A SIMULATION OF THE SURFACE-ENERGY BUDGET AND SOIL-WATER CONTENT OVER THE HYDROLOGIC ATMOSPHERIC PILOT EXPERIMENTS-MODELIZATION DU BILAN HYDRIQUE FOREST SITE

The surface energy budget and soil water content variation are simulated for a two month period during the 1986 Hydrologic Atmospheric Pilot Experiment-Modelisation du Bilan Hydrique (HAPEX-MOBILHY) (J.C. Andre et al., 1988) using the Oregon State University Coupled Atmosphere Plant Snow (CAPS) model. When initialized and driven by the observed soil water content and atmospheric forcing, the CAPS model closely simulated the observed surface energy budget and the variation of soil water content. Lateral transports of soil water may cause significant impacts on the simulated soil water content during the later part of the simulation. Vegetation appears to significantly affect the surface energy budget and temporal variation of soil water content. We found two major mechanisms by which vegetation controls the surface energy budget and soil water content: (I) dependencies of transpiration on ambient conditions, and (2) direct hydrological coupling between the atmosphere and deep soil water through the root structure. Compared to the simulation without vegetation, net effects of vegetation are (1) sustained evaporation over the entire simulation period, (2) more gradual variation of near surface soil water content, and (3) continued reduction of deep soil water content through transpiration. Soil heat flux becomes an important component of the surface energy budget over a bare soil surface. The simulated surface energy budget is strongly dependent on the initial soil water content, however, dependence of the simulated surface energy budget on the initial soil temperature was relatively minor.