echnical note c.1 ater resources and environment · element of our lending, supporting one of the...

Environment DepartmentThe World Bank1818 H Street, N.W.Washington, D.C. 20433, U.S.A.www.worldbank.orgFor information on these publications contact theESSD Advisory Service at [email protected] call 202.522.3773

The World Bank

Environmental Flows:Concepts and Methods

Water Resources and EnvironmentTechnical Note C.1

Water Resources and EnvironmentTechnical Note C.1

Series EditorsRichard Davis

Rafik Hirji

Series EditorsRichard Davis

Rafik Hirji

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WATER RESOURCES

AND ENVIRONMENTTECHNICAL NOTE C.1

Environmental Flows:Concepts and Methods

The World BankWashington, D.C.

SERIES EDITORS

RICHARD DAVIS, RAFIK HIRJI

2

A. Environmental Issues and LessonsNote A.1 Environmental Aspects of Water Resources ManagementNote A.2 Water Resources Management Policy Implementation: Early Lessons

B. Institutional and Regulatory IssuesNote B.1 Strategic Environmental Assessment: A Watershed ApproachNote B.2 Water Resources Management: Regulatory DimensionsNote B.3 Regulations for Private Sector Utilities

C. Environmental Flow AssessmentNote C.1 Environmental Flows: Concepts and MethodsNote C.2 Environmental Flows: Case StudiesNote C.3 Environmental Flows: Flood FlowsNote C.4 Environmental Flows: Social Issues

D. Water Quality ManagementNote D.1 Water Quality: Assessment and ProtectionNote D.2 Water Quality: Wastewater TreatmentNote D.3 Water Quality: Nonpoint-Source Pollution

E. Irrigation and DrainageNote E.1 Irrigation and Drainage: DevelopmentNote E.2 Irrigation and Drainage: Rehabilitation

F. Water Conservation and Demand ManagementNote F.1 Water Conservation: Urban UtilitiesNote F.2 Water Conservation: IrrigationNote F.3 Wastewater Reuse

G. Waterbody ManagementNote G.1 Groundwater ManagementNote G.2 Lake ManagementNote G.3 Wetlands ManagementNote G.4 Management of Aquatic Plants

H. Selected topicsNote H.1 Interbasin TransfersNote H.2 DesalinationNote H.3 Climate Variability and Climate Change

Water Resources and Environment Technical Notes

Copyright © 2003

The International Bank for Reconstruction and Development/THE WORLD BANK

1818 H Street, N.W., Washington, D.C. 20433, U.S.A.

All rights reserved.

Manufactured in the United States of America

First printing March 2003

3

AuthorsCatherine Brown, Jacqueline King

Technical AdviserStephen Lintner

EditorRobert Livernash

Production StaffCover Design: Cathe Fadel

Design and Production:The Word Express, Inc.

Cover photo byCurt Carnemark, World Bank

River, Chile

This series also is available onthe World Bank website(www.worldbank.org).

CONTENTSForeword 5

Acknowledgments 7

Introduction 9

Environmental Flows and River Management 11Environmental flows—the water left in a river ecosys-tem, or released into it to manage the condition ofthe ecosystem—are critical for maintaining ecosys-tems.

The Significance of Different Flows 13The flow regime of a river can be divided into baseflows, small floods that occur every year, and occa-sional large floods that spread out onto floodplains.Identifying these flow components—and understand-ing the ecosystem consequences of their loss or modi-fication—is central to a flow assessment.

Methods for Quantifying Environmental Flows 16Many methods have been developed over the last20 years to establish environmental flows. There is aconsiderable body of experience for temperate andsemi-arid rivers, but only limited experience in the ap-plication of these methods to tropical rivers.

Environmental Flows in the Decisionmaking Process 24Flow assessments are increasingly used as part of en-vironmental assesments, as well as tools in water re-source management that display the wider costs aswell as the benefits of development.

Implementation 25Environmental flows should be only one part of an in-tegrated set of environmentally sensitive water resourcedevelopments.

Conclusion 27

Further Information 28

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WATER RESOURCES AND ENVIRONMENT • TECHNICAL NOTE C.1

Boxes1. Terminology 112. Examples of valued features of rivers that could be protected

through environmental flows 123. Effects of different components of a flow regime on rivers 134. Actions that impact flow and consequences for aquatic

ecosystems 145. Instream flows that are not environmental flows 156. Features of prescriptive and interactive methodologies 167. Incorporation of social data into flow assessments 238. Desirable features for a successful environmental flows implementation 26

Figures1. An annual hydrograph of daily flows in a river 132. Hypothetical schematic illustrating the general relationships between

ecosystem use and condition 153. Wetted-perimeter method 194. The “building blocks” of the modified flow regime created using the BBM 205. Conceptualization of how PHABSIM calculates habitat values as a

function of flow 226. Basic components of a DRIFT assessment 23

Tables1. Relative data and time requirements of selected flow assessment methods 172. Tennant Method: percentage of average annual flow (AAF) required to

achieve different objectives 173. Phases of IFIM and DRIFT 21

5

ENVIRONMENTAL FLOWS: CONCEPTS AND METHODS

FOREWORD

The environmentally sustainable development andmanagement of water resources is a critical andcomplex issue for both rich and poor countries. Itis technically challenging and often entails difficulttrade-offs among social, economic, and political con-siderations. Typically, the environment is treatedas a marginal issue when it is actually key to sus-tainable water management.

According to the World Bank’s recently approvedWater Resources Sector Strategy, “the environmentis a special ‘water-using sector’ in that most envi-ronmental concerns are a central part of overallwater resources management, and not just a partof a distinct water-using sector” (World Bank 2003:28). Being integral to overall water resources man-agement, the environment is “voiceless” when otherwater using sectors have distinct voices. As a con-sequence, representatives of these other water us-ing sectors need to be fully aware of the importanceof environmental aspects of water resources man-agement for the development of their sectoral in-terests.

For us in the World Bank, water resources man-agement—including the development of surface andgroundwater resources for urban, rural, agriculture,energy, mining, and industrial uses, as well as theprotection of surface and groundwater sources, pol-lution control, watershed management, control ofwater weeds, and restoration of degraded ecosys-tems such as lakes and wetlands—is an importantelement of our lending, supporting one of the es-sential building blocks for sustaining livelihoods andfor social and economic development in general.Prior to 1993, environmental considerations of suchinvestments were addressed reactively and prima-rily through the Bank’s safeguard policies. The 1993Water Resources Management Policy Paper broad-ened the development focus to include the protec-tion and management of water resources in anenvironmentally sustainable, socially acceptable,and economically efficient manner as an emerging

priority in Bank lending. Many lessons have beenlearned, and these have contributed to changingattitudes and practices in World Bank operations.

Water resources management is also a critical de-velopment issue because of its many links to pov-erty reduction, including health, agriculturalproductivity, industrial and energy development,and sustainable growth in downstream communi-ties. But strategies to reduce poverty should not leadto further degradation of water resources␣ or eco-logical services. Finding a balance between theseobjectives is an important aspect of the Bank’s in-terest in sustainable development. The 2001 Envi-ronment Strategy underscores the linkages amongwater resources management, environmentalsustainability, and poverty, and shows how the 2003Water Resources Sector Strategy’s call for usingwater as a vehicle for increasing growth and re-ducing poverty can be carried out in a socially andenvironmentally responsible manner.

