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CLIMATE CHANGE AND WATER RESOURCES IN BRITAIN

NIGEL W. ARNELLDepartment of Geography, University of Southampton, Highfield, Southampton, SO 17 1BJ, U.K.

Abstract. This paper explores the potential implications of climate change for the use and managementof water resources in Britain. It is based on a review of simulations of changes in river flows,groundwater recharge and river water quality. These simulations imply, under feasible climate changescenarios, that annual, winter and summer runoff will decrease in southern Britain, groundwaterrecharge will be reduced and that water quality – as characterised by nitrate concentrations anddissolved oxygen contents – will deteriorate. In northern Britain, river flows are likely to increasethroughout the year, particularly in winter. Climate change may lead to increased demands for water,over and above that increase which is forecast for non-climatic reasons, primarily due to increased usefor garden watering. These increased pressures on the water resource base will impact not only uponthe reliability of water supplies, but also upon navigation, aquatic ecosystems, recreation and powergeneration, and will have implications for water quality management. Flood risk is likely to increase,implying a reduction in standards of flood protection. The paper discusses adaptation options.

1. Introduction

Droughts and floods in recent years in many countries have stimulated furtherinterest in global warming and its possible effects on water resources. The SecondAssessment of the Intergovernmental Panel on Climate Change (IPCC, 1996) statesthat a human influence on global climate is discernible, and that recent variabilityis unlikely to be entirely due to natural causes. An increasing concentration ofgreenhouse gases in the atmosphere is likely to lead to an increase in globalaverage temperature of between 0.15 and 0.3�C per decade (IPCC, 1996), withregionally-variable effects on precipitation and evaporation rates.

This paper provides an overview of the potential implications of climate changefor water resources in Britain. It is partly based on a desk study conducted for theNational Rivers Authority (Arnell et al., 1994), supplemented with more recentsimulations and analysis undertaken as a contribution to the U.K. Climate ChangeImpacts Review Group (CCIRG, 1996; Arnell, 1996). The paper outlines possiblechanges in the resource base, and then considers the implications for the manage-ment of water resources in different sectors. First, however, it is necessary to makesome general points about climate change and water resources, and outline thestructure of water management in Britain.

1.1. THE CONTEXT: CLIMATE CHANGE AND WATER RESOURCES

Climate change has a physicaleffecton water quantity and quality. This effect mayhave animpacton water resources and their management: an impact can be seen

Climatic Change39: 83–110, 1998.c 1998Kluwer Academic Publishers. Printed in the Netherlands.

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as an effect with value. The translation from effect to impact is not necessarilylinear or simple. In some circumstances, a large physical effect can have a verysmall impact – where there is currently plenty of excess slack in the managementsystem, for example – but in other cases a very small effect can have a significantimpact – where, for example, the management system is already under extremepressure. The characteristics of the water resource infrastructure influence the wayan effect is converted into impact. A supply system based on a large number ofsmall, independent reservoirs will be more sensitive to change than one which hasjust one large reservoir with the same total capacity. There may also be criticallevels within the water management system, beyond which change cannot beaccomodated by adaptation within the system (Parry et al., 1996). It may be possibleto cope with a certain amount of change in water quantity or quality within existingoperating practices, but with some greater change it may be necessary to upgradeinfrastructure or introduce alternative practices. The critical levels within a systemare determined by the tolerance of that system to change (Parry et al., 1996), whichdepends on how the system is configured and managed: different systems will havedifferent tolerances.

Virtually all studies into the impact of climate change on water resources haveassessed the implications offuture climate for thecurrent water managementsystem (Kaczmarek, 1996). This can be seen as a worst-case approach to climateimpact assessment, as in practice the water management system will have evolvedand adapted by the time the future climate arrives. Some of this evolution willbe in response to non-climatic trends. These include changes in population andthe demand for water, changes in the legislative framework (including nationallaws and international – for example European Union – policies), and changes inpublic and professional attitudes to water resources and their management. Suchchanges may mitigate – incidentally – the consequences of climate change, ormay exacerbate them. Water management agencies in many countries are movingtowards flexible, robust water management involving a mix of supply-side anddemand-side measures, and such a change is likely to lessen the future impact ofclimate change because it results in more adaptable strategies. Over the next fewdecades, however, water managers may also make some deliberate adaptations to achanging climate. These will probably be in response to extreme events, and wouldbe based on uncertain forecasts of future climate: the decisions may not be, withhindsight, optimal. The future impact of climate change therefore depends on theevolution of water management over time, and the adaptive actions that are takenby water managers, and the cost of climate change over the next few decades willtherefore be equal to the additional cost of adaptation plus the cost of impacts thatcannot be mitigated (Arnell and Dubourg, 1995). These impacts and costs will bevery difficult to assess.

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1.2. WATER MANAGEMENT IN BRITAIN

Water supplies to domestic, municipal and many industrial consumers in Englandand Wales are provided by a number of regional and local private-sector waterutilities, and the ten largest regional utilities also treat sewage effluent. These util-ities have two regulators. The Office of Water Services (OFWAT) regulates thebusiness of water supply and treatment, including services to consumers, pricesand dividends to shareholders. The National Rivers Authority (NRA) – since April1996 the Environment Agency – issues licences to allow both abstractions from anddischarges to surface and groundwater sources, and has a general duty to managewater resources. It is also responsible for the maintenance of water quality throughpollution control and consents to discharge effluent, and has powers to undertakeworks to prevent coastal and riverine flooding. To these three major functions areadded duties to maintain and enhance fisheries, recreation, conservation and nav-igation. In the most general sense, the Environment Agency seeks to manage theriver environment, reconciling competing demands. Britain’s canal network andsome of its navigable rivers are managed by British Waterways, a public-sectoragency. Electrical power is largely generated by a small number of large private-sector companies. Most generate electricity from fossil fuels, but two companiesgenerate electricity by nuclear power, and one – in Scotland – generates a substan-tial amount of hydroelectric power. In total, hydropower provides just 2% of theelectricity generated in Britain (DTI, 1995), and virtually all of this comes fromlarge-scale hydropower plants in Scotland and Wales. A small proportion comesfrom a number of small-scale hydropower plants throughout Britain.

In Scotland, water supplies have since April 1996 been provided by threeregional public water authorities: previously water was supplied to consumersby regional councils. The Scottish Environmental Protection Agency (SEPA) wascreated in April 1996 from regional River Purification Boards and other agencieswith environmental interests, and has a similar role in the water sector as theEnvironment Agency for England and Wales.

