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39

Water Scarcity and the Roleof Storage in Development

Andrew KellerR. SakthivadivelandDavid Seckler

ISSN 1026-0862ISBN 92-9090-392-9

Research Reports

IWMI’s mission is to contribute to food security and poverty eradication by fosteringsustainable increases in the productivity of water through better management ofirrigation and other water uses in river basins. In serving this mission, IWMIconcentrates 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.

The publications in this series cover a wide range of subjects—from computermodeling to experience with water user associations—and vary in content from directlyapplicable research to more basic studies, on which applied work ultimately depends.Some research reports are narrowly focused, analytical, and detailed empirical studies;others are wide-ranging and synthetic overviews of generic problems.

Although most of the reports are published by IWMI staff and their collaborators,we welcome contributions from others. Each report is reviewed internally by IWMI’sown staff and Fellows, and by external reviewers. The reports are published anddistributed both in hard copy and electronically (http://www.iwmi.org) and wherepossible all data and analyses will be available as separate downloadable files. Reportsmay be copied freely and cited with due acknowledgment.

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Water Scarcity and the Roleof Storage in Development

Research Report 39

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

Andrew KellerR. SakthivadivelandDavid Seckler

ii

The authors: Andrew Keller is a Fellow of the International Water Management Institute(IWMI), and R. Sakthivadivel and David Seckler are on the staff of IWMI.

Keller, Andrew; Sakthivadivel, R.; Seckler, David. 2000. Water scarcity and the roleof storage in development. Colombo Sri Lanka: International Water ManagementInstitute (IWMI), vii, 20p. (Research report 39).

/ groundwater resources / water storage / water scarcity / reservoir storage / costs /aquifers / conjunctive use / river basins / surface water / dams / India / China / USA /

ISBN 92-9090-399-6ISSN 1026-0862

Copyright © 2000 by IWMI. All rights reserved.

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

Please send inquiries and comments to: iwmi-research-news@cgiar.org

The International Irrigation Management Institute, one of sixteen centers supported bythe Consultative Group on International Agricultural Research (CGIAR), was incorpo-rated by an Act of Parliament in Sri Lanka. The Act is currently under amendment toread as International Water Management Institute (IWMI).

IWMI gratefully acknowledges the financial support for its research work from thefollowing Governments, Development Banks, Agencies, and Foundations: Australia,Brazil, Canada, China, Denmark, European Union (EU), France, Germany, India,Iran, Japan, Netherlands, Norway, Pakistan, Peru, South Africa, Sweden,Switzerland, Taiwan, Turkey, United Kingdom, and United States of America;African Development Bank (AfDB), Asian Development Bank (ADB), Food andAgriculture Organization of the United Nations (FAO), Ford Foundation, and WorldBank. The Governments of India, Iran, Mexico, Nepal, Pakistan, Sri Lanka, andTurkey also provided program support for IWMI-related activities in these countries.

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Contents

Summary v

Introduction 1

Kinds of Storage 4

Groundwater Storage 4Small Reservoirs 6Large Reservoirs 7Comparison of Large and Small Reservoirs 9Complementarities 10

Costs 12

Conclusion 14

v

One-third of the developing world will face severewater shortages in the twenty-first century eventhough large amounts of water will continue toannually flood out to sea from water-scarceregions. The problem is that the sporadic, spatialand temporal distribution of precipitation rarelycoincides with demand. Whether the demand isfor natural processes or human needs, the onlyway water supply can match demand is throughstorage.

There are four major ways of storing water—in the soil profile, in underground aquifers, insmall reservoirs, and in large reservoirs behindlarge dams. Storage in the soil profile isextremely important for crop production, but it isrelatively short-term storage, often only sufficientfor a period of days. In this paper, the authorsconcentrate on the three kinds of technologies that

Summary

store water for periods of months, in smallreservoirs, or years, in aquifers and largereservoirs. These three technologies are comparedfrom the hydrological, operational, and economicstandpoints. Some of the environmental aspects ofthese options are also mentioned, but theseaspects are very location specific and are notdiscussed in detail.

The two principle conclusions of this analysisare: 1) aquifers and small and large reservoirs allserve an indispensable role in water storage, andeach technology has strong comparativeadvantages under specific conditions of time andplace; and 2) where it is possible to do so,substantial gains can be achieved by combiningall three storage technologies in an integratedsystem.

1

Water Scarcity and the Role of Storagein Development1

Andrew Keller, R. Sakthivadivel, and David Seckler

1Andrew Keller originally presented the subject of this paper at the 1998 World Bank Water Week Conference, 15 December 1998, Annapo-lis, Maryland, USA, in a session on dams. The title of that presentation was “Water Scarcity and the Role of Dams in Development.” For thispaper, we changed the title, substituting the broader term “storage” for “dams,” to reflect the importance of increasing storage, regardless oftype, to address water scarcity.2Flow to sinks is an economic as well as physical concept. Sometimes it can be prevented, but at an unacceptably high cost.

Introduction

By 2025, one-third of the population of thedeveloping world will face severe water shortages(Seckler et al. 1998). Yet, even in many water-scarce regions, large amounts of water annuallyflood out to the sea. Some of this floodwater iscommitted flow to flush salt and other harmfulproducts out of the system and to maintain theecological aspects of estuaries and coastal areas(Molden 1997). However, in many cases, thefloodwater is not fully utilized; and, of course, thefloods themselves can do a great deal of harm.This problem is epitomized in India (see box onpage 2) where annual precipitation isconcentrated in the 4 months of the monsoon,and then in only a few hours of these months.