Over the past few decades, many nations have beensubjected to the ravages of either droughts or floods.Unsustainable land and water use practices havecontributed to the degradation of the water resourcesbase and are undermining the primary investmentsin water supply, energy and irrigation infrastruc-ture, often also contributing to loss of biodiversity.In response, new policy and institutional reformsare being developed to ensure responsible and sus-tainable practices are put in place, and new predic-tive and forecasting techniques are being developedthat can help to reduce the impacts and managethe consequences of such events. The Environmentand Water Resources Sector Strategies make it clearthat water must be treated as a resource that spansmultiple uses in a river basin, particularly to main-tain sufficient flows of sufficient quality at the ap-propriate times to offset upstream abstraction andpollution and sustain the downstream social, eco-logical, and hydrological functions of watershedsand wetlands.

6


With the support of the Government of the Nether-lands, the Environment Department has preparedan initial series of Water Resources and Environ-ment Technical Notes to improve the knowledgebase about applying environmental managementprinciples to water resources management. TheTechnical Note series supports the implementationof the World Bank 1993 Water Resources Manage-ment Policy, 2001 Environment Strategy, and 2003Water Resources Sector Strategy, as well as theimplementation of the Bank’s safeguard policies.The Notes are also consistent with the MillenniumDevelopment Goal objectives related to environmen-tal sustainability of water resources.

The Notes are intended for use by those withoutspecific training in water resources managementsuch as technical specialists, policymakers andmanagers working on water sector related invest-ments within the Bank; practitioners from bilateral,multilateral, and nongovernmental organizations;and public and private sector specialists interestedin environmentally sustainable water resourcesmanagement. These people may have been trainedas environmental, municipal, water resources, ir-rigation, power, or mining engineers; or as econo-mists, lawyers, sociologists, natural resourcesspecialists, urban planners, environmental planners,or ecologists.

The Notes are in eight categories: environmentalissues and lessons; institutional and regulatory is-sues; environmental flow assessment; water qual-ity management; irrigation and drainage; waterconservation (demand management); waterbodymanagement; and selected topics. The series maybe expanded in the future to include other relevantcategories or topics. Not all topics will be of inter-est to all specialists. Some will find the review ofpast environmental practices in the water sectoruseful for learning and improving their perfor-mance; others may find their suggestions for fur-ther, more detailed information to be valuable; whilestill others will find them useful as a reference onemerging topics such as environmental flow assess-ment, environmental regulations for private waterutilities, inter-basin water transfers, and climatevariability and climate change. The latter topics arelikely to be of increasing importance as the WorldBank implements its environment and water re-sources sector strategies and supports the next gen-eration of water resources and environmental policyand institutional reforms.

Kristalina GeorgievaDirector

Environment Department

7


ACKNOWLEDGMENTS

The Bank is deeply grateful to the Government ofthe Netherlands for financing the production of thisTechnical Note.

Technical Note C.1 was drafted by Catherine Brownand Jacqueline King of the Southern Waters Eco-logical Research and Consulting Pty (Ltd) in CapeTown, South Africa. The authors wish to thank Pe-

ter Cullen of the Cooperative Research Centre forFreshwater Ecology in Australia.

This Technical Note was reviewed by the followingBank staff: Hans-Olav Ibrekk, Alessandro Palmieri,Tor Ziegler, and Jean-Roger Mercier. We are grate-ful for their suggestions.

9


INTRODUCTION

eries will be considered in decisions concerningthe operation of reservoirs and the allocation of wa-ter.” The World Bank’s environmental assessmentpolicy (Operational Policy 4.01) is triggered if modi-fications to river flows lead to adverse environmentalrisks and impacts. If changes in flow have the po-tential to cause significant loss or degradation ofnatural habitats, borrowers must also comply withthe Bank’s natural habitats policy (OperationalPolicy 4.04) in order for a loan to be approved.

Technical Notes C.1 to C.4 deal with environmen-tal flows. Although changes in flow will affect wa-ter quality—for example, by increasing or decreasingturbidity—the focus in these notes is primarily onthe direct effects of flow on the ecological function-ing of rivers and the management of water quan-tity. Note C.1 introduces concepts and methods fordetermining environmental flow requirements forrivers, including a description of how different sortsof river flows contribute to the maintenance of riv-ers, the practicalities of undertaking a flow assess-ment, the need for balancing environmental andoffstream demands for water, and the challengesfaced in implementing environmental flows. Note

C.2 reviews some impor-tant case histories. NoteC.3 describes the rein-statement of flood re-leases from reservoirs forfloodplain inundation.Note C.4 addresses thedownstream social issuesarising from changes inflows.

The flows of the world’s rivers are increasingly be-ing modified through impoundments such as damsand weirs, abstractions for agriculture and urbansupply, maintenance of flows for navigation, drain-age return flows, and structures for flood control.These interventions have had significant impacts,reducing the total flow of many rivers and affectingboth the seasonality of flows and the size and fre-quency of floods. In many cases, these modifica-tions have adversely affected the ecological andhydrological services provided by water ecosystems,which in turn has increased the vulnerability ofpeople—especially the poor—who depend on suchservices. There is now an increasing recognitionthat modifications to river flows need to be balancedwith maintenance of essential water-dependent eco-logical services. The flows needed to maintain theseservices are termed “environmental flows,” and theprocess for determining these flows is termed “en-vironmental flow assessment,” or EFA.

The recognition that modifications to river flows arean important source of riverine, floodplain, and insome cases estuarine degradation is relatively re-cent. The methodology linking downstream re-source degradation andtheir social consequencesis also in its early stagesof development. TheWorld Bank acknowl-edged the issue in its 1993Water Resources Manage-ment Policy, which in-cluded as an objective that“the water supply needs ofrivers, wetlands, and fish-

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BOX 1.TERMINOLOGY

Several terms are used to describe flows for ecologicalmaintenance of rivers. “Environmental flows” is acomprehensive term that encompasses all componentsof the river, is dynamic over time, takes cognizance ofthe need for natural flow variability, and addresses socialand economic issues as well as biophysical ones. Otherterms include:

n Instream flow requirements (IFRs): an earlier, less-com-prehensive term for environmental flows, usually fo-cused on flows for fish.

n Maintenance IFR: a flow regime required to maintainall river ecosystem functions, and to provide sufficientaccess to water to allow plants and animals to repro-duce in most years.

n Drought IFR: a drastically reduced flow regime forrecognized drought years that is sufficient to main-tain species in a system without necessarily support-ing reproduction.

n Minimum flow: a general term used to describe aflow required to maintain some feature of a riverecosystem. The concept of minimum flow originatedin the United States as a streamflow standard to limitthe abstraction of water during the dry season,and may not be relevant in arid and semi-aridregions.

All of the above terms describe the maintenance ofhealthy conditions in a river.

ENVIRONMENTAL FLOWS AND RIVER MANAGEMENT

Environmental flows are the water that is left in ariver ecosystem, or released into it, for the specificpurpose of managing the condition of that ecosys-tem (Box 1).

The failure to maintain such flows has led to a de-cline in the health of many of the world’s water-dependent ecosystems, largely as a result ofincreasing pressure from water and catchment de-velopments. These ecosystems include not just in-river fauna and flora, but also the floodplains andwetlands watered by floods, groundwater-depen-dent ecosystems replenished through river seepage,and estuaries.

Not only does the decline in water-dependent eco-systems threaten environmental values such asmaintenance of biodiversity and protection of threat-ened species, but it directly affects many economicsectors that rely on such ecosystems. In many partsof the world, people depend on properly function-ing rivers and estuaries for fish and navigation;floodplain vegetation for grazing, fiber, and food;and wetlands for sediment trapping and pollutionremoval. Biophysical changes impact livelihoods.

