EFFECT OF SIMULATED CLIMATE-CHANGE ON SNOWMELT RUNOFF MODELING IN SELECTED BASINS

The projected increase in the concentration of CO2 and other greenhous e gases in the atmosphere is likely to result in a global temperature increase. This paper reports on the probable effects of a temperature increase and changes in transpiration on basin discharge in two differ ent mountain snowmelt regions of the western United States. The hydrol ogical effects of the climate changes are modeled with a relatively si mple conceptual, semi-distributed snowmelt runoff model. Based on the model results, it may be concluded that increased air temperatures wil l result in a shift of snowmelt runoff to earlier in the snowmelt seas on. Furthermore, it is shown that it is very important to include the expected change in climate-related basin conditions resulting from the modeled temperature increase in the runoff simulation. The effect of adapting the model parameters to reflect the changed basin conditions resulted in a further shift of streamflow to April and an even more si gnificant decrease of snowmelt runoff in June and July. If the air tem peratures increase by approximately 5-degrees-C and precipitation and accumulated snow amounts remain about the same, runoff in April and Ma y, averaged for the two basins, is expected to increase by 185 percent and 25 percent, respectively. The runoff in June and July will decrea se by about 60 percent each month. Overall, the total seasonal runoff decreases by about 6 percent. If increased CO2 concentrations further change basin conditions by reducing transpiration by the maximum amoun ts reported in the literature, then, combined with the 5-degrees-C tem perature increase, the April, May, June, and July changes would averag e +230 percent, +40 percent, -55 percent, and -45 percent, respectivel y. The total seasonal runoff change would be +11 percent.