Precipitation infiltration in the simplified SiB land surface scheme

A global two-dimensional implementation of the simplified Simple Biosphere (SSiB) land surface scheme is integrated offline for two years as part of t he Global Soil Wetness Project (GSWP). A climatology of soil wetness and su rface fluxes has been produced. This climatology is compared to a number of sensitivity studies that have been performed to investigate how the partit ioning of precipitation between runoff and evapotranspiration is affected, when aspects of the soil parameterization and the treatment of convective p recipitation are altered. The control integration has a reasonable spatial distribution of the surfac e hydrologic balance components, and shows realistic seasonal and interannu al variations. Evaporation from the soil surface accounts for a majority of the water fluxes from the soil over all but heavily forested areas, where transpiration dominates. The sensitivity studies show that in general the m ost sensitive terms on seasonal time scales appear to be runoff, direct eva poration from the soil, and the seasonal change of water storage in the soi l matrix. A realistic distribution of convective precipitation in space and time is necessary to simulate at the grid scale adequately high values of runoff, and to not over-represent direct evaporation of rainfall intercepte d by the canopy. Sensitivity is found to the choice of thickness of the sur face soil layer - a parameter often assigned arbitrarily in land surface mo dels. Little sensitivity is found when imposing a vertical profile of soil porosity intended to account for surface soil aeration and deep soil compac tion.