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<ul><li><p>Hydrology and Water Resources</p><p>4</p><p>NIGEL ARNELL (UK) AND CHUNZHEN LIU (CHINA)</p><p>Lead Authors:R. Compagnucci (Argentina), L. da Cunha (Portugal), K. Hanaki (Japan), C. Howe(USA), G. Mailu (Kenya), I. Shiklomanov (Russia), E. Stakhiv (USA)</p><p>Contributing Author:P. Dll (Germany)</p><p>Review Editors:A. Becker (Germany) and Jianyun Zhang (China)</p></li><li><p>Executive Summary 1 9 3</p><p>4 . 1 . I n t roduction and Scope 1 9 5</p><p>4 . 2 State of Knowledge of Climate ChangeImpacts on Hydrology and Wa t e r R e s o u rc e s :P ro g ress since the Second Assessment Report 1 9 54 . 2 . 1 I n t r o d u c t i o n 1 9 54 . 2 . 2 Estimating the Impacts of Climate Change 1 9 54 . 2 . 3 . Increased Awareness of the Effect of</p><p>Climatic Variability on Hydrologyand Water Resources 1 9 6</p><p>4 . 2 . 4 . Adaptation to Climate Changein the Water Sector 1 9 6</p><p>4 . 3 . E ffects on the Hydrological Cycle 1 9 74 . 3 . 1 . I n t r o d u c t i o n 1 9 74 . 3 . 2 . P r e c i p i t a t i o n 1 9 74 . 3 . 3 . E v a p o r a t i o n 1 9 84 . 3 . 4 . Soil Moisture 1 9 94 . 3 . 5 . Groundwater Recharge and Resources 1 9 94 . 3 . 6 . River Flows 2 0 0</p><p>4 . 3 . 6 . 1 . Trends in Observed Streamflow 2 0 04 . 3 . 6 . 2 . E ffects of Climate Change</p><p>on River Flows 2 0 24 . 3 . 7 . L a k e s 2 0 44 . 3 . 8 . Changes in Flood Frequency 2 0 54 . 3 . 9 . Changes in Hydrological</p><p>Drought Frequency 2 0 64 . 3 . 1 0 . Water Quality 2 0 74 . 3 . 11 . Glaciers and Small Ice Caps 2 0 84 . 3 . 1 2 . River Channel Form and Stability 2 0 94 . 3 . 1 3 . Climate Change and Climatic Va r i a b i l i t y 2 0 9</p><p>4 . 4 . E ffects on Wa t e r Wi t h d r a w a l s 2 0 94 . 4 . 1 . I n t r o d u c t i o n 2 0 94 . 4 . 2 . World Water Use 2 1 04 . 4 . 3 . Sensitivity of Demand to Climate Change 2 11</p><p>4 . 5 . Impacts on Wa t e r R e s o u rces and Hazards 2 1 24 . 5 . 1 . I n t r o d u c t i o n 2 1 24 . 5 . 2 . Impacts of Climate Change on</p><p>Water Resources: AGlobal Perspective 2 1 34 . 5 . 3 . Catchment and System Case Studies 2 1 34 . 5 . 4 . Impacts of Climate Change on</p><p>Water Resources: An Overview 2 1 7</p><p>4 . 6 . Adaptation Options andManagement Implications 2 1 84 . 6 . 1 . I n t r o d u c t i o n 2 1 84 . 6 . 2 . Water Management Options 2 1 94 . 6 . 3 . Implications of Climate Change</p><p>for Water Management Policy 2 2 14 . 6 . 4 . Factors A ffecting Adaptive Capacity 2 2 24 . 6 . 5 . Adaptation to Climate Change</p><p>in the Water Sector: An Overview 2 2 3</p><p>4 . 7 . Integration: Wa t e r and Other S e c t o r s 2 2 44 . 7 . 1 . The Nonclimate Context 2 2 44 . 7 . 2 . Water and Other Related Sectors 2 2 4</p><p>4 . 7 . 2 . 1 . Ecosystems (TAR Chapter 5) 2 2 44 . 7 . 2 . 2 . Coastal and Marine Zones</p><p>( TAR Chapter 6) 2 2 44 . 7 . 2 . 3 . Settlements (TAR Chapter 7) 2 2 44 . 7 . 2 . 4 . Financial Services</p><p>( TAR Chapter 8) 2 2 54 . 7 . 2 . 5 . Health (TAR Chapter 9) 2 2 5</p><p>4 . 7 . 3 . Water and Conflict 2 2 5</p><p>4 . 8 . Science and Information Needs 2 2 54 . 8 . 1 . I n t r o d u c t i o n 2 2 54 . 8 . 2 . Estimating Future Impacts</p><p>of Climate Change 2 2 54 . 8 . 3 . Adapting to Climate Change 2 2 6</p><p>R e f e re n c e s 2 2 7</p><p>CONTENTS</p></li><li><p> There are apparent trends in streamflow volumebothincreases and decreasesin many regions. These trendscannot all be definitively attributed to changes in regionaltemperature or precipitation. However, widespread acceleratedglacier retreat and shifts in streamflow timing in many areasfrom spring to winter are more likely to be associated withclimate change.</p><p> The effect of climate change on streamflow and groundwaterrecharge varies regionally and between scenarios, largelyfollowing projected changes in precipitation. In some partsof the world, the direction of change is consistent betweenscenarios, although the magnitude is not. In other parts ofthe world, the direction of change is uncertain. </p><p> Peak streamflow is likely to move from spring to winter inmany areas where snowfall currently is an importantc o mponent of the water balance. </p><p> Glacier retreat is likely to continue, and many small glaciersmay disappear. </p><p> Water quality is likely generally to be degraded by higherwater temperature, but this may be offset regionally byincreased flows. Lower flows will enhance degradation ofwater quality.</p><p> Flood magnitude and frequency are likely to increase in mostregions, and low flows are likely to decrease in many regions. </p><p> Demand for water generally is increasing as a result ofp o pulation growth and economic development, but it isfalling in some countries. Climate change is unlikely tohave a large effect on municipal and industrial demands butmay substantially affect irrigation withdrawals. </p><p> The impact of climate change on water resources dependsnot only on changes in the volume, timing, and quality ofstreamflow and recharge but also on system characteristics,changing pressures on the system, how the management ofthe system evolves, and what adaptations to climate changeare implemented. Nonclimatic changes may have a greaterimpact on water resources than climate change. </p><p> Unmanaged systems are likely to be most vulnerable toc l imate change. </p><p> Climate change challenges existing water resourcesm a nagement practices by adding additional uncertainty.Integrated water resources management will enhance thepotential for adaptation to change. </p><p> Adaptive capacity (specifically, the ability to implementintegrated water resources management), however, isd i stributed very unevenly across the world.