Because of the sporadic spatial and temporaldistribution of precipitation, the only way watersupply can be controlled to match demand isthrough storage. This is true whether the demandis for natural processes or human needs. Innatural systems, precipitation may be interceptedby vegetation and temporarily stored on plantsurfaces and on the soil surface. When waterinfiltrates the ground, it is stored in the soil andmay percolate to groundwater storage. On theland, surface water is stored in watercourses,lakes, and other water bodies and in frozen formas snow and ice. Man can create and enhancewater storage by such activities as water

conservation tillage, constructing dams and dikesto impound water, and artificially recharginggroundwater. Regardless of the method or typeof storage, the purpose is to capture water whenand where its marginal value is low—or, as in thecase of floods, even negative—and reallocate itto times and places where its marginal value ishigh. Here, “marginal value” includes all of theeconomic, social, and environmental values ofwater.

As competition for water increases in manyregions of the world, an increasingly higherproportion of normal flow of water is likely to beconsumed, and the risk of shortages in periodsof low flow will increase. For this reason, theneed for additional storage as a proportion of thetotal water consumed will increase in the future.

In evaluating various kinds of water storagesystems, it is useful to think in terms of threedistinct hydrological situations in river basins(Seckler 1996; Keller et al. 1996; and Perry1998):

· Open basins are those that have an excessof water, over and above all committedecological and environmental requirements,flowing to the seas, saline aquifers, or similarsinks2 during the low-flow seasons of theyear. In open basins, the excess water has

2

“P. R. Pisharoty, one of India’s leadingmeteorologists, points out that the nature ofIndian rainfall is completely different from thatof the middle latitude countries of Europe.

India receives some 400 million hectaremetres (mham) of rain annually over ageographical area of 329 million hectares(mha). This, if evenly spread, would uniformlysubmerge the entire land surface to a depth ofabout 1.28 m. However, rainfall distributionvaries widely across the land, both spatiallyand temporarily. Some areas like the TharDesert receive less than 200 mm annually,whereas Cherrapunji in the Northeast receivesas much as 11,400 mm each year. However,as Pisharoty emphasises, there is almost noarea where rainfall is less than 100 mmannually, and even this is sufficient to meetlocal drinking water needs, provided it isharvested properly and where it falls.

Another problem is that unlike a number ofEuropean countries, India does not receiverainfall all through the year. It is largelyconcentrated during four months of the year.But then too, rainfall does not occur daily; inmost parts of the country, there is precipitationduring not more than 50 days. On the dayswhen rainfall occurs, it doesn’t fall over a periodof 24 hours. In fact, heavy showers are

no opportunity cost, its marginal value is zeroor negative, and all that needs to beconsidered is the cost of utilizing more of thewater relative to the benefits of doing so.

· Closed basins are at the opposite end of thespectrum. Here there is no excess waterflowing into sinks at any time of the year.

A meteorologist’s view ofIndia’s water storage problems

common. Most of the country receives rain forjust about 100 hours each year. Pisharotyclaims that a thumb-rule is that the number ofhours of rain a place receives in a year isequal to the number of centimetres of rain itreceives annually. Delhi, with an annual rainfallof 80 cm receives this in just 80 hours; Nagpurreceives 100 hours of rain in a year, andJodhpur receives rain for just 40 hours a year.

Moreover, Pisharoty stresses that half theannual rainfall is precipitated in just one-fifthof the total hours of rain in a year. Thus, if atown receives 80 cm of rain, half of it, that is40 cm, falls in just 16 hours. In the country asa whole, half of the total annual rainfall isreceived in about 20 hours.

Pisharoty recalls that Ahmedabad, with800 mm annual rainfall, once received 320mm in six hours in a massive downpour. Partsof Rajasthan have in the past received twicetheir average rainfall in a period of just twodays. This type of rainfall naturally generatesa large runoff within short bursts of time,making it imperative to store this water if it isto be of use. Of the total rain that falls on theIndian subcontinent, only a small quantitypercolates into the ground.”

(Agrawal and Narain 1997)

This case represents a zero-sum game inphysical terms. Additional water use by oneparty means reduced use by another party.Here the only options are to:

· reduce nonproductive evaporation andtranspiration losses out of the basin, forexample by reducing the non-beneficial

3

3Structures less than 15 m high and embankment volume less than 0.75 mcm.

uses by weeds, shrubs, and trees (as isbeing done in South Africa)

· increase the total productivity of water byreallocating water from lower- to higher-valued uses

· minimize effective water depletion due tosalinization and pollution and losses tosinks

· augment water supply with transbasindiversions or desalinization (Keller et al.1998)

· Semi-closed basins represent the majoropportunities for adding value to water throughstorage. In these basins, there is no excessoutflow to sinks during the low-flow season,but there is excess outflow during the high-flow season. Thus, storing water andreallocating it between seasons can achievepotentially large increases in the value ofwater.

Two of the largest river basins, the Amazonand the Zaire, are open basins, but there arevery few open basins left in the highly populatedarid regions of the world. In China, for example,the Yangtze River, in the wet south, is open butthe Yellow River in the arid north is closed. Chinais now creating transbasin diversions from theYangtze River basin to the Yellow River basin toalleviate the problem of water shortage in thenorth. Other examples of completely closed riversare the Colorado River in the United States andMexico and the Cauvery in south India. Suchlarge and important rivers as the Indus inPakistan, the Narmada in India and the Gangesof Nepal, India and Bangladesh are semi-closed.

The Ganges represents a classical problem ofinternational waters, with the catchment areasand major new dam sites largely in Nepal, amajor need for water in the low-flow season inBangladesh, flood control storages in the headreaches, and a large demand for hydropower andirrigation in India. Development of the NarmadaRiver (the Sardar Sarovar project) in India hasbecome an international cause celebre becauseof concerns over resettlement and environmentalissues, not withstanding its enormous economicand social benefits (Seckler 1992).

