trade in agricultural commodi-ties requiring large volumes of water willbe restricted.

There is much that can be done to improvethe productivity of water on technicalgrounds. The institutional, social and eco-nomic aspects of these improvements needto be carefully investigated to determine thefeasibility of these improvements. But,given the fact that existing irrigation andother water-using systems are not nearly asinefficient as they are commonly thoughtto be at the level of global efficiency, therewill remain a need for further water devel-opment projects. This will require betterconjunctive use of surface and subsurfacewater supplies, water conservation tech-niques, small and large dams, and possi-bly, trans-basin diversions to areas of highfuture potential and need. Here is anotherchallenge: to improve the planning anddesign of water development projects, like

the Sardar Sarovar Project in India, so thatthe negative environmental impacts ofthese projects are ameliorated and peopleadversely affected by the projects are prop-erly compensated (Seckler 1992).

Ten years ago I published a paper with atitle similar to this one (Seckler 1985). Afterfinishing that paper I considered ending mywork on water problems and turning toother research interests because, I thought,there was not much more of fundamentalinterest to learn. But that paper turned outto be a new beginning, not the end, of myresearch interests in this field. In the newera of water management, the field of learn-ing is wide open. Indeed, one of our chal-lenges is to unlearn what we thought weknew so well and to start afresh.

Conclusion

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Literature Cited

Agcaoili, M., and M. W. Rosegrant. 1995. Global and regional food supply, demand, and trade prospects to2010. In Population and food in the early twenty-first century: Meeting future food demand of an increasing popula-tion, ed. Nurul Islam. Washington, D.C.: International Food Policy Research Institute.

Bhuiyan, S. I., M. A. Sattar, and M. A. K. Khan. 1995. Improving water use efficiency in rice irrigation throughwet-seeding. Irrigation Science 16(1): 1–8.

Carruthers, I. 1993. Going, going, gone! Tropical agriculture as we knew it. Occasional Paper no. 93/3. London:Wye College, University of London.

Department of Water Resources, State of California. 1994. California water plan update. Vol. 1, Bulletin 160–93.CA, USA: Department of Water Resources, State of California.

Frederiksen, H. D., and C. Perry. 1995. Needs and priorities in water-related research. Draft paper. Colombo, SriLanka: International Irrigation Management Institute.

Hargreaves, G. H., and Z. A. Samani. 1986. World water for agriculture—precipitation management. Washington,D.C.: Agency for International Development.

Keller, A., and J. Keller. 1995. Effective efficiency: A water use efficiency concept for allocating freshwater resources.Water Resources and Irrigation Division (Discussion Paper 22). Arlington, Virginia, USA: Winrock Interna-tional.

Keller, A., J. Keller, and D. Seckler. 1995. Integrated water resource systems: Theory and policy implications. Colombo,Sri Lanka: International Irrigation Management Institute.

Seckler, D. 1985. The new era of irrigation management in India. Journal of the Indian Water Resources Society 29–39) January.

Seckler, D. 1992. The Sardar Sarovar Project in India: A commentary on the report of the independent review. WaterResources and Irrigation Division (Discussion Paper 8). Arlington, Virginia, USA: Winrock International.

Seckler, D. 1993. Designing water resources strategies for the twenty-first century. Water Resources and IrrigationDivision (Discussion Paper 16). Arlington, Virginia, USA: Winrock International.

Seckler, D. 1994. Trends in world food needs: Toward zero growth in the 21st century. Water Resources and IrrigationDivision (Discussion Paper 18). Arlington, Virginia, USA: Winrock International.

Willardson, L. S., R. G. Allen, and H. D. Frederiksen. 1994. Universal fractions and the eliminations of irriga-tion efficiencies. Draft paper presented at the 13th Technical Conference, United States Commission onIrrigation and Drainage, Denver, Colorado, 19–22 October 1994.

World Resources Institute. 1994. World resources 1994–95. New York: Oxford University Press.

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Research Reports

1. The New Era of Water Resources Management: From “Dry” to “Wet” Water Savings. DavidSeckler, 1996.

Forthcoming

2. Altemative Approaches to Cost Sharing for Water Service to Agriculture in Egypt. C.J. Perry,1996.

3. Integrated Water Resource Systems: Theory and Policy Implications. Andrew Keller, JackKeller, and David Seckler, 1996.

4. Results of Irrigation Management Turnover in Two Irrigation Districts in Colombia. Dou-glas L. Vermillion and Carlos Garces-Restrepo.

Research Report

The New Eraof Water ResourcesManagement: From “Dry” to “Wet”Water Savings

David Seckler

1

INTERNATIONAL IRRIGATION MANAGEMENT INSTITUTEP O Box 2075 Colombo, Sri Lanka

Tel (94-1)867404 Fax (94-1) 866854 E-mail IIMI@cgnet.comInternet Home Page http:/ /www.cgiar.org