Water managers in Britain are also affected by directives from the EuropeanCommission. To date, these directives have exclusively concerned the quality ofdrinking and bathing water, and water utilities have had to invest significantly toensure river and coastal water quality meets European standards.

2. Changes in the Resource Base

This section outlines possible changes in the water resource base in Britain. Itlargely draws on a number of inter-connected simulations of the effect of climatechange on river flows (Arnell and Reynard, 1993, 1996; Arnell, 1996). These studiesfollowed the conventional linear impact assessment approach of perturbing theclimatic inputs to the hydrological system according to climate change scenarios,

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and simulating changes in river flows using calibrated catchment hydrologicalmodels (Arnell et al., 1996). Details of the hydrological model and the scenariosused are provided in Arnell and Reynard (1996) and Arnell (1996): this sectionsimply summarises the results, focusing on one climate change scenario.

In the absence of climate change, the quantity and quality of water will beaffected by changes in land use (such as urbanisation, agricultural change and thenew National Forest in the English Midlands). Available resources will also varyfrom year to year and decade to decade because of natural climatic variability.Other factors which affect the resource base – including the demand for water andthe pressures on water managers – will also evolve over the next few years forreasons other than climate change.

2.1. CHANGES IN STREAMFLOW AND GROUNDWATER RECHARGE

Figure 1 shows the change by the 2050s across Britain in average annual, winter(December to February) and summer (June to August) precipitation, together withchange in annual potential evaporation, under the scenario developed for the U.K.Climate Change Impacts Review Group (CCIRG, 1996; Hulme, 1996) from theHadley Centre transient climate change experiment (Murphy, 1995). The climatemodel output has been interpolated down to a grid resolution of 0:5� � 0:5�.Annual average rainfall increases under the scenario across the whole of Britain,but rainfall in summer decreases in the south and east. The change in potentialevaporation was calculated from the change in temperature, humidity, net radiationand windspeed, using the Penman formula. In southern Britain, annual potentialevaporation increases by up to 30%, whilst in the far north west a large increasein relative humidity results in a slight decrease in potential evaporation. Figure 2shows change in average annual runoff (30-year average) under this scenario,determined using a daily hydrological model applied independently to each gridsquare (Arnell, 1996).

According to the scenario shown in Figure 1, average annual runoff woulddecrease in the south and east of Britain – by up to 20% – and increase in the northand west. In the south and east, the increase in potential evaporation offsets thegeneral increase in rainfall (the change by the 2020s would be approximately halfthat by the 2050s). Other scenarios (Arnell and Reynard, 1996; Arnell et al., 1997)show a range of responses, with increases in runoff across the whole of Britainunder a ‘wet’ extreme, and a reduction across the whole of Britain under a ‘dry’extreme. Snowfall and snowmelt are not major components of the water balance inBritain, and under all the scenarios considered would be reduced considerably.

The change in annual average runoff, however, is not evenly distributed throughthe year. Figure 3 shows the average monthly runoff under the current climate andthe CCIRG scenario for the 2050s for six catchments distributed around Britain(Arnell, 1996: this time the hydrological model is applied with catchment data).There is a substantial decline in runoff during summer, by up to 50%, in the


Figure 1. Percentage change in average winter rainfall, summer rainfall, annual rainfall, and annualpotential evaporation, under the 1996 CCIRG scenario (CCIRG, 1996), by the 2050s.

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Figure 2. Percentage change in average annual runoff across Britain, under the 1996 CCIRG scenario(CCIRG, 1996).

catchments in the south and east, and little or no change during winter. Furthernorth, runoff tends to increase in every month, but seasonality is still increased:a greater proportion of runoff occurs during winter. Again, different scenariosgive different changes, but most suggest an increase in seasonality of flow, withreductions during summer (Arnell and Reynard, 1996; Arnell et al., 1997).

Low flows in southern catchments show an even greater percentage decreasethan monthly average flows. Under the CCIRG scenario, the flow exceeded 95%of the time would be reduced by over 50% by 2050 in many catchments: thisflow is frequently used to define licences to abstract water or discharge effluent.


Figure 3. Average monthly runoff in six British catchments, under the current climate and 1996CCIRG scenario (CCIRG, 1996).

Figure 4 shows flow duration curves for two study catchments. In the Don (northeastScotland), the flow duration curve would be little affected by climate change. In theHarper’s Brook (eastern England), however, low flows are considerably reduced.

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Figure 4. Flow duration curves for two British catchments, under the current climate and the 1996CCIRG scenario (Arnell, 1996).


Groundwater resources in Britain are recharged during winter, once soil moisturedeficits have been filled and before they begin to develop again in spring. Anincrease in winter rainfall would appear to increase groundwater recharge, but thehigher evaporation reduces the length of the recharge season. The effect of thereduced recharge season is likely to outweigh the increased winter rainfall – atleast under the CCIRG scenario – leading to a reduction in groundwater recharge,particularly in southern Britain (Arnell, 1996; Cooper et al., 1995).

2.2. CHANGES IN WATER QUALITY

Water quality describes the chemical characteristics of water, as well as physicalproperties such as temperature, colour and sediment concentration. In Britain, itis convenient to distinguish between upland and lowland systems (Jenkins et al.,1993). Upland systems tend to be fast flowing and oligotrophic, with high dissolvedoxygen and generally high quality. Lowland systems are slower flowing, with lowerturbulence, and hence lower dissolved oxygen. They also tend to be more heavilyutilised for agriculture and have larger urban populations, so have high nutrientconcentrations.

A change in climate will affect streamwater quality in Britain in five ways. A risein water temperature will affect the rate of operation of biogeochemical processeswhich determine water quality. Changes in flow volumes will alter residence timesand dilution. Increased atmospheric CO2 will affect the rate at which CO2 isdissolved in water, and hence the rate of operation of many processes, and a changein soil properties and flow pathways will alter the transport of chemical load to theriver. Finally, changes in inputs of chemicals to the catchment – perhaps due to theeffects of climate change on agriculture – will alter streamwater chemistry.

The temperature of river water in Britain is largely determined by air temper-ature, although groundwater is cooler than direct runoff and water temperatureis lower in shady streams. Webb (1992) used empirical relationships between airand water temperature to estimate the effects of global warming on river watertemperature in Britain. With the increase in temperature as assumed under theCCIRG scenario, water temperature would rise by the 2050s by around 1.6�C insummer and 1.8�C in winter in southeast England, with lesser increases to thenorth and west (there would also be lower increases in catchments dominated bygroundwater, and greater increases in small sensitive headwater streams). Such arise in temperature would increase substantially the frequency of high tempera-ture extremes, reduce the number of low temperature extremes, and alter thermalhabitats within streams.

