Droughts and arid zones

On a global level, the area affected by droughts is likely to increase due to a reduction in (summer) rainfall combined with increased evaporation (Sheffield, 2008). Especially in subtropical regions and the Mediterranean, reduced rainfall is predicted to lead to more frequent and more intense droughts. By the end of the 21st century, the land surface affected by extreme droughts at one time could increase by 10-fold to 40-fold (Bates et al, 2008). The regions most affected by increased droughts will be the Mediterranean, Central Asia, Central America, and Southern and West Africa (Sheffield, 2008). The southern parts of the US and Australia - already regularly affected by droughts - are likely to see more frequent and more severe droughts.

Increased droughts will cause significant problems for the water resources of large urban areas. For example, in Australia, almost all major urban centres will face reduced water availability and will therefore need to look for alternatives to satisfy domestic and industrial water demands. In Perth, large reductions in dam inflow are predicted and, as a result, alternative water resources are now being developed (see Chapter 13).

It is not only a reduction in rainfall that influences the number of droughts. Changes in seasonality and variability are also likely to cause an increase in drought frequency around the world. For example, in Northern Europe, no reduction in average rainfall is predicted by the global climate models; yet, summer droughts are likely to increase due to a higher variability in summer rainfall (less days with rainfall) and increased temperatures. These more frequent dry periods in summer, in combination with less runoff from snowmelt in summer, are expected to increase the frequency of low flows of all major river systems in Europe (Middelkoop et al, 2001). These low flows will affect navigation, ecosystems, water quality, and agricultural, industrial and domestic water supply.

All areas that depend upon snowmelt for summer river flows will see more frequent droughts. The combination of less rainfall, reduced stream flow and higher temperatures will put increased pressures on water resources in regions such as the north-western US. Higher temperatures and lower rainfall cause a higher water demand by agriculture, domestic (watering gardens and swimming pools) and industrial use (cooling water). As different pressures come together here, these impacts are likely to be already felt in the near future, and summer water stress and shortages are likely to increase rapidly in several parts of the world. This is especially the case in regions where water demands are increasing due to population growth and the expansion of economic activities - for example, in Perth and Cape Town. In both towns, climate projections indicate lower water availability, while at the same time demand is rapidly growing.

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