</p><p>EXECUTIVE SUMMARY</p></li><li><p>4.1. Introduction and Scope</p><p>This chapter assesses our understanding of the implications ofclimate change for the hydrological cycle, water resources, andtheir management. Since the beginnings of concern over thepossible consequences of global warming, it has been widelyrecognized that changes in the cycling of water between land,sea, and air could have very significant impacts across manysectors of the economy, society, and the environment. Thecharacteristics of many terrestrial ecosystems, for example, areheavily influenced by water availability and, in the case ofinstream ecosystems and wetlands, by the quantity and qualityof water in rivers and aquifers. Water is fundamental to humanlife and many activitiesmost obviously agriculture but alsoi n d u s t r y, power generation, transportation, and wastem a n a g ementand the availability of clean water often is aconstraint on economic development. Consequently, there havebeen a great many studies into the potential effects of climatechange on hydrology (focusing on cycling of water) and waterresources (focusing on human and environmental use ofwater). The majority of these studies have concentrated onp o ssible changes in the water balance; they have looked, forexample, at changes in streamflow through the year. A smallernumber of studies have looked at the impacts of these changesfor water resourcessuch as the reliability of a water supplyreservoir or the risk of floodingand even fewer explicitlyhave considered possible adaptation strategies. This chaptersummarizes key findings of research that has been conductedand published, but it concentrates on assessing opportunitiesand constraints on adaptation to climate change within thewater sector. This assessment is based not only on the few studiesthat have looked explicitly at climate change but also onc o nsiderable experience within different parts of the waters e ctor in adapting to changing circumstances in general.</p><p>This chapter first summarizes the state of knowledge of climatechange impacts on hydrology and water resources (Section4.2), before assessing effects on the hydrological cycle andwater balance on the land (Section 4.3). Section 4.4 examinespotential changes in water use resulting from climate change,and Section 4.5 assesses published work on the impacts ofc l imate change for some water resource management systems.Section 4.6 explores the potential for adaptation within thewater sector. The final two sections (Sections 4.7 and 4.8)c o nsider several integrative issues as well as science andi n f o rmation requirements. The implications of climate changeon freshwater ecosystems are reviewed in Chapter 5, althoughit is important to emphasize here that water management isincreasingly concerned with reconciling human and environmentaldemands on the water resource. The hydrological system alsoaffects climate, of course. This is covered in the WorkingGroup I contribution to the Third Assessment Report (TAR);the present chapter concentrates on the impact of climate onhydrology and water resources.</p><p>At the outset, it is important to emphasize that climate changeis just one of many pressures facing the hydrological systemand water resources. Changing land-use and land-management</p><p>practices (such as the use of agrochemicals) are altering thehydrological system, often leading to deterioration in the resourcebaseline. Changing demands generally are increasing pressureson available resources, although per capita demand is fallingin some countries. The objectives and procedures of watermanagement are changing too: In many countries, there is anincreasing move toward sustainable water management andincreasing concern for the needs of the water environment. Forexample, the Dublin Statement, agreed at the InternationalConference on Water and the Environment in 1992, urg e ss u stainable use of water resources, aimed at ensuring that neitherthe quantity nor the quality of available resources are degraded.Key water resources stresses now and over the next fewdecades (Falkenmark, 1999) relate to access to safe drinkingw a t e r, water for growing food, overexploitation of water resourcesand consequent environmental degradation, and deterioriationin water quality. The magnitude and significance of these stressesvaries between countries. The late 1990s saw the developmentof several global initiatives to tackle water-related problems:The UN Commission on Sustainable Development publishedthe Comprehensive Assessment of the Freshwater Resourcesof the World (WMO, 1997), and the World Water Councilasked the World Commission for Water to produce a vision fora water-secure world (Cosgrove and Rijbersman, 2000). Aseries of periodical reports on global water issues was initiated(Gleick, 1998). The impacts of climate change, and adaptationto climate change, must be considered in the context of theseother pressures and changes in the water sector.