This chapter first summarizes the state of knowledge of climate change impacts on hydrology and water resources (Section 4.2), before assessing effects on the hydrological cycle and water balance on the land (Section 4.3). Section 4.4 examines potential changes in water use resulting from climate change, and Section 4.5 assesses published work on the impacts of climate change for some water resource management systems. Section 4.6 explores the potential for adaptation within the water sector. The final two sections (Sections 4.7 and 4.8) consider several integrative issues as well as science and information requirements. The implications of climate change on freshwater ecosystems are reviewed in Chapter 5, although it is important to emphasize here that water management is increasingly concerned with reconciling human and environmental demands on the water resource. The hydrological system also affects climate, of course. This is covered in the Working Group I contribution to the Third Assessment Report (TAR); the present chapter concentrates on the impact of climate on hydrology and water resources.
At the outset, it is important to emphasize that climate change is just one of many pressures facing the hydrological system and water resources. Changing land-use and land-management practices (such as the use of agrochemicals) are altering the hydrological system, often leading to deterioration in the resource baseline. Changing demands generally are increasing pressures on available resources, although per capita demand is falling in some countries. The objectives and procedures of water management are changing too: In many countries, there is an increasing move toward sustainable water management and increasing concern for the needs of the water environment. For example, the Dublin Statement, agreed at the International Conference on Water and the Environment in 1992, urges sustainable use of water resources, aimed at ensuring that neither the quantity nor the quality of available resources are degraded. Key water resources stresses now and over the next few decades (Falkenmark, 1999) relate to access to safe drinking water, water for growing food, overexploitation of water resources and consequent environmental degradation, and deterioriation in water quality. The magnitude and significance of these stresses varies between countries. The late 1990s saw the development of several global initiatives to tackle water-related problems: The UN Commission on Sustainable Development published the Comprehensive Assessment of the Freshwater Resources of the World (WMO, 1997), and the World Water Council asked the World Commission for Water to produce a vision for a water-secure world (Cosgrove and Rijbersman, 2000). A series of periodical reports on global water issues was initiated (Gleick, 1998). The impacts of climate change, and adaptation to climate change, must be considered in the context of these other pressures and changes in the water sector.
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