There are four major ways of storing water—in the soil profile, in underground aquifers, insmall reservoirs,3 and in large reservoirs behindlarge dams. Storage in the soil profile isextremely important for crop production, but it isrelatively short-term storage, often only sufficientfor a period of days. Here we concentrate on thethree kinds of technologies that store water forperiods of months, in small reservoirs, or years,in aquifers and large reservoirs. These threetechnologies are compared from the hydrological,operational, and economic standpoints. Some ofthe environmental aspects of these options arealso mentioned, but these aspects are verylocation specific and are not discussed in detail.The two principle conclusions of this analysis are:

· Aquifers and small and large reservoirs allserve an indispensable role in water storage,and each technology has strong comparativeadvantages under specific conditions of timeand place.

· Where it is possible to do so, substantialgains can be achieved by combining allthree storage technologies in an integratedsystem.

4

TABLE 1.Comparative advantages, limitations, and key issues associated with groundwater, small reservoir, and large dam waterstorage.

Groundwater storage Small surface water reservoirs Large dam reservoirs

Little evaporation loss Ease of operation Large, reliable yieldUbiquitous distribution Responsive to rainfall Carryover capacityOperational efficiency Multiple use Low cost per m3 water storedAvailable on demand Groundwater recharge MultipurposeWater quality Flood control and hydropower

Groundwater recharge

Slow recharge rate High evaporation loss fraction Complexity of operationsGroundwater contamination Relatively high unit cost SitingCost of extraction Absence of over-year storage High initial investment costRecoverable fraction Time needed to plan and construct

Declining water levels Sedimentation Social and environmental impactsRising water levels Adequate design SedimentationManagement of access and use Dam safety Dam safetyGroundwater salinization Environmental impactsGroundwater pollution

4For aquifers with specific storage capacities of 10%, a typical value, a 2-meter decline in water level represents about 200 mm of actualwater. Thus, where groundwater levels are falling 2 m per year, extractions are exceeding recharge by approximately 200 mm per year.

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Kinds of Storage

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Table 1 summarizes the comparativeadvantages, limitations, and essential issuesassociated with a quifers, small reservoirs, andlarge reservoirs.

Groundwater Storage

One of the major advantages of storing water inunderground aquifers is that it can be stored foryears, with little or no evaporation loss, to beused in drought years as a supplementarysource of water supply. It also has theadvantage that storage can be near or directlyunder the point of use and is immediatelyavailable, through pumping, on demand. Thetubewell revolution that has swept throughagriculture capitalizes on these advantages. For

example, crop yields under tubewell irrigation inIndia are frequently two to three times greaterthan crop yields from irrigation by canal systemsalone (see table 2).

Another great advantage of groundwater isthat as water slowly percolates down into theaquifer it is usually purified of biologicalpollutants. Thus, groundwater is usually the bestsource of drinking water, especially in rural areasof developing countries where water treatmentfacilities are not available.

The critical issue facing many groundwateraquifers today is that the volume of waterwithdrawal exceeds long-term recharge, resultingin rapidly declining groundwater levels in manyareas. Closely related to this is the key issue ofmanaging groundwater access and utilization,because groundwater is a common property

5

TABLE 2.Average food grain yields, in tons per unirrigated hectare and per net irrigated hectare, by irrigation source in four Indianstates.

State Year(s) Yield (tons per hectare)

Unirrigated Groundwater Canal Tank

Punjab 77-79 1.08 5.46 3.24 –

63-65 0.75 3.06 1.18 –

50-51 0.37 1.75 0.94 –

Haryana 76-770.38 5.74 2.39 –

78-79

Andhra Pradesh 77-79 0.42 5.69 3.43 1.96

57-59 0.47 3.11 2.27 1.35

Tamil Nadu 77-79 0.49 6.53 2.60 2.33

64-66 0.61 4.00 2.14 2.08

56-58 0.66 3.78 1.69 1.86

Source: Chambers 1988

concern with individual benefits and collectivecosts.

Declining groundwater levels are often on theorder of 2 meters per year.4 The extraction ofwater from aquifers in some districts of India(North Gujarat, Southern Rajasthan, Saurashtra,Coimbatore and Madurai Districts in Tamil Nadu,Kolar District in Karnataka, and the whole ofRayalasema Region in Andhra Pradesh) exceedsrecharge by a factor of two or more. As theseaquifers are depleted, the resulting cutbacks inirrigation could reduce India’s harvest by25 percent or more (Seckler et al. 1998, andShah 1993). Groundwater levels in the PishinLara Basin, Pakistan have steadily declinedapproximately 2 meters per year since 1987(Prathapar 1998). In China, groundwater levelsare declining almost everywhere there is pumpirrigation. Under much of the north China Plain,where nearly 40 percent of China’s grain isharvested, water levels are dropping roughly1.5 meters per year (Worldwatch Institute 1999).

Groundwater depletion also has seriousequity implications since falling water tables take

the resource out of reach of small and marginalfarmers. Falling water tables can make wells fordomestic water supply run dry. An especiallydangerous aspect of falling groundwater tables isillustrated in Bangladesh, where toxic levels ofarsenic are being found in the drinking water ofmillions of people. One theory is that fallinggroundwater tables have permitted oxidizationand mobilizations of natural deposits of arsenicin these areas.

Other important problems of groundwaterstorage are water quality, the cost of pumping toextract groundwater, and the recoverable fractionof recharge. From a basin-wide perspective,nearly all of the groundwater recharge may berecoverable, but, from a more local perspective,there are some losses. Typically, groundwaterrecovery under artificial recharge averages 75percent of the recharge volume (OAS 1997).

