The impact of climate change in the water sector is a function of biophysical changes in water quantity and composition, the use to which the water is put, and the way in which those uses are managed. The implications of climate change for water resources therefore must be considered in the context of the many other pressures on water resources and their management. These pressures—and management responses to them—are evolving rapidly, and the water management system (legal, infrastructural, and institutional) in the future may be very different in many countries from that at present. Considerable efforts are underway in many international agencies and organizations (e.g., Global Water Partnership, World Bank) to improve the way water is used and managed; these actions will have very significant consequences not only for economies, access to safe water, and the environment but also for the impacts of climate change. Adaptation to climate change in the water sector must be considered in the context of these other changes—and, of course, climate change must be considered as a factor in the development of improved management techniques.

Water is a fundamental component of many economic activities. The impact of climate change on the quality and quantity of water therefore will be felt by such economic activities in one way or another. Examples of such linkages are given in the following subsections.

4.7.2.1. Ecosystems (TAR Chapter 5)

Changes in hydrological characteristics will lead to changes in aquatic and wetland ecosystems (as reviewed in Chapter 5)—as, indeed, may some of the actions taken by water managers to adapt to climate change. In practice, much water management increasingly focuses on ensuring that human use of water does not adversely impact the water environment, and maintaining and enhancing environmental quality is regarded as a legitimate management goal. Environmental demands, of course, will alter as climate changes.

Agriculture also will be affected by water availability, and actions taken by farmers in response to climate change may impact the water environment. For example, climate change may increase demands for irrigation from the agricultural sector, and if these extra needs are withdrawn from rivers or aquifers, there will be an effect on hydrological and ecological regimes: The “direct” effect of climate change on hydrological regimes and ecosystems may be enhanced. On the other hand, a lack of water resulting from climate change might mean that increased irrigation demands cannot be met, and changes in the water sector therefore are impacting directly on agricultural response to climate change. In addition, changes in agricultural land use resulting directly or indirectly from climate change may affect catchment water balance and water quality. These effects may be more substantial than the direct effects of climate change on hydrology.

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