Chemical reactions in water between species depend on water temperature,acidity and the concentration of particular species and ions. Higher temperaturesgenerally mean that processes operate more rapidly, but different processes aresensitive in different ways. Denitrification, for example, would be more affected bya rise in temperature than the rate of nitrification, so nitrate concentrations should

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fall (Jenkins et al., 1993). However, this might be offset by reduced dilution due tolower flow volumes, and a rise in temperature could increase the mineralisation oforganic nitrogen in soil, leading to increased inputs to the river channel.

Dissolved oxygen concentrations are influenced by biogeochemical oxygendemand (BOD) and water temperature. Warmer water can hold less oxygen, soglobal warming would lead to lower dissolved oxygen contents. The rate of deox-idation is also more sensitive to temperature than the rate of reaeration, enhancingthe effects of a rise in water temperature. Jenkins et al. (1993) conducted simulationstudies in several British rivers, which showed that rivers currently receiving largeamounts of effluent – with high BOD – would experience the greatest reduction indissolved oxygen.

Algal blooms can make a major contribution to BOD, and also have healthand aesthetic impacts. Higher temperatures might encourage blooms to form, andincreased residence times due to lower river flows would also stimulate bloomgrowth.

Streamwater quality in Britain is heavily influenced by the use of water, andparticularly by the discharge of sewage effluents and pollution from agriculturalchemicals. Actions taken by the water utilities and the Environment Agency totreat effluent and minimise pollution should improve water quality over time, andwill lessen sensitivity to climate change.

2.3. CHANGES IN THE DEMAND FOR WATER

In 1994, the National Rivers Authority forecast an increase in demand for publicwater supplies in England and Wales by 2021 ranging between 2 and 25% (NRA,1994). This increase would vary regionally, with the greatest increases in the southand east, and possibly small declines in parts of the north. The increase in demandis due largely to an increasing concentration of population in the south and east,decreasing household size, increased use of domestic appliances, and, particularly,an increasing usage of water in the garden. Large-scale industrial use is predictedto decline.

Climate change can be expected to add to this increasing demand. Herrington(1996) explored the potential effect of climate change on demands in southeastEngland, and concluded that per capita domestic demand would rise by an extra5% by 2021, over that forecast by the NRA, due to climate change (a temperatureincrease of just over 1�C). This increase would be partly due to increased show-ering, but largely due to an increase in garden watering. Peak demands would alsobe increased, and Herrington (1996) concluded that the peak 7-day ratio (the ratioof the maximum 7-day demand to the average 7-day demand) would rise frombetween 1.16 and 1.27 to between 1.32 and 1.52 by 2021. The hot, dry summerof 1995 illustrated the effect of warm temperatures on peak and garden demands:many supply companies reported record peak demands (OFWAT, 1996). Estimatesof future demand are, of course, highly dependent on policies which may be intro-


duced to curb increases or encourage efficiency of use. New standards for applianceefficiency would alter future demands, but more important will be the increaseduse of meters and variable tariff structures to charge for domestic water supplies.At present, the vast majority of domestic customers in Britain pay a flat rate forwater, based on property value. The effects of increased metering on demand willdepend significantly on the tariff structure.

The other area of demand most sensitive to climate change is irrigation. Atpresent, spray irrigation is largely done in order to ensure high quality horticuluralcrops. Between 1991 and 2021 the demand for spray irrigation water is predictedto rise by 69% (Weatherhead et al., 1993), and with climate change the increasemight be 115% (Herrington, 1996): this increase is largely concentrated in the eastand south. However, demands for irrigation water will be affected by the cropsgrown by farmers, and by the price of both crops and irrigation water.

Finally, around 23% of the water put into the public supply system is currentlylost through leakage (OFWAT, 1996). In southeast England, the amount currentlylost in leakage is around eight times the increase in demand anticipated due toclimate change (230 Mld�1). Efforts to reduce leakage will therefore have a veryimportant effect on the impact of climate change, and will reduce vulnerability tochange.

2.4. CHANGES IN WATER MANAGEMENT

Over the next few decades, the legislative and political context of water managementin Britain will change. In recent years, there have been three major drivers ofchange in water management practices. The first two are connected, and relate tothe privitization of water supply and treatment in 1989. Private-sector utilities nowprovide and treat water, and since 1996 a number of mergers have taken placecreating combined energy and water utilities. These utilities serve customers andare regulated by public bodies, but they are also responsive to the demands ofshareholders. There is widespread anecdotal evidence that customers are much lessforgiving of shortcomings in the private-sector utilities than they were of problemswhen water was supplied by public bodies. This was particularly apparent duringthe drought of 1995, when some private-sector utilities were heavily, and publicly,criticised. Public expectations of water management are therefore changing.

Parallel with the creation of the private-sector utilities has been the establishmentof a strong public-sector agency – initially the National Rivers Authority and subse-quently the Environment Agency – which has explicitly adopted an environmentalagenda. The Environment Agency sees itself as a protector of the environment (themotto of its predecessor, the National Rivers Authority, was ‘Guardians of the waterenvironment’), and since 1995 has been able to apply for emergency restrictionsduring droughts in order to protect the aquatic environment.

The third driver of change is the European Union, which since the early 1980shas issued a number of legally-binding directives through the European Com-

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mission. These directives have so far focused on water quality, but the proposeddirective on the Ecological Quality of Water (European Commission, 1994) placesa duty on water managers to maintain and enhance aquatic habitats. The proposednew European Commission Framework Directive on Water Resources (EuropeanCommission, 1996) will bring together all existing directives concerning water,with a focus on sustainable water resources development and an increasing con-cern with the management of water quantity – particularly the management ofsupplies during droughts.

3. Impact on Water Resources and their Management

Changes in the water resource base would affect many sectors of the Britisheconomy, and adaptations taken in one area will affect impacts in another. Thissection explores the principal implications of climate change for water resourcesand their management, focusing on individual water management sectors: linksbetween sectors will be emphasised, and the final part of the section looks atcompetition and conflict between sectors. The section is organised around theseven operational NRA functions (with, as in the NRA, the four smallest functionstreated together), with the addition of power generation. The Environment Agencyhas a slightly different structure, but has the same operational responsibilities andpowers in the water sector.