While falling groundwater tables are a majorproblem in many areas, many other areas sufferfrom the opposite problem of rising water levels,with waterlogging and salinization as aconsequence (Prathapar 1998). Rising water

6

tables also prevent effective sewage disposal inrural villages, with latrines overflowing andpolluting the drinking water in wells.

The problems of rising and falling watertables are among the most important issues inwater policy. Declining groundwater levels inmany metropolitan cities such as Mexico andBangkok, and in many parts of Japan cause landsubsidence. It is commonly thought thatgroundwater withdrawal should be decreased tothe sustainable rate of natural recharge. In somecases, this is correct, but the problem is that thisreduces production from this valuable resource. Itis much better to artificially recharge the aquiferswith excess water wherever possible. However,much more research and development areneeded in the field of artificial recharge beforethis will be a widely used technology. Theproblem of rising water tables is more tractable,in most cases, using well known but oftenexpensive drainage techniques. Care should betaken to prevent the problem of rising watertables in the first place, for example, by notirrigating highly saline areas unless anacceptable drainage plan is in place.

Small Reservoirs

Here we use the standard definition of smalldams as structures less than 15 meters high andwith an embankment volume generally less than0.75 million cubic meters (BOR 1987 and ICOLD1998). Included within our discussion of smallsurface reservoirs are small tanks and micro-storage facilities such as dug cisterns and farmponds.

Small reservoirs have the advantage of beingoperationally efficient. They are flexible, close tothe point of use, and require relatively few partiesfor management. Because of these attributes,they can be responsive to demands, the supplyto demand mismatch can be small, andmanagerial and institutional issues are easier tohandle. Because of their limited storage capacity,

small reservoirs respond rapidly to precipitationrunoff, often refilling several times a year. Thus,the actual amount of water delivery from a smallreservoir can be several times its one-timestorage capacity. The great operational benefit ofsmall storages is their rapid response times. Likegroundwater systems, they can respond torainfall on fields, thus maximizing effective rainfalland minimizing operational losses. Smallreservoirs often serve multiple uses such asbathing, washing, animal husbandry, andaquaculture in addition to irrigation. Smallreservoir storage is ideal from the standpoint ofoperational efficiency, but generally less effectivethan groundwater or large dams for waterconservation.

The high surface area to volume ratio ofsmall reservoirs leads to high evaporation loss.Micro-storage facilities lose, on average, 50percent of their impoundments to evaporation inarid and semi-arid areas (Gleick 1993 andSakthivadivel et al. 1997). Other limitations arethat their small storage volume does not allowfor seasonal or annual carryover and, in addition,there are the cost and safety problems ofhandling overflow during extreme storm events.

The seepage and percolation “losses” fromsmall tanks in Sri Lanka account for 20 percentof reservoir volume (Tasumi 1999) against5 percent of reservoir volume in large dams.These small reservoirs can act as percolationtanks, recharging aquifers and retarding runoff.Since seepage “loss” can be both an advantageand disadvantage of small reservoirs dependingon perspective, it is not listed in table 1; from abasin-wide hydrologic standpoint, it is generallyan advantage. In fact, in India, small reservoirsthat have high percolation rates, “percolationtanks”, are often preferred because of theircontribution to groundwater recharge.

Perhaps the greatest threat facing existingreservoirs, both large and small, issedimentation. While highly variable, it isestimated that 1 percent of the total globalfreshwater surface storage capacity is lost each

7

year to sediment (Palmieri 1998).5 This does notseem like much until it is realized that the worldneeds to increase the amount of storage by 25percent just to stay where we are over the next25 years!

Often small dams are built without adequateclimate and hydrologic analysis. Due to smallcatchment areas and large variation in rainfallsome small tanks in Sri Lanka, for example, donot get sufficient water 3 out of 10 years.Inadequate hydrologic analysis can also result ininsufficient spillway capacity and lead to damfailure due to breaching of the embankment.

An issue facing large and small dams alike,but primarily small dams, is dam safety. Of 8,818high-hazard, non-federal dams inspected by theUnited States Army Corps of Engineers, one-third (2,925) was determined to be unsafe (FEMA1996). It is unknown whether the fraction ofdams outside of the United States with safetyproblems is greater or less than this.6

Contrary to common opinion, it is verydifficult to construct safe small dams. First, inorder for them to store as much water aspossible, it is desirable to have a largecatchment area. But large catchment areas havelarge runoff, exceeding storage capacity inextreme storm events. The water musttherefore be spilled over or around the dam.However, it is very expensive to buildconcrete and steel spillways, and manysmall dams, especially in developingcountries, do not have them. Consequently,water spillage can breach the dam. Inaddition, small dams often are constructed inthe dry season when there is inadequate soilmoisture and water to properly compact soilduring construction. Consequently, water seepsthrough the dam creating “pipes” that canbreach a small dam from within.

Large Reservoirs

By 1997, there were an estimated 800,000 damsin the world, 45,000 of which qualify as largedams. More than half of these large dams wereconstructed in the past 35 years. In 1997, anestimated additional 1,700 large dams wereunder construction (WCD 1998). The aggregatedesign storage capacity of the world’s large damsis about 6,000 km3 (LeCornu 1998). Thiscompares with total water withdrawals of 3,800km3 (Gleick 1998). Considering loss of storagedue to sedimentation, lack of filling, etc., perhapsone-half of the design storage (or the totalwithdrawals of about one year) is actuallyachieved. However, given that a large percentageof withdrawals is from recycled water, theaggregate design storage capacity of 6,000 km3

seems to us to be an incredibly high amount.It is interesting to note that of all the

registered large dams in the world only 5 percentis in Africa where most of the severe economicwater scarce countries are located. Fifty-fivepercent of the large dams is in North Americaand Europe, where, largely because of this, thereare not likely to be severe shortages (LeCornu1998, and Seckler et al.1998).

