Sensitivity of the diurnal and seasonal course of modeled runoff to three different land surface model soil moisture parameterizations

Land surface models (LSMs) used in climate modeling include detailed above-ground biophysics but usually lack a good representation of soil hydrological processes. While evapotranspiration can be modeled and measured at a wide range of scales, runoff is a local scale process linked to topography and can only be measured at the catchment-scale. Both processes are closely linked through soil moisture, which is treated as a subgrid-scale process in climate modeling. To explore this connection, catchment-scale LSM simulations are performed with the use of three different soil moisture parameterizations over the Rhone catchment for the years 1986-1988. Results show that the use of a multilayer soil in comparison to the widely used 3-layer soil allows a better reproduction of the seasonal dynamics of runoff. Including lateral soil moisture flow significantly enhances monthly runoff performance and provides an effective means to recover from the dry soil moisture conditions at the end of summer. Snowmelt runoff in the Alpine part of the catchment is sensitive to upscaling and none of the used parameterizations can account for this process. Runoff in the continental part of the Rhone performs well at larger scales without using lateral soil moisture flow. Overall, accuracy in timing and magnitude of simulated runoff is substantially increased by the use of lateral soil moisture flow, especially at the daily time-scale. However evapotranspiration is not sensitive to the different parameterizations of soil moisture processes in the Rhone catchment.