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precipitation increases would be offset by increased evapotranspiration, with the net effect being a reduction in runoff ranging from 8 percent to 20 percent. · Of the three GCMs used to develop climate scenarios in this study, the GFDL model results in the most extreme decreases in runoff for all the sub-basins studied (-10 to -24 percent) because it predicts a relatively large regional temperature increase and no change in precipitation. The least extreme effects are generated by using either the UKMO or the GISS grid points, which incorporate respective increases in precipitation of 30 and 20 percent and lead to increases in runoff of 0 to 10 percent. · High-elevation basins appear to be more sensitive to changes in temperature and precipitation than low-elevation basins. Of the three sub-basins studied, the East River near Almont, Colorado is the most sensitive to changes in temperature and precipitation because of its higher elevation. . In general, runoff in the Upper Colorado River basin is slightly more sensitive to a 10 percent change in precipitation than to a 20C change in temperature. Thus, while increased temperatures will cause significant decreases in runoff, the overall response of the basin will ultimately depend upon the direction and magnitude of changes in precipitation. In summary, the hydrologic modeling results suggest that large changes in streamflow may occur in the Colorado River basin as a result of plausible climatic changes. GCM scenarios indicate that runoff in the basin is likely to decrease. The impacts of these potential changes in streamflow would be felt throughout the basin as changes in water deliveries, reservoir storage, and hydroelectricity production. Changes in the Colorado River Water Supply System The changes in runoff determined in the first part of the project were then used to evaluate impacts on several water-supply parameters, including salinity, reservoir levels, deliveries to users, and hydroelectricity generation. Some quite severe effects were seen, assuming no changes in the operating rules of the basin. For example, a 20 percent reduction in natural runoff would cause mean annual reductions in storage of 60 to 70 percent, reductions in power generation of 60 percent, and an increase in salinity of 15 to 20 percent. In contrast, a moderate increase in temperature (20C) and a large increase in precipitation (20 percent) would result in roughly a 20 percent increase in mean annual runoff, a 30 to 60 percent increase in storage, a 40 percent increase in power production, and a 13-15 percent decrease in salinity. The principal impacts on water supply identified with the CRSS model include the following: · Changes in mean annual actual streamflow along the River range from -31 percent to +32 percent for the scenarios studied. Decreases in runoff are relatively smaller in magnitude in the Lower Basin because they are cushioned by additional reservoir releases. For example, a decrease in natural flow of 20 percent causes a 31 percent decrease in mean annual streamflow at the Upper Basin station of Green River, but only an 11 percent decrease at Imperial Dam near the Mexican border. · Decreases in natural runoff cause severe changes in minimum runoff. For example, the -10% scenario causes mean annual runoff in the Upper Basin to decline by about 15%, but minimum flows at Lees Ferry drop 86%. · In the base case (i.e., under current hydrology), annual releases from Lake Powell never drop below the objective minimum of 8.23 million acre-feet per year (maf jyr); however a runoff decrease of 10% causes releases from Lake Powell to fall below 8.23 mafjyr in several years. x