Large surface water reservoirs have theadvantage of greater yield relative to theavailable inflow than small reservoirs, and theiryield is generally more reliable. This is becauseof lower evaporation loss fractions in largereservoirs due to their greater depth. Because oftheir depth, many large reservoirs can storewater for multiyear carryover to weatherdroughts. In monsoonal climates, large reservoirsstore excess flows in the wet season for use inthe dry season.

Other advantages of large surface storagefacilities include their relatively low cost per

5We note that some have serious reservations about the validity of this sedimentation figure. While we were unable to validate the number,we believe that if correct, it is alarming and important to point out.6Many of the “unsafe” dams in the US were rendered so by changes in the applicable design standards—especially the switch to probablemaximum flood (PMF) for spillway capacity. In addition, many dams in the US were built privately with less control of standards than is oftenthe case outside the US.

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unit of utilizable water (see table 5) andmultipurpose qualities—e.g., hydropower andirrigation. According to the Secretary Generalof the International Commission on Large Dams(ICOLD), 30 percent of the world’s registeredlarge dams is multipurpose (LeCornu 1998).7

An emerging new use of large reservoirs inthe United States is “mimicry” of the naturalhydrograph to mitigate environmental impactsassociated with water development. By releasingartificial flood flows from large storage dams, thehydraulics of the natural river system can beimitated and the dynamic conditions of anenvironmentally healthy system recreated withless water than under virgin conditions.

Large dam reservoirs are more complex tooperate than small reservoirs and groundwatersystems from the standpoint of meeting theneeds of individual users. Because theycommand large areas, they are often far fromthe points of use. This distance, measured asthe water travel time from the dam to the pointof use at around 3 km per hour,8 can be weekslong. Therefore, large dam operations cannot beresponsive to individual demands that deviatefrom their expected values and so there ispotential for large mismatches of supply todemand. For example, water released from theHigh Aswan Dam on the Nile in Egypt takes 10days to reach irrigated areas in the Nile Delta. Ifthere is an unexpected rainfall event in a portionof the Nile Delta that temporarily reduces thedemand for irrigation, the water released atAswan will likely be spilled directly into theMediterranean Sea unused. On the other hand,unexpected rises in demand may not be met,causing water stress to crops. The flexibility oflarge storage structures is further reduced whenthey are multipurpose and potentially conflicting

demands (for example, hydropower generationand irrigation) exist. Other factors limiting theflexibility of large dam operations are the manyparties and levels involved in their managementand countless institutional prerequisites.

Reservoirs that are sited upstream of majordemands have maximum operational flexibility toshift water among competing uses, for example,taking advantage of rainfall in one area toconserve water for use at another location ortime. Where reservoirs are too far downstream inrelation to basin demands, surplus flows maybecome unusable. The Oum er R’bia in Moroccois a case in point, where storage available in theupper catchment is insufficient to meet thedemands of irrigation facilities in the area, whileexcess water accumulates in large downstreamreservoirs with limited potential uses furtherdownstream.

An important general issue facing large damsis their social and environmental impacts. Theintense social and environmental debate overlarge dams led to the establishment of the WorldCommission on Dams, which started its work inMay 1998. The Commission’s report is due byJune 2000 (WCD 1998).

Many of the negative impacts associated withlarge dams occur because they are constructedon-stream where they obstruct fish passage,inundate important aquatic and riparian habitats,dislocate historic communities, etc. Consequently,many of the new dams being planned andconstructed, particularly in the United States (forexample, the recently completed Los VaquerosDam in California), are for off-stream storage.

In the United States, beginning in 1997,decommissioning of large dams has exceededtheir construction rate.9 Between June 1997 andJuly 1998, the Secretary of the United States

7We believe this number is an anomaly of the reporting in the ICOLD dam registry and that the percentage of multipurpose large dams islikely much greater than 30.8Based on an allowable critical flow velocity in earthen channels of around 1m/s. Lined sections may have twice this flow velocity. Wheregradients are shallow, such as in Egypt’s Nile Delta, flow rates are much slower.9Most of the recently decommissioned dams in the United States are hydropower dams, which, besides having adverse environmental im-pacts, suffer from dam safety and other issues and are not economical to repair or upgrade.

9

Department of Interior, Bruce Babbitt,symbolically took his sledgehammer to six largedams (Babbitt 1998). In response to the dambusting furor, the International Commission onLarge Dams (ICOLD) has prepared a positionpaper on dams and the environment (ICOLD1998). The ICOLD paper discusses sustainabledevelopment of water resources and the role ofdams and reservoirs

Comparison of Large and SmallReservoirs

Both large and small reservoirs are appropriatetechnologies under specific conditions of timeand place. Table 3 provides a means ofexamining these issues by comparing themassive High Aswan Dam (HAD) and itsreservoir, Lake Nasser, on the Nile in Egypt withthe more than 17,000 small tanks in Sri Lanka.10

Several observations may be made from thefigures in table 3:

· The storage capacity behind HAD is168.9 km3, three times Egypt’s annualallocation from the Nile, and sufficient tomeet 3 years of total water needs for all ofEgypt. HAD literally saved Egypt from thedisasters that afflicted most of Africa duringthe great drought of the late 1980s.

· In comparison, small reservoirs are usedprimarily to meet water demands within aperiod of a few months. The storage capacitybehind HAD is over 240 times the aggregatecapacity of all 17,000 minor tanks in SriLanka. HAD commands 3.4 million irrigatedhectares compared to around 700,000 ha inSri Lanka, and supplies water to meet thedomestic and industrial needs of 60 millionpeople.

