Projected Changes to Extreme Runoff and Precipitation Events for a Downscaled Simulation over the Western United States

Extreme precipitation and runoff events, which often impact natural and social systems more than mean changes, generally occur over regional scales. Future climate projections can be used to estimate how the hydrologic cycle may change, but the coarse resolution of global climate models (>1°) makes it difficult to evaluate regional impacts, such as over a single watershed or mountain range. To estimate changes in hydroclimatic variables at finer spatial resolutions, we dynamically downscale the Community Climate System Model (CCSM) version 4 with the Weather Research and Forecasting (WRF) regional climate model over the western United States at 9 km spatial resolution. We use a version of WRF with improvements to the runoff generation parameterization, making simulations of runoff from western mountains more realistic. By running WRF at a higher spatial resolution, we estimate future climate conditions over four watersheds (Columbia, Lower Colorado, Upper Colorado, and the Upper Missouri/Yellowstone) and four mountain ranges (Cascades, Great Basin, Sierra Nevada, and U.S. Rockies). Over each region, we compare the historical period (water years (WYs) 1996 through 2005) with mid-century (WYs 2041 through 2050) and end-century (WYs 2091 through 2100) to understand how runoff, precipitation, and snowfall may change under the RCP8.5 scenario. In particular, we consider how extreme events may change throughout the 21^st century; by comparing the historical 95^th, 99^th, and 99.9^th percentiles of daily runoff and precipitation to values for future decades, we can estimate how the wettest days are likely to change. From the WRF/CCSM4 simulations, the Columbia and the Lower Colorado watersheds are both expected to experience more extreme wet days, with the 99^th percentile of daily precipitation estimated to increase by 8% or more. For the Upper Colorado, however, the 99^th percentile of daily runoff is projected to decrease by over 30%. We also consider the causes of these changes by comparing historical and future WRF-simulated temperature, snow accumulation, and other hydroclimatic processes. Though only one realization from a single global climate model, the downscaled simulation presented here shows interesting results regarding how extreme events may change; these results can be tested by downscaling other global models with WRF to create an ensemble of dynamically downscaled future projections.