Impacts of climate change due to increased CO2 on the frequency of hydrologic extremes are important concerns in the western US as it affects directly natural disasters and water resources. Previous companion studies have suggested that the climate changes induced by increased atmospheric CO2 are likely to increase cold season precipitation and runoff in the mountainous watershed region in the western US, and the climate change signals exhibit strong influences of orography.

In this presentation, the impacts of climate change induced by increased CO2 on extreme hydrologic events in the western US are examined from a regional-scale climate change scenario obtained by downscaling HadCM2-generated global signals using an RCM, coupled MAS-SPS. Analyses of the downscaled data so far suggest substantial increases in the frequency of heavy and extreme precipitation events, as well as overall increases in rainfall intensity in an altered climate. The altered precipitation and snowmelt in turn increases high runoff events during cold seasons. Orographic effects appears to intensify existing spatial contrasts in precipitation distribution.