The land surface model in the GEOS-5 AOGCM is a catchment-based hydrological model described in Koster et al. (2000). In this model, subgrid heterogeneity in surface moisture state is treated statistically. The applied subgrid scale distributions are related to the topography, which exerts a major control over much of the subgrid variability. The catchment model is coupled to the multi-layer snow model described in Stieglitz et al. (2001).