TABLE 3.Contrast of characteristics of the High Aswan Dam and it reservoir, Lake Nasser, with a typical minor tank in Sri Lanka.

Characteristic High Aswan Dam Typical minor tank in Sri Lank

Storage capacity 168.9 km3 (16.89 million ha-m) 4.1 ha-m

Surface area 6,500 km2 (650,000 ha) 5.0 ha

Net irrigated area 2.648,000 ha 5.0 ha

Storage fraction of area times depth 0.29 0.4

Annual evaporation loss 14 km3 (1.4 million ha-m) 2.0 ha-m

Annual evaporation depth 2.7 m 1.0 m

Dam height 111 m 2 m

Crest length 3,830 m 170 m

Embankment volume 44,300,000 m3 2,600 m3

Travel time to command area 10 days to 60% of total command Few hours

Command area 3.4 million irrigated hectares <10 ha

10HAD statistics are from Gleick 1993 and the minor tank numbers are derived from Sakthivadivel et al.1997.

10

· The ratio of HAD’s Lake Nasser surfacearea to Egypt’s irrigated area is about5:1; the ratio with small tanks is near1:1. This means that the evaporationfrom the small tanks exceeds that of thearea they irrigate.

· The dispersion of area inundated by smalltanks may be better, in terms ofenvironmental impact, than the concentratedinundation that occurs with large reservoirs.On the other hand, small tanks oftensubmerge the best agricultural lands.11

· The high operational flexibility of small tanksand high overall effectiveness of cascadesystems (noted below) can providesubstantial benefits over large reservoirs.

The point in comparing these two surfacestorage systems is that they are both verydifferent yet appropriate technologies in theirrespective settings. Small dams could notcollectively capture the surplus flows of the Nileas effectively as the High Aswan Dam. On theother hand, a single large water impoundment

11Small tanks (by definition) only submerge a few feet up the sides of a valley—the rest is valley floor. Large dams flood a lot of non-valleyfloor area that is usually less productive land.12Note that there are several large storage facilities in Sri Lanka.

TABLE 4.Characteristics of storage structures.

Storage type Conservation Operational Adequacy Reliabilitypotential flexibility

Large reservoir H L H L

Small reservoir L H L L

Groundwater storage H H L H

Large and small reservoirs combined H H H L

Large and small reservoirs combined with groundwater storage H H H H

Notes: H = High; L = Low; Adequacy = Sufficiency of yield to meet needs of command area; Reliability = Assuredness of water deliveries.

with the combined capacity of all the small tanksin Sri Lanka would not be effective in servicing allthe associated small irrigation systems.12 This isnot to say that there is no room for improvementin either case or that either is optimally designedor sited to maximize the capture of flows thatwould otherwise be lost to the sea.

Complementarities

It is important to consider complementaryopportunities among different types of storagesystems to improve conservation and productivityof water. Water conservation per se may notincrease water productivity because of inefficientoperation and mismatches with crop waterrequirements. Table 4 presents the characteristicsof storage types for providing the neededconservation and operational efficacies. Amongthe alternatives available, combinations of storagesystems are most likely to produce superiorresults. The suitable combinations of storagetypes depend on a number of factors, includingtopography, hydrology, and the existence ofsuitable aquifers.

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A number of combinations already exist andwork satisfactorily. The combination of small andlarge reservoirs is nicely demonstrated by the“melons on a vine” irrigation schemes in China,Sri Lanka, and other countries. Here, a few largestorage facilities supply water to numerous smalltanks within a river basin. In this manner, small

Conjunctive use ofgroundwater and small

reservoir water

Oosambadi Peria Eri is situated 10 km fromThiruvannamali in Tamil Nadu, India. This smallreservoir has an 80-hectare command area, 53farmer beneficiaries, and 60 wells, mostly dug.Prior to 1986, only one crop was grown. Eventhis crop could not be successfully irrigatedwithout supplemental well water, becausereservoir water, when directly used for irrigation,is sufficient only for about 70 days when thereservoir is full.

In 1986, only four farmers in the commandarea did not own wells. It was decided by theWater Users Association that these four farmerswould be provided with water at the commoncost and that the reservoir water would be usedonly for recharging the aquifer. In 1986, thesluices of the reservoir were permanentlyclosed. From then on, farmers have grown twocrops, paddy and another crop. Conjunctive useof surface water and subsurface water has beenpracticed for the last 14 years. Similar switchingover to conjunctive use has taken place in morethan 16 minor irrigation reservoirs in the drydistrict of Coimbatore, Tamil Nadu.

reservoirs act to dampen supply and demandmismatches from large reservoirs. In the ImperialIrrigation District in southern California, smallregulator reservoirs of 500,000 m3 save morethan 12 million cubic meters annually of canalflows that otherwise spill to the Salton Sea; thisresults in an annual 25:1 water conservation tostorage volume ratio. In southern Sri Lanka,construction and linking of a large storagereservoir at Lunugamvehera with five small,existing, cascading reservoirs resulted in a 400percent increase in crop production. In fact,cascading small reservoirs can significantlyincrease crop water use by capturing drainage,return flow, and surpluses from upstreamreservoirs.

Complementarities also occur where surfacestorage, particularly in the form of micro-reservoirs, retards runoff and enhancesgroundwater recharge. With improved tubewelltechnology now available and within reach ofsmall farmers, many storage reservoirs, whichwere previously used as irrigation tanks in thearid and semiarid tracts of India, have now beenconverted to recharge ponds, and tubewells havetaken the place of irrigation canals.

These successful experiments indicate thatcombinations of big and small reservoirs alongwith effective aquifer management can provideefficient solutions for conserving water andincreasing its productivity. Hitherto, this concepthas not been effectively put into practice fromthe planning stage, although it has beenpracticed in many areas of the world. With waterbecoming scarce, use of such integratedplanning for conserving water could lead tohigher water productivity while maintainingenvironmental and ecological balance.