</p><p>4.2 State of Knowledge of Climate Change Impactson Hydrology and Water Resources: Progresssince the Second Assessment Report</p><p>4.2.1 Introduction</p><p>Over the past decadeand increasingly since the publicationof the Second Assessment Report (SAR) (Arnell et al ., 1996;Kaczmarek, 1996)there have been many studies into climatechange effects on hydrology and water resources (see theonline bibliography described by Chalecki and Gleick, 1999),some coordinated into national programs of research (as in theU.S. National Assessment) and some undertaken on behalf ofwater management agencies. There are still many gaps andunknowns, however. The bulk of this chapter assesses currentunderstanding of the impacts of climate change on waterresources and implications for adaptation. This section highlightssignificant developments in three key areas since the SAR:methodological advances, increasing recognition of the effectof climate variability, and early attempts at adaptation to climatechange.</p><p>4.2.2 Estimating the Impacts of Climate Change</p><p>The impacts of climate change on hydrology usually areestimated by defining scenarios for changes in climatic inputs toa hydrologicalmodel from the output of general circulation models</p><p>195Hydrology and Water Resources</p></li><li><p>(GCMs). The three key developments here are constructingscenarios that are suitable for hydrological impact assessments,developing and using realistic hydrological models, andu n d e rstanding better the linkages and feedbacks between climateand hydrological systems.</p><p>The heart of the scenario problem lies in the scale mismatchbetween global climate models (data generally provided on amonthly time step at a spatial resolution of several tens ofthousands of square kilometers) and catchment hydrologicalmodels (which require data on at least daily scales and at ar e solution of perhaps a few square kilometers). A variety ofdownscaling techniques have been developed (Wilby andWi g l e y, 1997) and used in hydrological studies. T h e s et e c hniques range from simple interpolation of climate modeloutput (as used in the U.S. National Assessment; Felzer andHeard, 1999), through the use of empirical/statistical relationshipsbetween catchment and regional climate (e.g., Crane andHewitson, 1998; Wilby et al., 1998, 1999), to the use of nestedregional climate models (e.g., Christensen and Christensen,1998); all, however, depend on the quality of simulation of thedriving global model, and the relative costs and benefits ofeach approach have yet to be ascertained. Studies also havelooked at techniques for generating stochastically climate dataat the catchment scale (Wilby et al., 1998, 1999). In principle,it is possible to explore the effects of changing temporal patternswith stochastic climate data, but in practice the credibility ofsuch assessments will be strongly influenced by the ability ofthe stochastic model to simulate present temporal patternsr e a listically.</p><p>Considerable effort has been expended on developingimproved hydrological models for estimating the effects ofc l imate change. Improved models have been developed tos i mulate water quantity and quality, with a focus on realisticrepresentation of the physical processes involved. These modelsoften have been developed to be of general applicability, withno locally calibrated parameters, and are increasingly usingremotely sensed data as input. Although different hydrologicalmodels can give different values of streamflow for a giveninput (as shown, for example, by Boorman and Sefton, 1997;Arnell, 1999a), the greatest uncertainties in the effects ofc l imate on streamflow arise from uncertainties in climatechange scenarios, as long as a conceptually sound hydrologicalmodel is used. In estimating impacts on groundwater recharge,water quality, or flooding, however, translation of climate intoresponse is less well understood, and additional uncertainty isintroduced. In this area, there have been some reductions inuncertainty since the SAR as models have been improved andmore studies conducted (see Sections 4.3.8 and 4.3.10). Theactual impacts on water resourcessuch as water supply,power generation, navigation, and so forthdepend not onlyon the estimated hydrological change but also on changes indemand for the resource and assumed responses of waterresources managers. Since the SAR, there have been a fewstudies that have summarized potential response strategies andassessed how water managers might respond in practice (seeSection 4.6).</p><p>There also have been considerable advances since the SAR inthe understanding of relationships between hydrologicalprocesses at the land surface and processes within the atmosphereabove. The...</p></li></ul>


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