Increases in Extreme Streamflow Events and Stream Temperature for the Sierra Nevada and Colorado River Basin

Some of the greatest impacts from global climatic changes on human life are expected from extreme hydrologic events, such as floods and droughts. Here we assess changes in the frequency of occurrence of extreme hydrologic conditions by the end of the century for the water-generating, mountainous basins of the Southwestern U.S., namely the Sierra Nevada and Upper Colorado River Basin. The extreme conditions considered are high flows, low flows, and elevated stream temperature as derived from historic and future simulations using the Soil and Water Assessment Tool hydrologic model and downscaled output from a GCM ensemble. Results indicate noteworthy differences in the frequency changes of extremes based on geographic region, season, elevation, and stream size. We found wide-spread increases in the occurrence of stream flows exceeding 150% of historic averages for winter by the end of the century, and extensive increases in the occurrence of both extreme low flows (representing <50% of historic averages), and elevated stream temperatures (> 3 °C of monthly averages) during the summer months, with some basins expecting extreme conditions 90-100% of the time by the end of the century. Understanding the differences in the changes of extreme conditions can assist in planning for climate change adaptation and mitigation.