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TABLE 5.Water supply costs (1998 US dollars).a

Technology Storage capital costs Lifetime delivery costs Source (US$/1,000 m3) (US$/1,000 m3)

Low Median High Low Medianb High

Large storage projects (storage andconveyance costs only) 110 270 1,600 2 5 32 Keller for this paper

Medium and small storage projects(storage and conveyance costs only) 130 320 2,200 7 17 110 Keller for this paper

Micro-storage projects (storage costs only) 160 390 2,500 7 17 110 Keller for this paper

Dug storage 500 800 1,200 22 35 60 OAS 1997

Artificial groundwater recharge 190 210 230 Gleick 1993

Groundwater development and pumping 20 40 110 Keller for this paper

Diversion projects (interbasin) 190 200 400 Gleick 1993

Conservation practices 40 105 300 Keller et al. 1998

Recycling wastewater (secondary treatment) 120 170 220 Gleick 1993

Reverse osmosis (for brackish water) 160 350 540 Gleick 1993

Recycling wastewater (advanced water treatment) 260 460 660 Gleick 1993

Desalinization of seawater 600 1,200 2,000 Keller et al. 1998aThe costs obtained from Gleick 1993 were indexed to 1998 US dollars using appropriate construction cost trends from BOR 1998 and

RSMeans 1998. The storage costs of small, medium, and large reservoirs were computed by applying BOR 1969 cost estimating guidelines todam statistics obtained from the BOR web site (1999) and from Gleick 1993. Indexed dam cost figures from the western US (BOR 1969) andthe State of Tamil Nadu, India (Sakthivadivel 1999) were used as a check.bMedian cost is taken as 2.5 times the low-end cost for large, medium and micro projects.

13These extremely low-cost dams are generally concrete arch or gravity dams. An example of such a low-cost (US$1.00/1,000 m3) dam is theKariba Dam in Zambia and Zimbabwe, which resulted in one of the world’s largest reservoirs (by volume, right behind HAD’s Lake Nasser).However, we note that Kariba had high environmental and social costs associated with it, which are not reflected in the dollar cost of thedam. (Kariba is a case study of the International Commission on Dams.)

The typical low, median, and high costs (in 1998US dollars) of various water supply technologiesare presented in table 5. The lifetime deliverycosts reflect the present value capital, operation,and maintenance costs over the economic life ofthe technologies divided by the total volume ofwater they produce and deliver.

The surface storage capital costs differgreatly between the low and high ends. This isdue to the wide variability in dam constructioncosts associated with site conditions, dam types,construction methods, spillway requirements, etc.We found some large dams that cost as little asUS$1.00 per 1,000 m3 of storage13 and others

that were more than US$15,000 per 1,000 m3.Rather than give these extremes, we present intable 5 typical low- and high-end costs. Asexpected, the distribution of surface storagecosts is positively skewed; that is, the averagecost of storage is greater than the median. Themedian cost of storage in table 5 is estimated at2.5 times the typical low-end cost, whereas theaverage cost of storage is closer to four timesthe low end.

It very well could be that dams can no longerbe built for the low-end costs listed in table 5given current dam safety requirements and thecosts of mitigating negative environmental

Costs

13

impacts. The cost of Los Vaqueros Dam, a keycomponent in the first major water project to bebuilt in California in the past decade, wasUS$346 per 1,000 m3 of storage (commensuratewith the median cost of large dams given in table5). The dam cost, however, represented only 10percent of the total project costs, which werelarge because of efforts to minimizeenvironmental impacts.14

The storage capital costs in table 5 are thecost of the storage facility per 1,000 m3 of grossreservoir capacity plus the associated cost of theconveyance system. For the storage capital coststo be comparable to the other water supplytechnologies listed in table 5, these costs mustbe adjusted to account for all the usable water areservoir will “produce” over its life. To do this,one has to divide the storage costs in table 5 byan estimate of how many times its capacity areservoir will release water over its life. This isaffected by the mean annual inflow to a reservoirrelative to its storage capacity, the rate ofsedimentation, and the evaporative loss fraction.For example, the combined large dam storagecapacity of the Colorado River system in thewestern United States is approximately fourtimes the mean annual flow of the river. Thus, onaverage, Colorado River storage cycles onceevery 4 years. So, in the course of 100 years, areservoir on the Colorado River will regulate andrelease 25 times its storage capacity. However,losses in capacity due to sedimentation15 andlosses of water due to evaporation reduce thetotal release volume to approximately 20 timesstorage capacity over 100 years.

In monsoonal climates, storage is only a smallfraction of the mean annual flow. (For example,Tarbela Dam on the Indus River in Pakistan has alive storage capacity less than10 percent of its mean annual inflow.) Since thesereservoirs are filling and releasing during the wet

14The Los Vaqueros Project was winner of the 1999 Outstanding Civil Engineering Achievement award; largely because of the way it ad-dressed environmental concerns (Hunt 1999).15Note that the effect of sedimentation on total reservoir yield for reservoirs with capacities two or more times their mean annual inflow isrelatively small compared to losses due to sedimentation in reservoirs with capacities smaller than their mean annual inflow.

season, they may realize 1.5 times their usablestorage capacity per year. However,sedimentation will reduce their relative yield by 50percent over a 100-year life of a typical reservoir.Thus, the yield of a large dam reservoir in amonsoonal climate might be 75 times its usablestorage capacity over the course of 100 years.