The understanding that flows are critical for main-taining ecosystems has triggered an internationalmove to understand and describe the links between

flows and ecosystem functioning, so that environ-mental flows can be specified that will halt or re-verse this decline, and to help minimize the loss ofvalued ecosystem features (Box 2). This understand-ing can be used to describe flows for a river thatwill:n minimize or mitigate the impacts of new water-

resource developmentsn rehabilitate systems impacted by past develop-

mentsn allow calculation of the costs of compensating

people for such impacts.

These flow descriptions can be as simple as thespecification of a water depth to provide wettedhabitat for a fish species, or as complex as a de-scription of a completely modified flow regime tomaintain a whole river and floodplain ecosystem.Armed with this knowledge, decisionmakers arebetter equipped to achieve a satisfactory balance be-tween consumptive uses and ecosystem uses of thewater resource. Of course, environmental flowsalone are seldom a sufficient prescription for healthyrivers. Environmental flow allocations should beconsidered in combination with other complemen-tary mitigation measures—such as water quality im-provements—in order to achieve a cost-effectivecombination of management interventions (seeSkagit River Case Study in Note C.2).

12


BOX 2.EXAMPLES OF VALUED FEATURES OF RIVERS THAT COULD BE PROTECTED THROUGH ENVIRONMENTAL FLOWS

Feature Explanation of value Examples of environmental flows required

Aquatic animals Freshwater fish are a valuable source of n flows to maintain the physical habitat;protein for rural people. Other valued fauna n flows to maintain suitable water quality;include: angling fish, rare water birds, or the n flows to allow passage for migratory fish;small aquatic life that forms the base of the n small floods to trigger life-cycle cues suchfood chain. as spawning or egg-laying.

Riparian vegetation Stabilizes river banks, provides food and firewood n flows that maintain soil-moisture levels infor rural people and habitat for animals, and the banks;buffers the river against nutrient and sediment n high flows to deposit nutrients on thelosses from human activities in the catchment. banks and distribute seeds.

River sand Used for building. n flows to transport sand and to separate itfrom finer particles.

Estuaries Provide nursery areas for marine fish. n flows that maintain the required salt/freshwater balance and ocean connectionto estuary.

Aquifers and Maintain the perennial nature of rivers acting n flows to recharge the aquifers.groundwater as sources of water during the dry season.

Floodplains Support fisheries and flood-recession agriculture n floods that inundate the floodplain at thefor rural people. appropriate time of the year.

Aesthetics The sound of water running over rocks, the smells n sufficient flow to maximize naturaland sights of a river with trees, birds, and fish. aesthetic features, including many of the

flows mentioned above.

Recreational and Clean water and rapids for river rafting or clean n flows that flush sediments and algae, andcultural features pools for baptism ceremonies or bathing. that maintain water quality – see also

Also features valued by anglers, birdwatchers, aquatic animals.and photographers.

Ecosystem services Maintain the capacity of aquatic ecosystems n flows that maintain biodiversity andto regulate essential ecological processes, for ecosystem functioning.instance to purify water, attenuate floods, orcontrol pests.

Overall environmental A wish to minimize human impacts and conserve n some or all of the above types of flows.protection natural systems for future generations.

13


THE SIGNIFICANCE OF DIFFERENT FLOWS

In general, the closer decisionmakers want the aquaticsystem to be to natural, the greater the volume of theoriginal flow regime that will be required as an en-vironmental flow. However, the pattern of flow overtime is as important as the overall quantity.

The flow regime of a river can be divided into baseflows (low flows), small floods that occur every year,and occasional large floods that spread out onto flood-plains (Figure 1). Different components maintaindifferent parts of aquatic ecosystems (Box 3). Theloss or degradation of one component of a flow re-gime will affect a system differently than the loss ofsome other component. Identifying these flow com-ponents—and understanding the ecosystem conse-quences of their loss or modification—is central to aflow assessment. The timing of these flow compo-nents within a year is also important, since tempera-ture and other temporal cues play an important partin ecosystem functioning. However, these cues canoften depend on real-time conditions that are notexactly fixed by calendar day or month. Consequently,some environmental flow prescriptions allow riveroperators to exercise discretion, within specifiedrules, to ensure that the flows are effective.

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large (infrequent) floods

small (relatively frequent) floods

lowflows

FIGURE 1.AN ANNUAL HYDROGRAPH OF DAILY FLOWS IN A RIVER,BEGINNING IN OCTOBER OF ONE YEAR AND CONCLUDING

IN SEPTEMBER OF THE FOLLOWING YEAR. THIS IS CALLED

A FLOW REGIME.

The temporal characteristics of the flow regime alsohave an important influence on the overall charac-ter of a river ecosystem. Fluctuations between lowflows and small and large floods change conditionsthrough each day and season, creating mosaics ofareas inundated and exposed for different lengthsof time. The more diverse the physical conditions,the higher the biodiversity and the greater the re-silience of the ecosystem to disturbance.

Flows Importance to river ecosystem

BOX 3.EFFECTS OF DIFFERENT COMPONENTS OF A FLOW REGIME

ON RIVERS

Low flows occur when the river is not inflood. They are larger and more variedin the wet season than in the dry, anddefine whether the river flows all year,only during the wet season, or just afterrains. They create different conditions indifferent seasons, dictating which (andhow many) biotic species occur at anytime of the year.

Small floods stimulate spawning in fish,flush out poor-quality water, cleanse theriverbed, and sort the river stones bysize, thereby creating different kinds ofhabitat. They trigger and synchronizeactivities as varied as upstream migra-tions of fish and germination of seed-lings on riverbanks.

Large floods trigger the same in-riverresponses as small ones, but alsoprovide scouring flows that shape thechannel. They move and cleansecobbles and boulders on the riverbed,and deposit silt, nutrients, eggs, andseeds on floodplains. They re-chargesoil moisture levels in the banks,enabling seedlings of riparian trees togrow, and maintain links with the sea byscouring estuaries. These floods inun-date backwaters, secondary channels,and floodplains. They trigger bursts ofgrowth in many floodplain species,including waterbirds such as ibis. NoteC.3 describes the release of water forlarge floods.

Low flows:their rangein dry andwetseasons

Smallfloods: size,numberper year,and timing

Largefloods: sizeand timing

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Different developments will affect different compo-nents of the flow regime and, in turn, elicit variousresponses from the aquatic ecosystem. However, notall changes to the flow regime arise from direct ma-nipulations of flows. For instance, deforestation of anupland catchment can significantly increase the en-ergy of floods, resulting in changes to the form of theriverbed as well as deposition of excessive sedimenton floodplains. Box 4 summarizes typical develop-

ments, the components of the flow regime most com-monly affected, and examples of the consequencesfor aquatic ecosystems. These consequences have adirect impact on human populations dependent onthese ecosystems. Thus, the reduction in small-to-medium-sized floods following the building of alarge dam can remove cues needed for fish breed-ing, and so affect the livelihoods of downstreamwater-dependent communities.

BOX 4.ACTIONS THAT IMPACT FLOW AND CONSEQUENCES FOR AQUATIC ECOSYSTEMS

Irrigation flows(using the riveras a conduit)

Run-of-riverdiversion

Large dams

Hydropowerstations

Afforestationof catchment

Deforestationof catchment

Dry-season lowflowsincreased, andseasonal variabilityreduced.

Wet and dry seasonlowflows reduced.

Frequency andduration of floodsreduced.

Timing and distribution offlows altered.Rate of change betweenhigh and low flowsdecreased.

Wet and dry seasonlowflows reduced andsmall floods attenuated.

