The sensitivity of runoff to soil moisture initial conditions: A numerical experiment over CONUS

Soil moisture is an important land surface state variable playing a crucial role in land-atmosphere interactions as it affects both precipitation and runoff generation. Soil moisture affects infiltration rates and thus has an important control on rainfall-to-runoff transformation process that defines a catchments flood response. Additionally, it can change the magnitude and spatial distribution of precipitation either through evapotranspiration affecting the moisture supply to the atmosphere, or through land surface heat fluxes influencing the evolution of the planetary boundary layer, which in turn affects runoff generation and flood characteristics. Consequently, understanding the role of soil moisture in land-atmosphere interactions across different climate conditions and basin characteristics is fundamental to climate research, water resources management, and other applications. In this study we investigate the direct and indirect effects (through precipitation) of soil moisture on runoff generation over the conterminous United States (CONUS). We selected two 30-day periods (a cold- and a warm-season period) to conduct simulation scenarios with the Weather Research and Forecasting (WRF) model at 3-km horizontal grid spacing over the CONUS, using initial soil moisture and temperature conditions derived from three different products: Global Forecast System (GFS) analyses, the National Center for Atmospheric Research (NCAR) continuously cycling ensemble Kalman filter Data Assimilation system, and the North American Land Data Assimilation System (NLDAS-2). Subsequently, we used precipitation and soil moisture states derived from the atmospheric simulation scenarios as forcing for the Ensemble Framework for Flash Flood Forecasting (EF5) modeling system to conduct event-based distributed hydrologic sensitivity simulations at 1km horizontal grid spacing over the CONUS. The results of this study elucidate the important role of soil moisture on storm runoff response. Our results indicate different characteristics of the soil moisture-precipitation-runoff relationship during the cold and warm periods and under different climate. It is also shown that different initial soil saturation conditions yielded strongly different soil moisture-precipitation-runoff relationship.