If we assume that on average medium andlarge dam storage projects deliver 50 times theirstorage capacity and small dams 20 times (dueto shorter lives and greater evaporation fractionsthan larger reservoirs), the effective cost per1,000 m3 delivered to a 50-hectare command isUS$2.00 to US$32.00 for large dams andUS$7.00 to US$110.00 for medium and smalldams.

We have also added cost estimates foralternative sources of water supply. Theconservation practices listed in table 5 includeprograms targeted at real water savings. Watersaved by such activities is transferable to otheruses or is available for expanded use within theproject area without adverse consequencesdownstream. An example of a water conservationpractice is canal lining where the seepage fromthe unlined canal is lost to non-beneficialevaporation or to a saline sink. The median costfor conservation practices listed in table 5 wasderived from the water conservation agreementbetween the Imperial Irrigation District (IID) andthe Metropolitan Water District (MWD) of southernCalifornia. Under the terms of this agreement,MWD paid for conservation at IID in exchange forthe water saved (Keller et al. 1998).

The costs of desalination of seawater givenat the bottom of table 5 include all approaches,of which multistage flash distillation and reverse-osmosis are the most common (Gleick 1998).The low-end desalination costs are engineeringestimates for cogeneration/reverse-osmosisdesalination (Keller et al. 1998).

14

The technologies presented in table 5 varywidely in scale and, therefore, to make the costscomparable, we have included the conveyancecost associated with delivering water from thefacility to within 50 hectares of its point of use inthe storage capital cost. For large surface storageprojects, we estimate the cost of conveyancefacilities to range between US$100 and US$600per 1,000 m3 of gross reservoir capacity. Formedium and small-scale surface storage projects,the costs in table 5 include US$80 to US$500 per

1,000 m3 of gross reservoir capacity. For waste-water recycling, interbasin diversion, reverseosmosis, and desalinization, the costs includeUS$0.00 to US$10.00 for conveyance of 1,000 m3

of developed water. The delivery costs in table 5include no conveyance costs for micro-storageand dug storage, groundwater development andrecharge, conservation practices, and distillation,as these technologies are generally scaled to the50-hectare command area or are additions toexisting systems.

Conclusion

Under all but the most optimistic scenarios, thereis a dearth of freshwater storage. If climatechange as a result of global warming manifests,the need for freshwater storage will become evenmore acute. Increasing storage through acombination of groundwater and large and smallsurface water facilities is critical to meeting thewater of the twenty-first century. This is especiallyso in monsoonal Asia and the developingcountries in the tropics and semitropics. As animmediate first step, we must assess the majorriver basins of the world, whether they are open,closed or semi-closed. The productivity of wateras presently used must also be assessed to

determine the extent to which increased demandsfor irrigated agricultural production can be met byincreasing water productivity, and the extent towhich increased demands will require increasedconsumption of water. The uncommitteddischarges from those basins that are open orsemi-closed must then be determined, and plansmade to effectively capture and put this water touse. Combinations of small and large storage andsurface water and groundwater recharge aregenerally the best systems where they arefeasible. In monsoonal Asia, research anddevelopment are needed on how to managewater under monsoonal conditions.

15

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Research Reports

26. Use of Historical Data as a Decision Support Tool in Watershed Management: ACase Study of the Upper Nilwala Basin in Sri Lanka. W. K. B. Elkaduwa and R.Sakthivadivel, 1998

27. Performance Evaluation of the Bhakra Irrigation System, India, Using AdvancedInformation Technologies. Wim Bastiaanssen and D. Molden, 1998.

28. Performance Evaluation of the Bhakra Irrigation System, India, Using Remote Sensingand GIS Techniques. R. Sakthivadivel, S. Thiruvengadachari, Upali Amerasinghe,W.G.M. Bastiaanssen, and David Molden, 1999.

29. Generic Typology for Irrigation Systems Operation. D. Renault, and G.G.A.Godaliyadda, 1999.

30. Mechanically Reclaiming Abandoned Saline Soils: A Numerical Evaluation. S. A.Prathapar and Asad S. Qureshi, 1999.

31. Gender Issues and Women’s Participation in Irrigated Agriculture: The Case of TwoPrivate Irrigation Canals in Carchi, Ecuador. Elena P. Bastidas, 1999.

32. Water Scarcity Variations within a Country: A Case Study of Sri Lanka. Upali A.Amarasinghe, Lal Mutuwatta, and R. Sakthivadivel, 1999.

33. Modernization Using the Structured System Design of the Bhadra Reservoir Project,India: An Intervention Analysis. R. Sakthivadivel, S. Thiruvengadachari, and UpaliA. Amarasinghe, 1999.

34. Assessment of Participatory Management of Irrigation Schemes in Sri Lanka: PartialReforms, Partial Benefits. M. Samad and Douglas Vermillion, 1999.

35. Modernizing Irrigation Operations: Spatially Differentiated Resource Allocations. D.Renault and I. W. Makin, 1999.

36. Institutional Change and Shared Management of Water Resources in Large CanalSystems: Results of an Action Research Program in Pakistan. D. J. Bandaragoda,1999.

37. Farmer-Based Financing of Operations in the Niger Valley Irrigation Schemes. CharlesL. Abernethy, Hilmy Sally, Kurt Lonsway, and Chégou Maman, 2000.

38. An Assessment of the Small-Scale Irrigation Management Turnover Program inIndonesia. Douglas L. Vermillion, Madar Samad, Suprodjo Pusposutardjo, SigitS. Arif, and Saiful Rochdyanto, 2000.

39. Water Scarcity and the Role of Storage in Development. Andrew Keller, R.Sakthivadivel, and David Seckler, 2000.

Research Report

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Water Scarcity and the Roleof Storage in Development

Andrew KellerR. SakthivadivelandDavid Seckler

ISSN 1026-0862ISBN 92-9090-392-9