Energy of medium-large floods increased;dry season flowsincreased.

n Can result in higher flows in the dry than in the wet season. Hydrau-lic and thermal conditions, in particular, can become mismatchedwith life-cycle requirements, causing species to decrease innumbers and abundance. Pests are often able to take advantageof such environmental conditions and increase in abundance.

n Reduces habitat availability and restricts movement of aquaticanimals, thus increasing competition for space and vulnerability topredation.

n Increases diurnal temperature fluctuations, concentrates effluents,and can lead to toxic algal blooms.

n Flood cues that trigger fish spawning or seed germination mayoccur at the wrong time of the year or not at all, resulting in a failureto produce new generations of individuals.

n Reduced wetting of banks stresses riparian vegetation and reducesestablishment of seedlings. Bank stability is weakened and soilerosion increases.

n Reduced flows into estuaries reduces access for marine fish usingestuaries as nursery areas.

n Reduced flooding of riparian wetlands and floodplains causes lossof fisheries and other attributes.

See Lesotho Highlands Water Project in Note C.2.

n Mismatched flows and abnormal flow fluctuations impact life-cyclestages of many animals and plants. See Skagit River case study inNote C.2.

n Reduces flood cues that trigger fish spawning or seed germination,and decreases wetted habitat through the year.

n Increases bank and bed erosion, which alters the available habitatfor aquatic species.

n Reduces habitat availability in the dry season.

n Increases the risk of animals being washed away.

Management Example of the Examples of ecosystemactions impact on flow consequences

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Water released down a river for nonenvironmentalpurposes (Box 5) may damage the aquatic environ-ment. Any flow assessment that is part of a devel-opment proposal should include the ecosystemimpacts of nonenvironmental flows.

Balancing the multiple and competing demands forwater is one of the greatest challenges facing water

managers. In the past, some development costs, par-ticularly those affecting the environment and borneby economically weak communities, have been ig-nored. As a general rule, the overall benefits fromexploitation of water resources increase to a pointbeyond which use is no longer sustainable, as thesystems degrade with exploitation and the costseventually outweigh the benefits (Figure 2).

FIGURE 2.HYPOTHETICAL SCHEMATIC ILLUSTRATING THE GENERAL RELATIONSHIPS BETWEEN ECOSYSTEM USE AND CONDITION

BOX 5.INSTREAM FLOWS THAT ARE NOT ENVIRONMENTAL FLOWS

Hydropower releases Water released to generate hydroelectricity creates wide fluctuations in downstream river flow,flooding, and drying out habitat for aquatic species such as fish (see Note C.2). To someextent, such flow surges can be controlled to mitigate their impact on the downstream river.

Irrigation releases Irrigation water releases can cause seasonal reversal of the flow regime, with flows that arehigher in the dry season than in the wet. Life-cycle cues for aquatic species, provided by theflow, become mismatched with temperature and other required conditions, causing loss ofspecies and other ecosystem imbalances.

Navigation Unnaturally high flows for navigation can cause bank and bed erosion, and can also dampenor remove flow variability (Box 3).

Dilution of pollution Diluting pollutants as a way to improve water quality is poor management. If pollutants arecontrolled at the source and not through high dilution flows, more water is available for otheruses.

Release of wastewater Same as for navigation, but with added pollution impact.

Interbasin transfers Water moved from one catchment to another can have many of the above effects. It can alsoimpact on biodiversity through the introduction to a catchment of competitive or alien species.

Increased degradation of the river system

Increased exploitation of the river system

Sustainable use Non-Sustainable use

Overall benefits of use for modern humans

High-riskuse

Ben

efit

s

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METHODS FOR QUANTIFYING ENVIRONMENTAL FLOWS

Many methods have been developed over the last20 years—primarily in Europe, the United States,South Africa, and Australia—to establish environ-mental flows. Some techniques were developed forprotection of specific (often threatened) species,while others were developed for broader ecosys-tem protection. These techniques have now beenapplied in over 25 countries, resulting in a consid-erable body of experience for temperate and semi-arid rivers, but only limited experience in theapplication of these methods to tropical rivers.

Environmental flow assessment methods fall intotwo categories, prescriptive and interactive (Box 6).Methods based on the prescriptive approach usu-ally address a narrow and specific objective andresult in a recommendation for a single flow valueor single component of the flow regime. Their out-comes tend not to lend themselves to negotiation,because effort is mostly directed to justifying thesingle value, and insufficient information is sup-plied on the implications of not meeting the rec-ommended value to allow an informed compromise.Interactive approaches, on the other hand, focus onthe relationships between changes in river flow andone or more aspects of the river. Once these rela-tionships are established, the outcome is no longerrestricted to a single interpretation of what the re-sulting river condition would be. Methods based onthe interactive approach are thus better suited foruse in negotiations. They do tend to be more com-plex, however, and have more onerous data and timerequirements, than do prescriptive approaches. Sev-eral methods have been developed in each category.

The methods presented in this Note are chosen toillustrate different degrees of data and time require-ments, as well as the reliability of the results andthe level of experience required to apply the method(Table 1).

PRESCRIPTIVE APPROACHES

These can be divided into four broad categories:n Hydrological index methods are mainly desk-

top approaches relying primarily on historicalflow records to make flow recommendations forthe future. Little, if any, attention is given to thespecific nature of the considered river or itsbiota.

n Hydraulic rating methods use the relationshipbetween the flow of the river (discharge) andsimple hydraulic characteristics such as waterdepth, velocity, or wetted perimeter to calcu-late an acceptable flow. These methods are animprovement on hydrological index methods,since they require measurements of the riverchannel, and so are more sensitive than thedesktop approaches to differences between riv-ers. However, judgment of an acceptable flowis still based more on the physical features ofthe river rather than on known flow-relatedneeds of the biota.

n Expert panels use a team of experts to makejudgments on the flow needs of different aquaticbiota.

n Prescriptive holistic approaches require collec-tion of considerable river-specific data and make

BOX 6.FEATURES OF PRESCRIPTIVE AND INTERACTIVE METHODOLOGIES

Prescriptive

Often provide a single flow regime to maintain a singleobjective (river condition).

Motivate for the inclusion of specific parts of the flow regime.

Not conducive to exploring options.

Suited for application where objectives are clear and thechance of conflict is small.

Interactive

Provide a range of flow regimes, each linked to adifferent river condition.

Explain the consequences of flow manipulations.

Conducive to exploring options.

Suited for application where the eventual environmentalflow is an outcome of negotiations with other users.

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TABLE 1.RELATIVE DATA AND TIME REQUIREMENTS OF SELECTED FLOW ASSESSMENT METHODS

Approximate RelativeData and time duration of confidence in Level of

Output Method requirements assessment output Experience

Prescriptive Tennant Method Moderate to low Two weeks Low USA/extensive

Wetted- Moderate 2-4 months Low USA/extensivePerimeterMethod

Expert Panels Moderate to low 1-2 months Medium South Africa,Australia/extensive

Holistic Moderate to high 6-18 months Medium Australia/Method very limited

Interactive IFIM Very high 2-5 years High USA,UK/extensive

DRIFT High to very high 1-3 years High Lesotho, SouthAfrica/very limited

structured links between flow characteristics ofthe river and the flow needs of the main bioticgroups (fish, vegetation, invertebrates). Twoexamples are included here.