3.1. WATER SUPPLY

The private-sector utilities and Scottish public water authorities supply water todomestic, municipal and commercial consumers, small industry and agriculture.Large industrial enterprises tend to abstract their own water, and farmers themselvesdraw water for irrigation directly from groundwater or rivers. All abstractions mustbe licenced by the Environment Agency, and historically most of these licences areindefinite. In England and Wales as a whole, approximately a third of supplies aretaken from groundwater, a third directly from reservoirs, and a third from rivers:some of these rivers are regulated by upstream reservoirs. The importance of eachsource varies regionally, however, and in parts of the south and east most wateris taken from chalk aquifers. The private-sector utilities also treat effluent, anddischarge it into water courses. These discharges must also be licenced by theEnvironment Agency, which imposes quantity and quality constraints.

The impacts of climate change on water supply will be felt through impacts onresources, on the treatment of raw water and on its distribution to customer, and onthe treatment of sewage effluent: this latter area is considered under the heading ofwater quality management.


3.1.1. Implications for ResourcesThe major potential effect of climate change on resources lies in changes in thereliability of individual sources and the network of sources. Studies into the sen-sitivity of hypothetical supply systems (generally single reservoirs) have shownthat sensitivity to change increases as yield increases as a proportion of averageflow, and as storage decreases as a proportion of annual flow (Cole et al., 1991;Arnell, 1996). Small, unconnected supply sources will be much more sensitive to achange in climate than a system based on inter-connected sources, or on one largereservoir. Direct river abstractions, with no upstream storage, will be most affected.

Under the 1996 CCIRG scenario, river flows in southern and eastern Englandwould be reduced during summer, with clear implications for the reliability ofabstractions direct from the river. The effects on reservoir reliability, however, willdepend on reservoir capacity, and whether additional winter flows can be storedto sustain yields through drier springs and summers. Although no simulationshave been conducted using real reservoirs and realistic operating rules, it is likelythat small reservoirs in southern and eastern Britain would experience a reductionin reliability: in other words, lower yields could be extracted for the same riskof failure. In northern Britain, reservoir yields might increase, as flows increasethrough much of the year. The possible reduction in groundwater recharge wouldreduce groundwater yields in southern England, compounding resource problems.

Sea level rise has two potential effects on water resources in Britain. First, arise in sea level would increase saline intrusion into aquifers. Many aquifers arepotentially at risk from sea level rise, but most of the 29 aquifer units at risk arealong the south coast. Clark et al. (1992), however, showed that the effects of sealevel rise on yield and hence reliability would be minor: a rise in sea level of 0.6 m(above current IPCC projections) would lead to a reduction in yield of coastalboreholes of only between 1 and 2%, and yields could easily be maintained bymoving boreholes further inland. The cost of relocation would be small relative toannual maintenance and operational costs (Clark et al., 1992).

The second potential effect of sea level rise is on freshwater intakes locatedclose to the current tidal limit. Studies have shown, however, that the effect of a sealevel rise would be small relative to the difference between spring and neap tides,or high and low tides (Clark et al., 1992; Dearnaley and Waller, 1993), and that fewintakes would be at risk. Lower freshwater inflows would exacerbate problems in afew estuaries (Dearnaley and Waller, 1993), but the risk posed by saline intrusionalong estuaries is not great.

The Environment Agency, the water supply utilities and those who extractwater directly for themselves have a range of adaptation options, which may besummarised hierarchically.Operational adaptationsare changes to day-to-day orshort-term management practices. An example in the water supply utilities wouldbe a change to the operating rules for individual sources. On the regulatory side, theEnvironment Agency could introduce time-limited licences, or licences which canbe easily amended.Institutional adaptationsare changes in the way the balance

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between water supply and demand are managed. The most obvious example isa shift from supply-side management (building reservoirs) to demand-side man-agement (curbing increases in demand and cutting leakage), and this tendencyis already apparent in British water management. The Environment Agency cur-rently looks unfavourably on proposals for new resources whilst leakage remainsat present levels. Climate change gives an extra incentive for such a shift.Localinfrastructureadaptationsare small-scale alterations to the water supply infrastruc-ture, at the scale of the water supply utility. For example, Yorkshire Water increasedthe interconnections between its small supply reservoirs in south and west York-shire, following the 1995 drought. Leakage control is a local infrastructural adap-tation. Another local adaptation – which is also being increasingly recommendedby the Environment Agency – would be for farmers to develop off-line irrigationponds which are filled during winter and can be used during summer.Strategicinfrastructure adaptationsare at the regional or national scale. In 1994 the NRAconducted a review of possible new strategic resource developments, which maybe necessary to meet increasing demand (Figure 5). Most of the strategic supplyoptions involve the transfer of water between basins, although some involve reser-voir development. Options excluded – at the strategic scale – included desalination,bulk shipments by tanker and effluent re-use. Each strategic option is expensive,and has environmental implications and a political dimension. The strategic optionsidentified in the NRA review were not formulated with climate change in mind,and may or may not be sufficient in themselves to meet the gap between supply anddemand – and indeed may not be practical under some climate change scenarios.However, they do provide a starting point for a review of strategies for coping withclimate change.

There are two other actors with the potential to influence the future balancebetween supply and demand. The first is the consumer, who may be persuadedon environmental, rather than just financial, grounds to save water. The secondinfluence is through the land use planning system. At present, availability of waterresources (and capacity to treat waste) is not seen as a constraint on regionalplanning and the location of new development. However, with a forecast increaseof 4.4 million households in England by 2016 (Department of the Environment,1996a), mostly in parts of the country where resource pressures are high and arelikely to increase, it is probable that planners will begin to see water resourcesas an influence on the amount and location of development. Some water supplycompanies in southern England are already calling for curbs on new developmentin their supply areas, following problems experienced in meeting demands duringthe drought of 1995.

3.1.2. Implications for DistributionWater is treated, then delivered to customers through distribution networks ofservice reservoirs and pipes. The capacity of water treatment works is controlledby peak weekly demands, whilst the characteristics of the distribution network


Figure 5. Strategic resource options identified by the National Rivers Authority (NRA, 1994).

is determined by peak daily demands. Many of the restrictions on use faced byconsumers are due to the inability of the treatment and distribution network tocope with peak demands: during the drought of 1995, for example, the distributionnetwork in some regions was insufficient to deliver the quantity of water demandedby consumers at peak times.

