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Aoolication of Climate Scenarios to the NWSRFS Model The hypothetical scenarios used in each of the model runs are shown in Table 4. In the absence of information on the distribution of annual changes throughout the year, mean annual changes were applied uniformly to all the historical data. Temperature changes were applied as absolute amounts, while precipitation changes were interpreted as percent differences: 6T = Tnew - TOld 6 P = ~ew-:.fold (1 ) (2) P01d Potential evapotranspiration (PET) rates were assumed to follow the general relationship to temperature of 4 percent per degree Celsius as derived by Budyko (1982:119). Wetherald and Manabe (1975) found that global evaporation increases by 3 percent when temperature increases by 10 C. Accordingly, for the Two-elevation model, additional sensitivity runs were done using a potential evapotranspiration rate of 3% per degree Celsius. As expected, the potential evapotranspiration rate is most important for temperature-ctependent scenarios (i.e. increases in temperature with no net change in precipitation). For a temperature increase of 4' C and no net change in precipitation, the use of a 4% per degree potential evapotranspiration rate rather than a 3% per degree rate decreases mean monthly runoff by an additional 3%. For other scenarios, the effect of the potential evapotranspiration rate was much less important. Temperature data in the model were altered by changing the mean elevation of the basin relative to the existing station data using an appropriate lapse rate. For standard calibration runs, the model normalizes temperature station data to the mean elevation of the basin being modeled. To convert this station data, the model uses minimum and maximum lapse rates (to convert minimum and maximum temperature data, respectively) For climate change runs, the elevation of the sub-basin was altered using an average lapse rate, usually between 0.5 and 0.7' C per 100 meters. It is important to note that model 17