The Regional Impacts of Climate Change

Other reports in this collection Impacts, Adaptations, and Vulnerabilities

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.

Table 8-5: Summary of annual runoff impacts from climate change scenarios

Region/River Basin
Scenario Method Hydrological Changes (annual) Reference(s)
East-Central Canada      
St. Lawrence, Ontario and Quebec GCM: CCC92 -34% Croley (1992)
Opinaca-Eastmain, Quebec GCM: GISS84, GFDL80 +20.2%, +6.7% Singh (1987)
La Grande, Quebec GCM: GISS84, GFDL80 +15.6%, +16.5% Singh (1987)
Caniapiscau, Quebec GCM: GISS84, GFDL80 +13.0%, +15.7% Singh (1987)
Moise, Quebec GCM: CCC92 -5% Morin and Slivitzky (1992)
Grand, Ontario GCM: GISS87, GFDL87, CCC92 -11%, -21%, -22% Smith and McBean (1993)
Canadian Prairie
Saskatchewan GCM: GISS87 (1) +28%, +35% Cohen et al. (1989); Cohen (1991)
  GCM: GFDL87 (1) -27%, -36%
  GCM: OSU88 (1) +2%, -4%
NorthWest Canada
Mackenzie GCM: CCC92, GFDL-R30 -3 to -7% Soulis et al. (1994)
  analog +7%
Mid-Atlantic USA
Delaware GCM: GISS, GFDL, OSU -5 to -38% McCabe and Wolock (1992)
  (soil moisture index)
Western USA
Upper Colorado GCM: GISS, GFDL, UKMO -33 to +12% Nash and Gleick (1993)

(1) Includes low and high irrigation.

Sources: CCC92 (Boer et al., 1992; McFarlane et al., 1992), OSU88 (Schlesinger and Zhao, 1988), GFDL87 (Manabe and Wetherald, 1987), GISS87 (Cohen, 1991), GISS84 (Cohen, 1991), GFDL80 (Cohen, 1991).


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