Hydrological index methods. The Tennant (or Mon-tana) Method is a desk-top approach that is rela-tively inexpensive, quick, and easy to apply. Itsdevelopment required considerable research and

input from experts. The results compare relativelywell with those from data-intensive techniques. Theapproach is based on trends derived from field ob-servations in the United States of the relationshipamong river condition, the amount of flow in theriver, and the resultant fish habitat. These are usedto recommend environmental flows for the main-tenance of fish, wildlife, recreation, and related re-sources (Table 2). For example, if the average annual

Recommended percentage of AAF

Objective Autumn-Winter Spring-Summer

Flushing or maximum flows 200 200

Optimum range of AAF 60-100 60-100

Percentage AAF required to maintain a required river condition

Outstanding 40 60

Excellent 30 50

Good 20 40

Fair or degrading 10 30

Poor or minimum 10 10

Severe degradation 10-zero flow 10-zero flow

TABLE 2.TENNANT METHOD: PERCENTAGE OF AVERAGE ANNUAL FLOW (AAF) REQUIRED TO ACHIEVE DIFFERENT OBJECTIVES

(AAF EXPRESSED AS INSTANTANEOUS FLOW)

18


flow (AAF) in a river is 100 x 106 cubic meters perannum (m3a-1), then for an “outstanding” river con-dition, the flow in the river in Autumn-Winter wouldneed to be 40 percent of the average instantaneousflow, or 1.3 cubic meters per second (m3s-1). Themethod is claimed to be applicable to a wide rangeof river types and sizes, and the general approach,at least, may be applicable in many parts of theworld. Once the initial relationship between rivercondition and flow has been established for a re-gion, the data requirements of the method are mod-erate, requiring measured or easily simulatedmonthly hydrological data.

As with all rapid assessment methods, the TennantMethod is most suited to the region—in this case,the western United States—where it was developed,where the hydrological and ecological characteris-tic of the rivers are well-studied and well-under-stood. It was designed principally for managing trouthabitat, which may limit its applicability to otherbiota in other parts of the world.

In new regions where time is a major constraint, aspecially tailored Tennant approach, based on fieldobservations of the habitat responses of the biotaof interest in that region, would provide a good me-dium-resolution technique for determining envi-ronmental flows. The outcome of such a “TailoredTennant” approach would be a table similar to thatin Table 2, but based on empirical observations thatare relevant to the country where they were taken.

Other examples of hydrological index methods in-clude the Flow Duration Curve Analysis, Range ofVariability Approach, and the Desktop Method.

Hydraulic rating methods. Like hydrological indexmethods, hydraulic rating methods also use the hy-drological record. However, they link this to simplecross-section data collected in the river of interest.The Wetted-Perimeter Method is a low-resolution,river-specific method that is used for determiningseasonal flows required to maintain fish popula-tions. It is relatively quick and cost-effective. Thenumber of measurements taken and field visitsmade will depend on the level of confidence requiredfor the study. It is useful as a planning method at

catchment scale or greater. Because it is widely usedin the United States, there is a great deal of exper-tise and experience to draw upon.

The method is based on the assumption that fish-rear-ing is related to food production, which in turn is re-lated to how much of the river bed is wet. It usesrelationships between wetted perimeter and discharge,depth, and velocity to set minimum discharges for fishfood production and rearing (including spawning).The relationships are constructed from measuring thelength of the wetted perimeter at different dischargesin the river of interest. The resulting recommendeddischarges are based on inflection points on the wet-ted-perimeter/discharge curve, which are assumedto represent the maximum habitat for minimum flowbefore the next inflection point (Figure 3).

The disadvantage of the method is that the observedrelationships between wetted-perimeter and dis-charge used to recommend suitable habitat for fishare based on general principles, and are not provento be relevant to the fish of a particular river. Toremedy this, detailed studies have to be undertakenon the relationship between wetted perimeter andthe survival and reproduction of particular fish spe-cies. Although these studies increase the reliabilityof the results, they also add considerably to the timerequired and the costs of the method.

Expert panel. The family of expert-based methodsdescribed here have the common feature that theyuse a team of experts to make judgments on theflow needs of different aquatic biota. The composi-tion of the panel will depend on the specific envi-ronmental and social features of the river inquestion, but typically includes a hydrologist, geo-morphologist, aquatic botanist, and fish biologist.In many cases, one or more community represen-tatives will join the panel. The collective experienceof the panel members is used in the absence of re-liable, predictive flow-ecology models. By puttingthese experts on a panel, rather than employingthem independently, it is expected that an integratedassessment of flow needs will emerge.

Although the procedure varies from panel to panel,it is usual for the panel to undertake field inspec-

19


tions at different points in the river. If the river hasupstream impoundments, it is also common fordifferent-sized flows to be released during these fieldvisits so that the experts can see the extent of inun-dation and, in some cases, the response of ecologi-cal compartments to these different flows. The panelmeets with stakeholders during the course of thefield visits to understand the water use requirementsof different communities along the river. The panelalso has access to the hydrological records for theriver as well as ecological data and reports. Basedon this array of information, the panel produces adraft report describing the likely ecological re-sponses of the river biota to different flow regimes,including low, medium, and high flows. The reportis discussed at one or more workshops attended bystakeholders and managers before being finalized.

The method has been widely applied in the easternstates of Australia with considerable success. Itsadvantages are its rapidity, ability to effectively cap-ture and integrate the knowledge of different ex-perts, and its flexibility. It does not rely on theexistence of models (although models can be em-ployed if available). However, the results are site-specific and non-reproducible, and therefore moreopen to challenge than traditional data-intensive/modeling approaches.

Holistic approaches. Holistic approaches are essen-tially ways of organizing and using flow-related dataand knowledge. They often incorporate some of themethods described above, particularly the expertpanel methods. They are better described as meth-odologies, which implies the linking of several dis-tinct procedures or methods to produce an outputthat none could have produced alone. They weredeveloped in the southern hemisphere, mainly be-cause northern hemisphere methods, which tendto target individual (often commercially valuable)species, were too limited when the aim was to man-age the health of the whole river ecosystem.

The Holistic Method in Australia and the BuildingBlock Methodology (BBM) in South Africa were de-veloped in collaboration and share the same basictenets and assumptions. Both require early identi-fication of the future desired condition of the river.An environmental flow regime is then constructed—on a month-by-month basis, through separate con-sideration of different components of the flow re-gime (Figure 4)—to achieve and maintain thiscondition. Each flow component is intended toachieve a particular ecological, geomorphological,or water-quality objective. Given the similarities be-tween the two methodologies, only the BBM is dis-cussed further here.

a)

Banktop

Water levels correspondingto break points

b)

Banktop

Break points in slope

Discharge

Wet

ted

per

imet

er

FIGURE 3.WETTED-PERIMETER METHOD: (A) HYPOTHETICAL CHANNEL CROSS-SECTION AND (B) GRAPH OF WETTED PERIMETER VERSUS

DISCHARGE. BREAKPOINTS IN SLOPE INDICATE THE MAXIMUM AVAILABLE FISH HABITAT FOR THE LEAST AMOUNT OF WATER, UNTIL

THE NEXT BREAKPOINT.

20


The BBM was designed to address the southern-African realities of limited data, money, and time.It depends on available knowledge, expert opinion,and limited new data, which are used in a struc-tured set of activities to describe an environmentalflow. The major components of a river ecosystem,both physical (hydrology, physical habitat, andchemical water quality) and biological (vegetation,fish and macro-invertebrates), are considered, asis subsistence use of the river by riparian people.For each of these disciplines, all available data aresynthesized and new data collected where neces-sary. Field measurements always include the sur-

veying of cross-sections at representative sites alongthe river, and development of the relationship be-tween flow and water depth, velocity, and area ofinundation. The biological specialists also conductfield studies, from which they develop an under-standing of the links between aquatic species andthe flow in the river at different times. After datacollection, a desired future condition for the riveris described in a specialist workshop.

The specialists then reach consensus on a modi-fied flow regime that would help achieve the de-sired condition.

