An Integrated High-Resolution Multi-Member Modeling Approach to Assessing Mid-Century Climate Change Impacts on Extreme Hydrological Events and Water Supplies in the Western United States

Precipitation in much of the Western U.S. occurs primarily in winter months causing densely populated cities and the billion dollar agricultural industry to heavily rely on imported snowmelt driven water supplies. This study assesses the hydrological impacts from climate change and subsequent influence on water supply availability for the Western U.S. A 10-member ensemble of coupled global climate models is dynamically downscaled forcing a regional (RegCM4) and hydrological model (VIC) resulting in a high-resolution 4-km output over the region. Greenhouse gas concentrations are prescribed according to the IPCC Representative Concentration Pathway 8.5 using the present-day period of 1966-2005 and future period of 2011-2050. Temperatures are projected to increase over the region with mid-elevations experiencing exacerbated changes caused by the snow-albedo feedback consequently increasing the probability of snow droughts. Evaporation and precipitation increase over most of the Western U.S., however, impacts on runoff depend on the specific region. Overall, runoff timing shifts as a result of decreasing snowpack with winter runoff increasing and spring runoff decreasing. The intensity and frequency of both wet and dry years increases throughout the region, which, when coupled with runoff timing shifts, poses a substantial uncertainties in water supply for agricultural and urban centers. The daily annual maximum runoff 100-year event, for example, becomes approximately nine times more likely in the Colorado River basin and twice as likely in the other basins. Regardless of positive or negative trending runoff amounts, the region's water supply is projected to diminish by mid-century as a result of runoff magnitude and frequency changes. On one hand, areas like the Sierra Nevada do not currently possess adequate storage to capture additional runoff resulting in water held in reservoirs to be released. On the other hand, the projected increased frequency of abnormally low annual runoff increases the regions susceptibility to droughts.