AN EFFICIENT APPROACH TO MODELING THE TOPOGRAPHIC CONTROL OF SURFACE HYDROLOGY FOR REGIONAL AND GLOBAL CLIMATE MODELING

The current generation of land-surface models used in GCMs view the so il column as the fundamental hydrologic unit. While this may be effect ive in simulating such processes as the evolution of ground temperatur es and the growth/ablation of a snowpack at the soil plot scale, it ef fectively ignores the role topography plays in the development of soil moisture heterogeneity and the subsequent impacts of this soil moistu re heterogeneity on watershed evapotranspiration and the partitioning of surface fluxes. This view also ignores the role topography plays in the timing of discharge and the partitioning of discharge into surfac e runoff and baseflow. In this paper an approach to land-surface model ing is presented that allows us to view the watershed as the fundament al hydrologic unit. The analytic form of TOPMODEL equations are incorp orated into the soil column framework and the resulting model is used to predict the saturated fraction of the watershed and baseflow in a c onsistent fashion. Soil moisture heterogeneity represented by saturate d lowlands subsequently impacts the partitioning of surface fluxes, in cluding evapotranspiration and runoff. The approach is computationally efficient, allows for a greatly improved simulation of the hydrologic cycle, and is easily coupled into the existing framework of the curre nt generation of single column land-surface models. Because this appro ach uses the statistics of the topography rather than the details of t he topography, it is compatible with the large spatial scales of today 's regional and global climate models. Five years of meteorological an d hydrological data from the Sleepers River watershed located in the n ortheastern United States where winter snow cover is significant were used to drive the new model. Site validation data were sufficient to e valuate model performance with regard to various aspects of the waters hed water balance, including snowpack growth/ablation, the spring snow melt hydrograph, storm hydrographs, and the seasonal development of wa tershed evapotranspiration and soil moisture.