The strength of the BBM lies in its ability to incor-porate any relevant knowledge, and to be used inboth data-rich and data-poor situations. It addressesa wide range of ecosystem components, and the fi-nal environmental flow is arrived at through con-sensus by the full BBM team of river specialists. Itis well documented and is widely used in South Af-rica. Holistic approaches are new, however, andjudging their effectiveness will take time.

INTERACTIVE APPROACHES

Flow-assessment methods that use an interactiveapproach tend to be more complex than prescrip-tive methods and are predominantly limited to twobroad types: the habitat simulation and holistic

methodologies. They are illustrated here byone of the oldest—the Instream Flow Incre-mental Methodology (IFIM)—and one of thenewest—Downstream Response to ImposedFlow Transformations (DRIFT).

Both are essentially problem-solving toolswith similar approaches (Table 3). Theoutput is a set of options—alternatives inIFIM terminology, or scenarios in DRIFTterminology. Each option quantitatively de-scribes:n a modified flow regimen the resulting condition of the river, or

species, whichever is being addressedn the effect on yield for offstream usersn the direct economic costs and benefits.Kihansi Gorge, Tanzania

Pho

to b

y Pe

ter D

ew

ee

s, W

orld

Ba

nk

Dis

char

ge

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

LOW FLOWS(first building blocks)

CHANNEL AND HABITAT MAINTENANCE FLOODS(second building blocks)

SPAWNING/MIGRATION

FRESHES(third

building blocks)

Months

FIGURE 4.THE “BUILDING BLOCKS” OF THE MODIFIED FLOW REGIME

CREATED USING THE BBM.

21


DRIFT also addresses the social costs and benefitsof changing river conditions, particularly for down-stream riparian users (the population at risk) of riverresources. Neither methodology provides a recom-mended environmental flow. Rather, each seeks to

provide objective, scientifically based input on theconsequences for rivers of a range of flow manipu-lations. These allow water managers to make moreinformed decisions on the equitable use of water.

Problemidentificationor issuesassessment

Studyplanning

Studyimplementation

Optionsanalysis

Problemresolution

n Identification of interested andaffected parties, their concerns,information needs and relativeinfluence or power.

n Identification of the broad studyarea, and the extent of probableimpacts.

n Identification of the main components of the projectand the interested and affected parties.

n Identification of the population at risk.

n Identification of the broad study area, and the extent ofprobable impacts.

n Identification of social concerns (local, national, andinternational) to be addressed in the biophysical studies.

Both approaches require:

n Assessment of existing biophysical, social, and economic data, and evaluation of the needfor further data.

n Selection of representative river reaches.

n Design of data collection procedures.

n Identification of key data collection sites.

Interdisciplinary integration of site selection and data collection avoids overlaps and gaps,and maximizes the usefulness of the data.


n Development of environmental flow alternatives or scenarios, each describing a possiblefuture flow regime and the resulting river condition.

n Yield analysis of water available for development with each scenario.

n Determination of the directcosts and benefits of thealternatives

n Determination of the direct costs and benefits of eachscenario.

n Additionally, for each scenario, determination of thesocial impacts and costs to the population at risk ofchanging river condition.

PHASES IFIM DRIFT

Addition of social considerations in selection of study areaand sites. In particular, compatibility ensured betweenbiophysical data (collected at river sites) and social data(collected in rural villages).

n Collection of hydraulic, chemical, geomorphological,thermal and biotic data, and analyses to developpredictive capacity on how flow changes will affecteach.

n Multidisciplinary workshop to compile a database ofbiophysical consequences of a range of flowmanipulations.

n Collection of hydraulic andbiotic data.

n Calibration of habitat model.


n Assessment of the bigger picture (for example, data on other costs/benefits of the water-resource development).

n Negotiation with offstream water users.

n Public participationn Transparent decisionmaking processes.

TABLE 3.PHASES OF IFIM AND DRIFT

22


Habitat simulation methodologies: the Instream FlowIncremental Methodology (IFIM). IFIM is the mostcommonly used flow assessment method worldwideand the best-documented method currently avail-able. It was developed by the U.S. Fish and WildlifeService’s Instream Flow Group in the late 1970s.It is founded on a basic understanding and descrip-tion of the water supply and habitats within riverreaches of concern.

IFIM is used to evaluate the effects of incrementalchanges in discharge on channel structure, waterquality, temperature, and availability of suitablemicrohabitat for selected target aquatic species. Bothmacrohabitat and microhabitat, as described below,are assessed for key species. These species are cho-sen either because they are the major species of

A: Site specific microhabitat data

PHABSIM

B: Habitat suitability criteria

C: Relationship betweendischarge and microhabitat for juveniles of Species A

Flow

SI

Velocity (ft/sec) Depth (ft)

Cover fractions

0 1 2 3 4 0 1 2 3 4

0 0.2 0.60.4 0.8 1.0

Cross section B

Cross section A

0

0.2

0.4

0.60.8

1.0

0

0.2

0.40.60.8

1.0

SI

Discharge

Wet

ted

usa

ble

are

a

00 100

100,000

FIGURE 5.CONCEPTUALIZATION OF HOW PHABSIM CALCULATES HABITAT VALUES AS A FUNCTION OF FLOW. (A) FIRST, DEPTH, VELOCITY,COVER CONDITIONS, AND AREA ARE MEASURED OR SIMULATED FOR A GRID OF CELLS OVER A RANGE OF FLOWS. (B) SUITABILITY

INDEX (SI) CRITERIA ARE USED TO WEIGHT THE SUITABILITY OF EACH CELL AS HABITAT FOR EACH SELECTED SPECIES OVER THE

SAME RANGE OF FLOWS. THE HABITAT VALUES FOR ALL CELLS IN THE STUDY REACH ARE SUMMED TO OBTAIN A SINGLE HABITAT

VALUE FOR EACH FLOW (C). THE OPTIMUM MICROHABITAT FOR JUVENILES OF SPECIES A IS NOTED WITH AN ARROW.

concern, or because they are deemed to representthe species and the general river condition desired.

Microhabitat is the small physical area in any placein a river that is directly relevant to the species be-ing studied. The availability of suitable microhabi-tat over a range of flows is modeled using PHABSIMII (Physical Habitat Simulation Model). This modelpredicts how the water depth, water velocity, andriverbed features change with changing flow, andthus their changing suitability for the chosen spe-cies (Figure 5). The model was designed for, and isusually applied to, fish habitat. The model requiresextensive field data and considerable understand-ing to apply. It also requires a fairly detailed under-standing of the habitat preferences of the chosenspecies during their different life stages. For ex-

23


ample, some fish species require attached vegeta-tion for egg laying, others need clean pebble beds;some require reeds or grasses for refuge, others needlarge woody debris. In spite of these data and knowl-edge demands, some of the PHABSIM programs canbe used with limited field data for reconnaissanceor area-wide planning studies.

The suitability of microhabitat is qualified, in themacrohabitat component, by the suitability of wa-ter quality and temperature. Thus, even if the physi-cal microhabitat requirements are met, some fishspecies will not breed if the correct water tempera-ture and flow cues are absent. The results are a timeseries of suitable habitat for a particular species overa period of changing flows (Figure 5).

IFIM has been subjected to extensive scientific cri-tique, but this is more a product of its widespreaduse in flow assessments than an indication of itsmerits relative to other methods. Its main drawbackslie in its complexity, difficulty of use, its extensivefield data demands, requirements for good under-standing of target species needs, and questionableapplicability outside its area of development. Itsauthors suggest that its strong scientific basis makeit appropriate for the most controversial project as-sessments and that other less onerous methods arebetter employed for other projects.

