Projected end-of-century changes in water available for runoff across mountain basins of western North America

For much of western North America, mountain snowpack acts as a natural water reservoir storing winter precipitation that accumulates as snow and releasing it gradually during the spring melt season. Winter precipitation events can both buffer and enhance flood risk for a given catchment and runoff event depending on rain-snow height, basin hypsometry, and antecedent snowpack conditions. For example, flood risk can be buffered when a substantial fraction of event precipitation falls as snow, reducing the water available for runoff, particularly at higher elevations. Flood risk can also be enhanced by additional snowmelt contribution to runoff during rain-on-snow. These competing factors - buffering and enhancing water available for runoff - often occur simultaneously within a given mountain river basin and precipitation event. How they respond to a warmer climate will largely determine changes in flood risk for snow-dominated regions. We present an assessment of possible future changes in the water available for runoff over western North America simulated at high-resolution by the Weather Research and Forecasting (WRF) model run for both a 13-year control time period and re-run with a 'business-as-usual' future climate scenario. We analyze the 10 largest five-day runoff events for the historical and future scenario for large mountain river basins and compare the results against snowfall climate sensitivity, hypsometry, precipitation seasonality, and event snowfall fraction. As the climate warms, the flood buffering capacity of winter snowfall is projected to be reduced for all mountain areas as rain-snow heights expand upward in elevation. The greatest increases in the 10 largest five-day runoff events occur for maritime regions of the U.S. and Canada where event snowfall fraction is historically high, snowfall is prone to a shift toward rain, and precipitation increases are projected. The results have implications on the frequency and severity of hydrologic extremes in historically snow-dominated watersheds.