An increase in background demands, together with an increase in peak demandratios, would place the treatment and distribution network under increasing strain.In less extreme circumstances, consumers would experience a reduction in water

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pressure during peak periods. Under more extreme conditions, temporary restric-tions on the use of garden hosepipes and sprinklers may be imposed. The regulator,OFWAT, currently allows water utilities to impose hosepipe bans on average oncein ten years. In order to cope with increasing pressures on the distribution network,and to maintain a target hosepipe ban frequency of once in ten years, water utilitieswould need either to invest to upgrade the distribution network – which may bevery expensive – or seek to curb rising demands, perhaps through metering and dif-ferential pricing tariffs. Several water supply companies have since 1995 adoptedpolicies of avoiding entirely restrictions on customer use, necessitating still furtherinvestment. A reduction in leakage would also serve to offset to a certain extent theeffects of climate change.

3.2. WATER QUALITY MANAGEMENT

The Environment Agency in England and Wales, and the Scottish EnvironmentalProtection Agency in Scotland, have the responsibility to maintain and enhance thequality of river, lake and groundwater. They do this both by regulating activitieswhich discharge effluent into water, and by responding to pollution emergencies.The main areas of concern with respect to climate change are the maintenanceof water quality through discharge consents, the management of algal blooms,minimising storm sewer overflows and managing pollution incidents (Arnell etal., 1994): possible changes in public health risks were also identified in the 1996CCIRG report.

3.2.1. Discharge Consents and Statutory Water Quality ObjectivesThe ten regional water utilities in England and Wales not only supply water toconsumers, but also treat sewage and discharge the treated effluent to water courses.Industrial users also return effluent to water courses. The Environment Agencyissues licences to discharge effluent, which specify the quantity and quality ofeffluent which may be discharged. These constraints are based on the flow andquality of the receiving waters. If climate change were to lead to a reduction indissolved oxygen content, for example, or flows in the receiving river were tobe reduced, then less effluent could be discharged to the river if current waterquality is to be maintained. This would lead to a reduction in the capacity of theexisting treatment works (and increased demand implies an increased need forsewage treatment). However, higher temperatures would mean that the biologicalprocesses involved in sewage treatment would accelerate, and treatment efficiencywould increase.

Since the early 1990s, the NRA and the Environment Agency have been settingwater quality objectives for individual river reaches. These Statutory Water QualityObjectives (SWQOs), expressed in terms of indicators such as dissolved oxygencontent, are determined for an individual reach on the basis of reach characteristicsand flow regime, together with an assessment of the water quality that the reach can


reasonably achieve. Discharge consents are increasingly being defined to ensurethat river reaches meet or maintain these SWQOs. Climate change can affect boththe feasible SWQO for a river reach – by changing flow volumes or chemicalconcentrations, for example – and can also affect the effluent discharges which canbe sustained without breaching the SWQO. If discharge consents are expressed interms of the quality of thereceivingwater, then the organisation discharging intothe water course may need, because of climate change, to improve the quality ofeffluent: the burden of coping with climate change falls on the discharger. If thedischarge consents are expressed in terms of the quality and volume ofeffluent,however, then it will be up to the Environment Agency to seek a variation to theconsent in order to maintain reach water quality. The uncertainty posed by climatechange implies that the most efficient management of water quality uses consentsexpressed in terms of the quality of the receiving water.

The Environment Agency is also defining an increasing number of source pro-tection zones, largely to protect groundwater abstraction points from becomingcontaminated by pollution from agricultural and other chemicals. Climate changeis unlikely to affect dramatically the definition of protection zones, except insofaras changes in recharge may affect catchment areas around a borehole.

3.2.2. Algal BloomsThe frequency of outbreak of blue-green algae in lakes, reservoirs and lowlandrivers is likely to increase with climate change. Not only are such blooms poten-tially toxic and aesthetically and ecologically damaging, but they can also clogwater intakes. Actions currently being taken to manage algal blooms by strippingphosphates from sewage effluent will help mitigate the effects of climate change.

A number of the strategic resource development options identified by the NRAinvolve the transfer of water from the lower reaches of one river to the headwatersof another. One of the most important ecological implications of such transfers isthe mixing of waters from different sources, and the increase in bloom-formingpotential in the river receiving water, possibly rich in phosphates. Climate changemight make this potential greater, and needs to be considered when evaluating thecosts and benefits of inter-basin transfer.

3.2.3. Sewer OverflowsUrban storm sewers are designed to remove storm runoff from urban surfaces. Inold urban areas the storm sewerage system is combined with the foul seweragesystem, and storm flows go through sewage treatment works. However, if suchcombined systems are surcharged, then untreated sewage is discharged into thereceiving water course along with the storm drainage. Urban storm drainage is alsooften extremely dirty, and contains heavy metals and hydrocarbon-basedpollutants.Finally, solids are deposited in the sewerage system during long dry spells, andare subsequently flushed out by heavy rainfall: the receiving water will be rapidlyde-oxygenated, possibly leading to fish kills.

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Changes in the frequency of storm sewer overflows and surcharges, and hencepollution, are dependent on changes in short-intensity rainfall characteristics, andthese are currently very difficult to define. However, under the 1996 CCIRG sce-nario, it is assumed that intense rainfalls will become more frequent, which willincrease the frequency of incidents due to sewer overflow.

It is difficult and expensive to upgrade existing sewerage systems, which areembedded in the urban infrastructure. One possibility is to install stormwaterdetention tanks at critical points in the system.

3.2.4. Pollution IncidentsPollution can arise from urban storm sewer overflows, but can also come from acci-dents or spillages (which will not be affected by climate change), and from diffusenon-point sources. The most significant non-point pollution source is agriculture,and particularly pollution from slurry. Higher winter rainfall would increase therisk of pollution from farm slurry, although increasingly rigorous efforts are beingmade to minimise farm-based pollution.

Even if the frequency of pollution incidents were to remain unchanged, thesensitivity of a river system to pollution might be altered by climate change. Lowerflows, for example, would mean less dilution for a pollutant, and if dissolvedoxygen levels are already lower due to climate change, aquatic systems might beeven more vulnerable to increased BOD resulting from pollution. On the otherhand, higher temperatures could mean that self-purification processes mitigate theeffects of pollution more rapidly.

3.2.5. Public Health RisksVirtually all domestic consumers in Britain receive treated water from water supplycompanies, and treatment processes generally protect the public against water-related diseases. However, in recent years there have been a number of outbreaks ofinfection arising from cryptosporidium in public water supplies. Cryptosporidiumin water supplies is associated with runoff from land used for dairy farming.Higher water temperatures may increase the opportunities for cryptosporidiumgrowth, and may increase the potential for the contamination of water sources.Existing procedures, with enhanced monitoring, should be able to cope with anyincreased risk to public water supplies. A greater risk is the increased likelihood ofcontamination of recreational water courses (CCIRG, 1996).

