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Important vulnerabilities of water resources to potential climate change scenarios involve changes in runoff and streamflow regimes, reductions in water quality associated with changes in runoff, and human demands for water supplies.
Seasonal and annual runoff may change over large regions as a result of changes in precipitation or evapotranspiration.
Runoff is simply the area-normalized difference between precipitation and evapotranspiration; as such, it is a function of watershed characteristics, the physical structure of the watershed, vegetation, and climate. Although most climate change models show increases in precipitation over much of North America, rates of evaporation and perhaps transpiration also are likely to increase with increasing temperatures. Therefore, regions in which changes in precipitation do not offset increasing rates of evaporation and transpiration may experience declines in runoff and consequently declines in river flows, lake levels, and groundwater recharge and levels (Schindler, 1997). Alternatively, regions that experience substantial increases in precipitation are likely to have substantial increases in runoff and river flows.
Projected changes in annual discharge (summarized in Table
8-5) for some river basins in North America using various climate change
scenarios indicate potential increases as well as declines. (Many of these hydrological
impact assessments, however, were developed using older climate change scenarios
of somewhat larger increases in global air temperature than the most recent
scenarios that include regional aerosol-cooling effects.) Seasonal changes in
runoff also could be substantial. Most climate change scenarios suggest increased
winter precipitation over much of North America, which could result in increased
runoff and river flows in winter and spring. Several climate change scenarios
show declines in summer precipitation in some regions (e.g., the southeastern
United States; IPCC 1996, WG I, Figure 6.11) or declines in summer soil-moisture
levels (e.g., over much of North America; IPCC 1996, WG I, Figure 6.12), which
could result in significant declines in summer and autumn runoff in these regions.
However, climate change scenarios showing summer declines in precipitation or
soil-moisture levels in these regions generally are produced from simulations
with doubled CO2 forcing alone; when aerosol forcing is included, summer precipitation
and soil-moisture levels increase only slightly. This pattern highlights the
large uncertainty in climate change projections of runoff.
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