Since the SAR, many studies have explored linkages between recognizable patterns of climatic variabilityparticularly El Niño and the North Atlantic Oscillationand hydrological behavior, in an attempt to explain variations in hydrological characteristics over time. These studies in North America (McCabe, 1996; Piechota et al., 1997; Vogel et al., 1997; Olsen et al., 1999), South America (Marengo, 1995; Compagnucci and Vargas, 1998), Australasia (Chiew et al., 1998), Europe (e.g., Shorthouse and Arnell, 1997), and southern Africa (Shulze, 1997) have emphasized variability not just from year to year but also from decade to decade, although patterns of variability vary considerably from region to region. Most studies focus on the past few decades with recorded hydrological data, but an increasing number of studies have reconstructed considerably longer records from various proxy data sources (e.g., Isdale et al., 1998; Cleaveland, 2000). Such research is extremely valuable because it helps in interpretation of observed hydrological changes over time (particularly attribution of change to global warming), provides a context for assessment of future change, and opens up possibilities for seasonal flow prediction (e.g., Piechota et al., 1998) hence more efficient adaptation to climatic variability. It also emphasizes that the hydrological baseline cannot be assumed to be constant, even in the absence of climate change.
Water management is based on minimization of risk and adaptation to changing circumstances (usually taking the form of altered demands). A wide range of adaptation techniques has been developed and applied in the water sector over decades. One widely used classification distinguishes between increasing capacity (e.g., building reservoirs or structural flood defenses), changing operating rules for existing structures and systems, managing demand, and changing institutional practices. The first two often are termed supply-side strategies, whereas the latter two are demand-side. Over the past few years, there has been a considerable increase in interest in demand-side techniques. International agencies such as the World Bank (World Bank, 1993) and initiatives such as the Global Water Partnership are promoting new ways of managing and pricing water resources to manage resources more effectively (Kindler, 2000).
This work is going on largely independently of climate change, but changes in water management practices will have a very significant impact on how climate change affects the water sector. Water managers in some countries are beginning to consider climate change explicitly, although the methodologies for doing so are not yet well defined and vary between and within countries depending on the institutional arrangements for long-term water resources planning. In the UK, for example, water supply companies were required by regulators in 1997 to consider climate change in estimating their future resource, hence investment, projections (Subak, 2000). In the United States, the American Water Works Association urged water agencies to explore the vulnerability of their systems to plausible climate changes (AWWA, 1997).
Clearly, however, the ability of water management agencies to alter management practices in general or to incorporate climate change varies considerably between countries. This issue is discussed further in Section 4.6.
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