Downstream Response to Imposed Flow Transfor-mations (DRIFT). DRIFT was developed for theassessment of environmental flows for the LesothoHighlands Water Project (see Note C.2). DRIFT cul-minates in one or more multidisciplinary workshopsthat are designed to produce an agreed number ofbiophysical and socioeconomic scenarios.

Specialists use methods that are specific to differ-ent components of the flow regime to collect dataand then, within the DRIFT structure, to predictthe consequences of flow changes. For instance,PHABSIM II could be used by the fish biologists tomodel changes in fish habitat arising from medium-level floods that affect in-stream fish habitat. DRIFTalso uses data on cultural and subsistence use ofthe river to predict the socioeconomic implicationsof river change (Figure 6, Table 3, and Box 7).

FIGURE 6.BASIC COMPONENTS OF A DRIFT ASSESSMENT

BOX 7.INCORPORATION OF SOCIAL DATA INTO FLOW ASSESSMENTS

In regions such as Africa, South America, and Asia,where large numbers of poor people rely directly onrivers for subsistence, flow assessments should includeconsideration of the social and economic implica-tions of changes in river flow. In some cases these willbe obvious, such as loss of a food fish or plant,deterioration in the quality of potable water, or filling inof a pool used for ceremonies. In others, the impactswill be less obvious. Vitamins and minerals supplied byriparian plants may contribute to the overall health ofa community, or certain levels of flow may dilute oraid decomposition of wastes entering the river, so thatthe water can be drunk without incurring health risks.So often externalized in water-resource planning, theseindirect costs of deteriorating river condition areusually borne by the poorest members of society.

DRIFT is essentially a system for managing dataand knowledge in a structured way, following fivemain steps.n Identification and isolation of wet-season and

dry-season low flows, and small and large floodsfrom the long-term hydrological record.

n Description of the consequences for the river ofpartial or complete removal of each of these flowcomponents (Box 3).

n Creation of a biophysical database detailing theconsequences of flow alterations.

BIOPHYSICALCURENT

CONDITION

PARCURENT

CONDITION

ECONOMICSCURENT

CONDITION

Flow-related areas of concern

Flow-related issues

FLOW SCENARIOS:-river condition-economics-social-yield

DECISION

Data collection and analysisto develop predictive

capacity

24


n Use of the database to describe how river con-dition will change with any future combinationof high and low flows.

n Description of the socioeconomic implicationsof the changes in river condition. This, togetherwith the previous step, constitutes the creationof environmental flow scenarios.

CHOOSING THE RIGHT TECHNIQUE

The purpose of a flow assessment and the intendeduse of the results should guide the selection ofthe assessment method. Project-specific flow as-sessments for large or controversial projects, whichare likely to call for considerable negotiationand tradeoffs between environment and develop-ment issues, require a more comprehensive ap-proach than do flow assessments for coarse-scale

planning studies, where a single number mightsuffice.

Within either category, the flow assessment methodeventually chosen will depend on technical consid-erations such as the quality and availability of dataon the study rivers, the location and extent of thestudy area, the prevailing time and financial con-straints, and the level of confidence required in thefinal output. Eco-hydrology is a relatively new sci-entific field, so there is only limited understandingand very few models of species responses to varyinghydrologic conditions. Most of the data and under-standing required for interactive approaches haveto be acquired on a site-by-site basis, considerablyadding to the time, funding, and expertise requiredfor a flow assessment. Probably because of this, mostapplications have used a prescriptive approach.

ENVIRONMENTAL FLOWS IN THE DECISIONMAKING PROCESS

ENVIRONMENTAL ASSESSMENT ANDENVIRONMENTAL FLOW ASSESSMENT

Environmental Assessment (EA) is the integrativeprocess of identifying and evaluating the likely bio-physical, social, and other relevant effects of devel-opment proposals prior to major decisions beingmade. Mitigation measures are sometimes includedin EA and sometimes described in separate Envi-ronmental Management Plans (EMPs).

In the case of water development projects, a flowassessment should be an essential component of anEA. Impacts arising from alteration of a river’s flowregime will always have the potential to be severe.These impacts can often be mitigated through thedesign of environmental flows or compensatedthrough resource substitution or community devel-opment programs, and this can be shown in an EMP.

ALLOCATING WATER

Increasingly, flow assessments are seen as tools inwater-resource management that display the wider

costs as well as the benefits of development, allow-ing more informed tradeoffs to be made.

Water policy and legislation can provide a vital sup-port and guide to decisionmakers. Where policy andlegislation define the need and objectives for envi-ronmental flows within the realm of sustainable uti-lization, flow assessments need only determine thevolume and temporal distribution of an environ-mental flow. Without such a framework, they havethe burden of not only defining environmental flowsbut also of giving legitimacy to them.

With such legal support, a structured, transparent,and widely accepted decisionmaking process canaddress the results of engineering, economic, andenvironmental studies, including flow assessments.From this, an agreed decision can emerge onwhether, and in what way, to proceed with a waterresource development. In the event such a devel-opment is pursued, agreement on the desired fu-ture river condition and the flow allocations requiredto maintain that condition will provide the legiti-macy for environmental flow allocations.

25


Flow-related effects that are not easily expressedin monetary terms need to be included in this deci-sion process. If not, effects of development arisingfrom changes in flow regime—such as the loss ofa fish species, or the declining quality of life of ri-

parian people—will be undervalued and lead to dis-proportionate costs being borne by those groups insociety who are not fully integrated into a marketeconomy, or fully represented in the decisionmakingprocess.

IMPLEMENTATION

Environmental flows should be only one part ofan integrated set of environmentally sensitivefeatures. Complementary mitigation (biophysical)features that could be considered include fishladders; multiple-level releases in reservoirs;water-chemistry and temperature sensors at thedifferent off-take structures; outlet pipes able totake the volume from all the off-takes simulta-neously if necessary; structures that minimize an-ticipated water-quality conditions such as anoxicor super-saturated water; and a facility for passingsediments through reservoirs and past the dam wallsor weirs.

Compensation and mitigation programs should bedeveloped on the basis of specific consideration ofdownstream issues, which are often different thanupstream issues. Downstream impacts relate notonly to the reduction in water flows, but also theassociated transformation from an aquatic environ-ment to a terrestrial environment. Downstream is-sues that may form part of the compensation andmitigation programs for riverine resource lossesmay include reduction in fish, vegetables, plants,animal forage, firewood, timber for other uses andwater supply for people, livestock and other usesfrom direct and indirect changes in the amount,quality, and timing of flows. The methodologies foraddressing downstream social issues has not beenwell established and the practice is still evolving.

Experience in a number of countries has shown thatrecommendations arising from environmental flowassessments are not always implemented. The fea-tures that are likely to lead to successful implemen-tation are summarized in Box 8. These features areextensive; few projects would be able to satisfy allof them. Nevertheless, the box provides a checklist

that a project manager may want to consider whenembarking on the implementation of an environ-mental flows assessment.

Conversely, some of the common reasons for thefailure to implement assessments are:n the perception among engineers and water

managers that “too much” water was requestedn lack of flow-related biological data that can be

used to justify the environmental flows, result-ing in a heavy reliance on expert opinion

n unwillingness or inability to incorporate inno-vative, and possibly more expensive, releasemechanisms into dams for environmental re-leases

n lack of political or legislative pressure to im-plement the environmental flows (usually be-cause other demands were seen as moreimportant)

n “last minute” or post-hoc flow assessments thatare commissioned after most (if not all) themajor decisions about the design and cost ofthe development, and the allocation of water,have already been made

n reluctance to move away from established prac-tices.

