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r in a 9% decrease in mean annual runoff. Our results show a 7% decrease in mean annual runoff for a 'Z C temperature rise and an 8% increase in potential evapotranspiration (refer to Table 8), which is in close agreement with the results from Schaake. For the range of scenarios presented here, mean annual runoff changes nearly linearly with precipitation, although this relationship begins to break down as precipitation increases by 20% at which point runoff begins to increase relatively faster. Results from Schaake indicate that, in the absence of temperature and potential evapotranspiration increases, this non-linearity occurs for a precipitation increase of only 10%, which causes a corresponding increase in runoff of 19%. Overall. our results are within the range reported by other investigators for semi-arid river basins (Table 10). The results derived from GCM scenarios fall within the range established by the hypothetical scenarios. Of the three GCMs, the GFDL model (T + 4.9' C, P + 0) results in the most extreme decreases in runoff for all basins (-10% to -24%) because it predicts a relatively large regional temperature increase and no change in precipitation. The least extreme effects are generated by either the UKMO 1 or the GISS 1 grid point, which incorporate respective increases in precipitation of 30% and 20% and lead to increases in runoff of 0 to 10%. Overall, however, the GCM scenarios suggest that decreases in runoff are much more likely than increases in this region. This is consistent with the work of Rind, et al. (1990), who have analyzed the frequency of droughts using GCM outputs other than soil moisture and have found increased drying. Moreover, it is only the GCM grid points which incorporate large increases in precipitation (20 to 30%) in which runoff does not decrease. The greater uncertainty associated with precipitation changes should be kept in mind. All the GCM scenarios suggest large regional increases in temperature. which would lead to decreased runoff, unless offset by precipitation increases of 20% or more. The GISS transient scenario implies increases in runoff in all three sub-basins and in the Two- elevation model. These range from 4% in the White River basin to 12% in the Animas River basin. In contrast, the GISS equilibrium scenarios imply decreases in runoff of -8% to -14%, except on the White River where runoff increases by 10%. This suggests the potential for short-term increases in runoff (due to 35