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were applied as absolutes (i.e. + ~ C), while changes in precipitation were applied as percentages (i.e. + 10% of precipitation in the base case).9 RESULTS OF HYDROLOGIC MODELING Annual Runoff For the three Colorado River sub-basins, the magnitude of changes in mean annual runoff induced by the hypothetical scenarios ranged from decreases of 33% to increases of 19%. The greatest decrease in runoff was seen in the East River for a ~ C increase in temperature in conjunction with a 20% decrease in precipitation. The greatest increase was seen in the White River basin when a ~ C increase was combined with a 20% increase in precipitation. In all cases, at least a 10% increase in precipitation was required to offset the effect on annual runoff of a ~ C temperature rise. A 20% increase in precipitation caused runoff to increase in every case. For the Two-elevation moclel, mean annual runoff decreased by 12% and 21 % when the respective hypothetical scenarios of T + ~ C and T + ~ C were applied with no change in precipitation. Tables 5 through 8 show these results. In general, the Two-elevation moclel was more sensitive to increases in temperature than the three sub-basin moclels. While this may be an artifact of the Two-elevation moclel itself, it may also be explained by the increased importance of evaporation in the lower elevation zones that the model encompasses. For the Animas and East rivers, all GCM scenarios led to decreases in runoff, ranging from -8% to -20%, which reflects the dominant effect of increased evaporation. For the White River, two out of the four GCM scenarios showed increases in runoff (of 10% to 12%), while the other two scenarios resulted in 9 Mean monthly changes (rather than mean annual changes) cannot be used in the NWSRFS without moclifications to the moclel. All historical temperature and precipitation data are stored in data files that are called upon by the calibration program. The program then normalizes these data for the basin being mocleled using a single coefficient. Mean annual temperature and precipitation data can therefore be easily moclified by altering these coefficients. In order to incorporate monthly changes, however, it would be necessary to alter the data associated with particular months by different amounts. While this can be done, it requires access to the actual program files, which were not available for this study. 21