A monitoring program is particularly importantgiven the generally poor understanding of thelinks between flow and ecological response.The implementation of an agreed flow regimeshould allow for adaptive management based onthe monitoring. The monitoring program shouldbe designed to provide essential feedback onwhether:n the agreed-upon flow is being releasedn the overall objective (desired river condition)

is being achieved

26


n the objectives for different components of theflow regime are being met

n the environmental flow allocation needs tobe modified in the light of the observed responses.

The monitoring program should be designed to al-low the effects of environmental flows on different

biota to be separated from the effects of other inter-ventions—for example, improved water quality fromsewage treatment plants—and from climatically in-duced variations in river flows. In practice, this is ex-tremely difficult to do, and the interpretation of anymonitoring program will always rely on the experi-ence of the hydrologists and ecologists involved.

Political will,legislation, andmanagementstrategies

Data and tools

Specialistexpertise

Funds

Timemanagement

n Recognition of tangible and intangible national costs of degraded rivers.n Acceptance of flow assessments as a tool for use in integrated river-basin management.n Supporting legislation to empower water managers to manage river flows according to

recommendations.n The necessary tools to implement and enforce legislation.n A structured and transparent decisionmaking process, whereby the results of engineering

and economic studies, environmental flow assessments, and stakeholder input are jointlyused to decide on future flow allocations and river condition.

n Ethical, moral, and other intangible considerations form important inputs to the finaldecisionmaking process.

n Commitment of politicians, developers, and water resource managers to adhere toagreed-upon environmental flow objectives.

n Long-term accurate hydrological data.n Hydrological models with daily time-steps.n Linked surface and groundwater models for intermittent rivers.n Long-term water chemistry records for rivers (and groundwater, where necessary), preferably

linked to hydrographs.n Appropriate flow assessment methodologies.n Comprehensive data on the distribution, life histories, and flow-related habitat requirements

of riverine species in the rivers of concern. Similar data for the abiotic aspects of rivers and,where relevant, for estuaries and coastal marine environments. Data on the toleranceranges of riverine biota to physical and chemical variables.

n A well-structured link between river and estuary flow assessments where appropriate.

n Senior specialists, with first-hand knowledge of the rivers of concern, in the flow-relatedaspects of the following disciplines: hydrology, geohydrology, hydraulics, geomorphology,sedimentology, water chemistry, biotic integrity, physical habitat, riparian and instreamvegetation, fish, invertebrates, and possibly herpetofauna and terrestrial wildlife.

n If socioeconomic aspects are to be included in the assessment, specialists in the followingdisciplines may be required: sociology, human geography, anthropology, public health,domestic-stock health, resource and project economics, and public participation proce-dures. Also required are specialists with knowledge of the flow-related aspects of waterbornediseases, and those of parasites and/or their hosts.

n Recognition that ecological and socioeconomic aspects of water resource developmentare as important as engineering and direct economic aspects.

n Sufficient planning to provide adequate funds for flow assessments.

n Suitable planning horizons for flow-related investigations

BOX 8.DESIRABLE FEATURES FOR A SUCCESSFUL ENVIRONMENTAL FLOWS IMPLEMENTATION.

27


CONCLUSION

Provision for environmental flows is central tointegrated water resources management. EFAmethods are still evolving and World Bank ex-perience in addressing downstream biophysicaland social impacts is limited, but developing. Arecent review of the impact of dams on ecosystemfunction for the World Commission on Damsconcluded that there were four principle ap-proaches to limiting the impacts of dams on natu-ral resources: avoidance; mitigation; compensation;and restoration.

Successful mitigation, compensation, and restora-tion of downstream effects are more likely if a thor-ough flow assessment has been undertaken.

This Technical Note has outlined the principlesbehind environmental flow assessments, provideda description of the methods that have been used toassist with such assessments, and highlighted the

features that will enhance the chance of successfulimplementation of environmental flows.

Although the theory has developed rapidly in thelast three decades, the practical application of en-vironmental flows has been retarded by a lack ofdata and understanding of hydrology-ecology link-ages; a lack of specialists in developing countries;a lack of legislative support; and a reluctance onthe part of water resource developers, designers,builders, and operators to move away from pastpractices. Provision of water for the environmentwill bring with it legal challenges from other po-tential users of the water, yet the scientific knowl-edge base needed to defend environmental flowsagainst such challenges remains poor. Much of thisis changing, however, and flow assessments arebecoming integrated with other tools such as EAand water allocation planning for guiding decisionson water resource developments.

28


FURTHER INFORMATION

Tharme, R.E. 19 96. “Review of the international meth-odologies for the quantification of the instreamflow requirements of riv ers.” Water law reviewfinal report for policy de velopment for the D e-partment of W ater Affairs and F orestry, P retoria.Cape Town: Freshwater Research Unit, Univ er-sity of C ape Town, Cape Town, South A frica.

Information on specific techniques described in thisTechnical Note can be obtained from:

Collings, M.R., R.W . Smith, and G.T . Higgins. 19 72. “Thehydrology of four streams in w estern Washing-ton as related to se veral Pacific salmon species. ”US Geological Service W ater Paper 1968. (Wet-ted-perimeter method)

King, J.M. and D. L ouw. 1998. “Instream flo w assessmentsfor regulated riv ers in South A frica using theBuilding Block Methodology .” Aquatic EcosystemsHealth and Mangement 1:109-124.

Stalnaker, C., B.L. L amb, J. Henriksen, K. Bo vee, and J.Bartholo w. 1995. “The Instream F low Incremen-tal Methodology: A primer for I FIM.” BiologicalReport 2 9, March 19 95. Reston, V A: US Depart-ment of the Interior, National Biological Service.

Swales, S . and J. H. Harris. 19 95. “The Expert P anelAssessment Method (E PAM): A new tool forDetermining En vironmental F lows in RegulatedRivers.” In Harper, D.M. and A.J.D. F erguson,eds. The Ecolog ical Basis for River Management .Chichester, U K: John W iley and Sons, pp 12 5-134.

Tennant, D.L. 19 76. “Instream flo w regimens for fish,wildlife, recreation and related en vironmentalresources.” Fisheries 1(4): 6-10.

The following reports provide contextual informa-tion on environmental flows :

Bergkamp, G., M. McC artney P. Dugan, J. McNeely and M.Acreman. 2 000. “Dams, ecosy stem functions andenvironmental restoration. ” Contributing paper toWorld Commission on D ams Thematic Re view.Environmental Issues I I. New York: UNEP. Down-loadable from www.dams.org or on C D fromEarthscan, L ondon, www.earthscan.co.uk

Bizer, J.R. (2 000). “International mechanisms for a void-ing, mitigating and compensating the impacts oflarge dams on aquatic and related ecosy stemsand species.” Submission E NV249 to World Com-mission on D ams. Ne w York: UNEP. Down-loadable from www.dams.org or on C D fromEarthscan, L ondon, www.earthscan.co.uk

Harper, D.M., and A.J.D. F erguson, eds. 19 95. The Eco-log ical Basis for River Management . Chichester,

UK: John W iley and Sons.

The following three documents provide compara-tive descriptions of environmental flow techniques:

Arthington, A. H., and J.M. Z alucki, eds. 19 98. “Compara-tive Evaluation of En vironmental F low Assess-ment T echniques: Re vie w of Methods. ”Occasional P aper No 27/98. Canberra, Australia:Land and Water Resources Research and D evel-opment C orporation.

Dunbar, M.J., A. Gustard, M.C.A creman, and C.R.N.Elliott.1998. Overseas Approac hes to Setting River Flo wObjectives . R&D Technical Report W6B(9 6)4.Wallingford: Institute of H ydrology.

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