3.3. FLOOD DEFENCE

One of the most high-profile potential impacts of climate change is on floodfrequency and risk. In England and Wales, the Environment Agency has powersto protect communities and land against coastal and riverine flooding, and localcouncils can build flood protection schemes on small streams: in Scotland, theScottish Environmental Protection Agency can protect against flooding.


The major areas of concern are changes in coastal and riverine flood risk, theintegrity of riverine flood defences, development control, urban storm drainage andthe potential implications for the flood insurance industry.

3.3.1. Change in Coastal Flood RiskA rise in sea level, together with a change in the frequency of storms, would havea clear effect on the frequency with which coastal defences are exceeded, and theEnvironment Agency has long been concerned. Indeed, since the early 1990s apolicy has been in force whereby new coastal flood defences can be designed sothat they can be extended in the future,as long as the costs of the scheme are stilllower than the benefits expected from it(Arnell et al., 1994). The EnvironmentAgency has a standard set of assumptions about rates of sea level rise, which mustbe used in planning coastal flood protection works (Arnell et al., 1994).

Raising flood defences, however, is just one way of coping with changes incoastal flood risk, and a more radical solution in some areas would be to abandonland to the sea, and move the defence line further inland (CCIRG, 1996; Burd,1995). The most appropriate solution for each segment of coastline must be based onan assessment of the economic and environmental costs and benefits of protectionor retreat, but important political forces are also at play.

On a much less dramatic note, a large number of lowlying coastal catchmentsin parts of eastern England rely on pumping to keep flood waters out and watertables low and thus permit agriculture. A rise in sea level will affect water levelswithin such catchments and hence affect pumping costs.

3.3.2. Change in Fluvial Flood RiskPossible changes in inland riverine flood risk have received far less attention,largely because it is currently difficult to estimate the effects of climate changeon extreme high flows. Increased winter rainfall implies an increase in winterflooding, whilst less frequent but more intense summer rainfall would suggest apossible increase in the occurrence of extreme floods during summer. The effectof a change in rainfall characteristics on flood frequency will depend on catchmentphysical characteristics. A small, responsive catchment, for example underlain byclay, would be very sensitive to changes in short-term intense rainfall. Floods ona larger, less-responsive catchment are more influenced by the accumulation ofrainfall over a longer period, so would be less sensitive to changes in short-termextremes but more sensitive to changes in accumulations over several days.

Naden et al. (1996) simulated the effects of climate change and land use changeon flood frequencies in the Severn, Trent and Thames, three large British catch-ments. Floods with return periods between 5 and 50 years would increase inmagnitude by between 8 and 11% by the 2050s, under a scenario numericallydifferent, but qualitatively similar, to the 1996 CCIRG scenario. Virtually all ofthe change would be due to the assumed increase in rainfall, and would reflect an

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increase in winter flooding. In all three catchments, the flood with a return periodof 20 years would by the 2050s be exceeded on average once every five years.

The major implication of a change in flood frequencies is that flood protectionworks will be exceeded with a greater frequency. The Environment Agency cur-rently attempts to provide a defined standard of service for each floodplain reach,based on the value of floodplain land and subject to the benefits of flood alleviationexceeding the costs. Class A land (highly urbanised), for example, has a target pro-tection standard of one in 50 years. Climate change might mean that the standardof service provided to a particular floodplain segment falls below the target for thatland use. Adaptation options include accepting a decline in standards (in whichcase the impact of climate change will be the extra flood costs), and maintainingtarget standards (in which case the impact of climate change will equal the costsof enhancing flood protection).

3.3.3. Integrity of Riverine Flood DefencesA change in river flows may also lead to changes in erosion and deposition, andhence in the performance of flood defence schemes. Embankments may be under-mined, or increased deposition during low flow periods may reduce channel capac-ity. Unfortunately, very little is known about possible future changes in erosion,sedimentation and channel stability in Britain (Newson and Lewin, 1991), bothbecause information on possible changes in extreme flows is lacking and there areno robust mechanistic models relating channel change to hydrological conditions.However, historical evidence suggests that river channels in Britain, particularlyin the uplands, have shown changes in response to variations in climate (Hooke,1995), although it is difficult to separate out the effects of land use and upstreamchannel change.

Warmer temperatures would probably lead to increased growth of weeds inwatercourses. In some parts of Britain, particularly in chalk streams, weed growthconsiderably increases the flood risk at certain times of the year, and weeds areregularly cleared by the Environment Agency. If weed growth were to be prolonged,the season of increased blockage would lengthen and may begin to encroach moreregularly on the season of increased river flows. Flood risk would then be increased.

3.3.4. Floodplain Development ControlLocal planning authorities are responsible for planning control in Britain. TheEnvironment Agency and Scottish Environmental Protection Agency are askedfor advice about development in flood-prone areas, and usually advise againstit, but their advice is not necessarily followed by the planning authorities. Agovernment circular (Department of the Environment, 1992) recommends thatplanning authorities generally discourage floodplain development, and remindsplanning authorities of the threat of climate change. Climate change is unlikelyto have an effect on policies or advice, but might affect the size of the designatedfloodplain. In practice, however, most floodplains in Britain are currently defined


on the basis of recorded experience rather than simulations of the extent of floodswith a defined return period.

3.3.5. Urban Storm DrainageUrban storm drainage systems are implemented by developers and local authorities.An increased frequency of extreme rainfalls would increase urban storm flooding,and the frequency with which design standards are exceeded would increase. Manyurban storm drainage systems have long lifetimes, so are difficult to upgrade, andwill be exposed to a changing climate. As with the control of urban pollution,stormwater detention basins may need to be installed at critical points in the stormdrainage network.

3.3.6. Flood InsuranceIn Britain, standard domestic and commercial insurance policies currently includecoverage against flood loss, usually at no extra charge and with no additionalconditions imposed. Increasingly, however, insurance companies are increasingpremiums or excesses on policyholders in known hazard areas.

An increase in flood risk, and hence an increase in flood insurance claims, willclearly have an impact upon the insurance industry. The industry is most concernedabout increases in coastal flood risk (Dlugolecki et al., 1995), because the potentialfor catastrophic loss, affecting thousands of policies, is greater than with riverineflooding. An increasing frequency of damaging events will probably trigger morechange in the insurance industry (whose past record on flood insurance has beendetermined by response to extremes: Arnell et al., 1984), leading perhaps to higherpremiums in risk areas, increased excesses or reduced availability of coverage.

3.4. FISHERIES, RECREATION, CONSERVATION AND NAVIGATION

These four aspects of water management are together less significant than any ofthe preceding three dimensions, but are closely interlinked and indeed are relatedclosely to water supply, water quality and flood defence.

