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This section summarizes the potential effects of climate change on the components of the water balance and their variability over time.
Precipitation is the main driver of variability in the water balance over space and time, and changes in precipitation have very important implications for hydrology and water resources. Hydrological variability over time in a catchment is influenced by variations in precipitation over daily, seasonal, annual, and decadal time scales. Flood frequency is affected by changes in the year-to-year variability in precipitation and by changes in short-term rainfall properties (such as storm rainfall intensity). The frequency of low or drought flows is affected primarily by changes in the seasonal distribution of precipitation, year-to-year variability, and the occurrence of prolonged droughts.
TAR WGI Section 2.5 summarizes studies into trends in precipitation. There are different trends in different parts of the world, with a general increase in Northern Hemisphere mid- and high latitudes (particularly in autumn and winter) and a decrease in the tropics and subtropics in both hemispheres. There is evidence that the frequency of extreme rainfall has increased in the United States (Karl and Knight, 1998) and in the UK (Osborn et al., 2000); in both countries, a greater proportion of precipitation is falling in large events than in earlier decades.
Current climate models simulate a climate change-induced increase in annual precipitation in high and mid-latitudes and most equatorial regions but a general decrease in the subtropics (Carter and Hulme, 1999), although across large parts of the world the changes associated with global warming are small compared to those resulting from natural multi-decadal variability, even by the 2080s. Changes in seasonal precipitation are even more spatially variable and depend on changes in the climatology of a region. In general, the largest percentage precipitation changes over land are found in high latitudes, some equatorial regions, and southeast Asia, although there are large differences between climate models.
Until recently, very few projections of possible changes in year-to-year variability as simulated by climate models have been published, reflecting both the (until recently) short model runs available and the recognition that climate models do not necessarily reproduce observed patterns of climatic variability. Recent developments, however, include the increasing ability of some global climate models to reproduce features such as El Niño (e.g., Meehl and Washington, 1996) and open up the possibility that it may be feasible to estimate changes in year-to-year variability. Recent scenarios for the UK, derived from HadCM2 experiments, indicate an increase in the relative variability of seasonal and annual rainfall totals resulting from global warming (Hulme and Jenkins, 1998).
Potential changes in intense rainfall frequency are difficult to infer from global climate models, largely because of coarse spatial resolution. However, there are indications (e.g., Hennessy et al., 1997; McGuffie et al., 1999) that the frequency of heavy rainfall events generally is likely to increase with global warming. Confidence in this assertion depends on the confidence with which global climate models are held. More generally, uncertainty in GCM projections of precipitation largely determines the uncertainty in estimated impacts on hydrological systems and water resources.
Increasing temperatures mean that a smaller proportion of precipitation may fall as snow. In areas where snowfall currently is marginal, snow may cease to occur—with consequent, very significant, implications (discussed below) for hydrological regimes. This projection is considerably less uncertain than possible changes in the magnitude of precipitation.
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