Relatively small climatic variations can create large problems for water resources, especially in arid and semi-arid regions. However, the climatic impacts on hydrology at the regional scale are uncertain, and will be influenced by a complex mix of temperature, precipitation, evaporation, soil moisture availability and run off changes. For many marginal areas, such as the Sahelian region in Africa, the rates of present desertification (Nicholson, 1989; Demarée & Nicolis, 1990) are likely to increase, having significant implications for agriculture, water storage, distribution and power generation. Other areas, such as NW Europe, may experience increases in precipitation, with resultant increases in the risk of flooding, as water tables rise.
Patterns of water demand will need to alter in response to changes in water supply (Fiering & Rogers, 1989). This will be true for areas at risk from both drought and flooding, and effective water management strategies will be required.