3.4.1. FisheriesA change in water temperature and flow regime will affect instream habitats andinfluence some aspects of fish physiology. Most British fish are well within theirthermal limits, so a change in water temperature is unlikely to have significanteffects. Exceptions are the native brown trout (salmo trutta) and the grayling (thy-mallus thymallus), which are both limited by high temperatures, and two cold-waterlake fish, the whitefish (coregonus lavaretus) and the charr (salvelinus alpinus)found in some northern lakes. Changes in river flow regimes are likely to havegreater effects on fish habitats than increased water temperatures. A change in thefrequency of drought is particularly important, and altered seasonal flow regimescould affect migration patterns (Arnell et al., 1994). Higher temperatures, how-ever, could mean an extension in the distribution of some exotic species, such

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as the zander (stizostedion lucroperca), which are currently constrained by lowtemperatures.

There are several possibilities for adaptation. Although river temperatures can-not readily be managed, it is possible in principle to maintain river levels in orderto sustain particularly important habitats. Also, it may be feasible by selectivebreeding to develop strains more tolerant to higher temperatures.

3.4.2. RecreationAlthough water managers seek to maintain aquatic ecosystems for environmentalreasons, much of the incentive and most of the funding comes from recreationalfishing. Fishing is Britain’s most common participatory sport, and funds fromfishing licences support Environment Agency fisheries work. A change in fishstocks will have knock-on effects on fishing and hence the recreational benefit ofa stretch of water. The brown trout is a particularly important sport fish in Britain,and trout fishing rights along some chalk streams in southern England are highlyprized and very expensive: a reduction in trout stocks could therefore have localisedeconomic effects.

Fishing is not the only recreational use of rivers and lakes. The demand forwater-based recreation – swimming, boating and riverside-walking – is likely toincrease with higher temperatures (CCIRG, 1996), and the recreational potential ofa watercourse is also likely to change. Water-based recreation potential is mostlyinfluenced by water quality (Burrows and House, 1989), and quality standards forwater used for contact recreation have been defined by the European Commission.Changes in the frequency of occurrence of algal blooms will be particularly impor-tant. The aesthetic quality of a water course – determined by quantity and quality ofwater – can be an important and highly-valued component of the river landscape.A reduction in flow, perhaps associated with a deterioration in quality, could leadto a reduction in the aesthetic value, and hence tourist potential, of a river.

3.4.3. ConservationThe Environment Agency and Scottish Environmental Protection Agency havestatutory duties to protect and enhance the environment. In practice, this meansthat both agencies undertake conservation work to improve the environment, andconsider conservation issues when fulfilling their other duties and obligations.Changes in water availability, temperature and quality will affect instream andriverine habitats and ecosystems, although the detailed effects are not yet known.

It is particularly difficult to prevent any adverse effects of climate change onnatural ecosystems – because any intervention would attempt to sustain an ecosys-tem out of balance with its controls – but it is feasible to allow natural adaptiveprocesses to operate most efficiently. For example, migration corridors could beestablished along water courses to allow species to migrate. Perhaps the greatestrole for conservation in water management, however, is to ensure that actions tak-en in other sectors to mitigate the effects of climate change do not have adverse


impacts on the environment. It may also be the case that actions taken in othersectors provide opportunities for the enhancement of the natural environment. Forexample, ‘soft’ engineering solutions to flood defence have many environmentalbenefits.

3.4.4. NavigationThe vast majority of inland navigation in Britain is for recreation: few waterwayshave significant commercial traffic. Navigation along navigable rivers is controlledby weirs and locks, and the effect of climate change on navigation potential dependson how this infrastructure can be operated to maintain water levels during lowersummer flows. With lower flows, the navigable upper limit of a river might movedownstream. Changed flows will also affect erosion and sedimentation, with impli-cations for the maintenance of navigable waterways.

The canal network in Britain – around 4000 km – takes its water from rivers,where rivers and canals run in parallel, and from many small supply reservoirs.These reservoirs tend to maintain supplies to high parts of the canal network, andare very sensitive to changes in inputs. There are already shortages in parts of thecanal network during dry summers (7.2% of the canal network suffered restrictionsduring the dry summer of 1990, for example: British Waterways, 1991), and it islikely that this will increase in the future.

3.5. POWER GENERATION

It was noted above that hydropower contributes less than 2% of the electricitygenerated in Britain, and most of this comes from a few large reservoir schemesin Scotland and Wales. Under the 1996 CCIRG scenario, these parts of Britain arelikely to experience increased runoff, implying firstly that reservoirs will be fullmore frequently and secondly that power generation potential will increase. How-ever, increased winter rainfall and runoff will increase the risk of winter flooding,and it is possible that hydropower reservoirs would need to keep more capacityavailable for storing flood waters, thus reducing power generation potential.

Small-scale hydropower schemes, either run-of-river or exploiting small headdifferences over a weir, generate a very small proportion of Britain’s electricity.For such schemes, power generation potential is a function of variability in riverflows over time, and is directly proportional to the slope of the flow durationcurve. Increased variability in flows, and lower flows during part of the year, wouldreduce power generation potential, thus making small-scale hydropower schemesless economically-justifiable. In northern Britain, however, the general increase inflows under the 1996 CCIRG scenario implies increased potential.

The vast proportion of electricity in Britain is generated from thermal powerstations, and these require cooling water. Many power stations – including allnuclear power stations – are situated on the coast, so would not be affected by achange in river flows and water temperature, but some important power stations

106 NIGEL W. ARNELL

take water from major rivers: the Trent provides cooling water to a number oflarge power stations generating electricity for the English Midlands. A reductionin summer flows along such rivers could limit the amounts of water which couldbe extracted, and higher water temperatures would both affect the efficiency ofcooling water and lower the temperature at which cooling water can be returnedto the river without discharges exceeding licence conditions. Over the next fewdecades, however, it is likely that power station technology will evolve to requireless cooling water (CCIRG, 1996), thus lessening the exposure of the power sectorto changes in water resources.

3.6. OVERVIEW: CONFLICT AND COMPETITION

A change in climate will, as shown above, affect both the water resource baseand the demands on that resource. This may lead to altered competition for water,and conflicts between water uses and users. Most particularly, it is likely to lead toconflicts between offstream uses – such as water supply – and the instream needs ofaquatic ecosystems, amenity and recreation. These conflicts are already increasingboth as public attitudes towards the environment shift and as the EnvironmentAgency increasingly adopts an environmental protection agenda. Another potentialarea of conflict is over the use of water by domestic consumers in the garden.As summer temperatures rise and the increasing trend towards garden wateringaccelerates, conflicts over the customers’ ‘right’ to water and the balance betweenessential and non-essential uses of water will become more frequent.

