THE IMPLEMENTATION AND VALIDATION OF IMPROVED LAND-SURFACE HYDROLOGY IN AN ATMOSPHERIC GENERAL-CIRCULATION MODEL

New land-surface hydrologic parameterizations are implemented into the NASA Goddard Institute for Space Studies (GISS) General Circulation M odel (GCM). These parameterizations are: 1 ) runoff and evapotranspira tion functions that include the effects of subgrid-scale spatial varia bility and use physically based equations of hydrologic flux at the so il surface and 2) a realistic soil moisture diffusion scheme for the m ovement of water and root sink in the soil column. A one-dimensional c limate model with a complete hydrologic cycle is used to screen the ba sic sensitivities of the hydrological parameterizations before impleme ntation into the full three-dimensional GCM. Results of the final simu lation with the GISS GCM and the new land-surface hydrology indicate t hat the runoff rate, especially in the tropics, is significantly impro ved. As a result, the remaining components of the heat and moisture ba lance show similar improvements when compared to observations. The val idation of model results is carried from the large global (ocean and l and-surface) scale to the zonal, continental, and finally the regional river basin scales.