4. Managing Water Resources in the Face of Climate Change

The future water resources of Britain cannot be predicted, but what is certain is thatwater managers cannot assume that the future will be like the past. Climate changeneeds to be factored into water management, recognising that there is uncertainty.There are three types of uncertainty. The first relates to the climate change scenarios,and arises due to uncertainty in the future rate of emission of greenhouse gases,the fate of gases in the atmosphere, the effect of global warming on climate, andthe effect of global changes on local weather and climate. This is the uncertaintythat has had the greatest attention. The second type of uncertainty arises from thetranslation of change in climate to effect on a system, such as a catchment or anaquatic ecosystem. Current hydrological models are reasonably good at simulatingaverage hydrological conditions under different climatic circumstances, but areless good at simulating behaviour during extreme conditions: effects of climatechange on extreme flows are therefore more uncertain than effects on averages.Effects of climate change in other aspects of the water environment are moredifficult to estimate, with problems in estimating changes in water quality and,most particularly, aquatic ecosystems. The previous sections catalogued a wide


range of possible effects of climate change, but did not indicate the magnitude orrelative significance of many of these effects, largely because the consequencesof climate change for many systems are currently difficult to predict. As a result,further simulation might show that some of the catalogued effects of climate changeare in fact very minor – or may indicate some currently unforeseen effects. Thethird uncertainty arises from the actions of managers and actors within the waterenvironment, who will respond – or not – both to perceived future climates and toother non-climatic influences, and thus influence the impact of a given change inclimate.

Over the short and medium terms (less than 20 years), a climate change sig-nal in average hydrological behaviour will remain small relative to year-on-yearvariability (Arnell and Reynard, 1996), and will probably be small in relation tothe other evolving pressures on water resources and managers. Extreme events,however, will be attributed to global warming (in some cases they already havebeen) and may stimulate management response, even though it is not possible toblame global warming for individual extreme events. In fact, the recent reviewof Britain’s water resources (Department of the Environment, 1996b) was largelytriggered by an extreme drought coupled with fears of climate change.

Over the long-term the effects of a climate change on resourceswouldbecomenoticeable. Although many aspects of water management do operate on shortand medium term horizons others, particularly those involving strategic planningor significant capital investment (in supply sources, distribution networks, sewersystems and flood defences, for example) are planned and implemented over periodsin excess of 20 years. Such activities currently need to consider explicitly thepossible effects of climate change. Indeed, both the Department of the Environmentin its 1996 review of future water resources management (Department of theEnvironment, 1996b) and the House of Commons Environment Select Committee(1996) urged water companies and regulators to take climate change into accountin water planning. A scenario-based approach to scheme and strategy assessment isneeded, and the Department of the Environment report explicitly stated that it wasthe Government’s view that estimates of future resources ought to be tested againstclimate change scenarios. Such an approach should also incorporate scenariosfor changes other than climate change, producing a multi-dimensional matrix ofpossible outcomes for a given system or strategy. A formal risk assessment ofthese alternative futures requires estimates of the chance of each outcome, butunfortunately it is not possible to produce quantitative figures for the likelihood ofalternative climate change scenarios. It will be easier to incorporate ‘supply-side’strategies (such as new resources or hard engineering flood defences) into a scenarioanalysis than ‘demand-side’ or ‘soft’ strategies, because their performance underdifferent conditions can generally be more readily determined. The performanceof demand-side strategies is more difficult to define, as it tends to depend onassumptions about behavioural response (the effect of metering, for example), orinherently uncertain science (predicting vegetation responses, for example).

108 NIGEL W. ARNELL

The management problem therefore becomes one of coping with uncertainty,where this uncertainty lies partly in the inability to predict the future and partly in theinadequacy of data and predictive models. ‘Better’ science and better simulationmodels will reduce the uncertainty in some impact areas – particularly waterquality and ecosystems – but some uncertainty is inherent in the system. There are,however, several possible ways of adapting to an uncertain future. One would beto consider the range of possible outcomes of a scheme or strategy as an indicationof the robustness of the proposal, and robustness could be used as a design orselection criteria. The better able a scheme to cope with alternative futures, themore attractive the scheme (although the economic justification will depend on thetime sequence of adaptive costs and the discount rate used).

Another management strategy would be to identify critical levels of change –beyond which a new supply source would be needed, for example – and to assessusing scenario studies the chance of these critical levels being exceeded.

A more wide-reaching reaction would be to shift towards adaptable manage-ment practices, which would mean moving from expensive capital investmenttowards investment in maintenance and soft, rather than hard, engineering, andadopting an incrementalist philosophy (Clark and Gardiner, 1994). An incremen-talist philosophy concentrates on modelling investment phasing so as to achievethe ideal balance between waiting for further information and immediate action,and allows adaptation to unforeseen circumstances at minimum cost (Clark andGardiner, 1994). This is not the same as short-termism: an incremental approachhas a long-term goal (to maintain a specific gap between supply and demand, forexample), but moves towards that goal in small steps. The goal may change overtime, as different uses are given different priorities. An incremental approach maynot work under all circumstances: beyond a certain point, a step change in actionmight be necessary.

Water management is inherently adaptable, and water managers in Britain haveresponded over recent years to changes in demands and pressures. Strategies andoptions for meeting new demands are well defined (as shown in the water supplyarea by the 1994 NRA review of strategic resource options, for example). Watersupply management in Britain is currently following a twin-track approach, withdemand management on the fast track, encouraged by government and the regula-tors, and resource improvement on the slower track (encouraged by the water sup-ply utilities). On the one hand, climate change encourages a demand-management,incrementalist approach, but on the other it may mean that demand managementwill not be enough to maintain security of supply: scenario analyses are thereforeessential when following the twin-track approach, and setting the speed of the twotracks.

Water managers in Britain are explicitly adopting a goal of sustainable waterresource development (NRA, 1994). This means ensuring that present actions donot limit the options of future generations. Climate change clearly needs to be


factored into any water resource policy based around sustainable developmentconcepts.

Acknowledgements

This paper is based on discussions with many water managers in Britain, anddeveloped from research funded by the National Rivers Authority of Englandand Wales, but the views and interpretations are those of the author. The authoracknowledges the helpful comments of the reviewers.

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(Received 7 February 1997; in revised form 21 October 1997)

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