Lawrence Berkeley National Laboratory

An understanding of the hydrologic interactions among atmosphere, land surface, and subsurface is one of the keys to understanding the water cycling system that supports our life system on earth. Properly modeling such interactions is a difficult task because of theinherent coupled processes and complex feedback structures among subsystems. In this paper, we present a model that simulates the landsurface and subsurface hydrologic response to meteorological forcing. This model combines a state of the art landsurface model, the NCAR Community Land Model version 3 (CLM3), with a variably saturatedgroundwater model, the TOUGH2, through an internal interface that includes flux and state variables shared by the two submodels. Specifically, TOUGH2, in its simulation, uses infiltration, evaporation, and rootuptake rates, calculated by CLM3, as source/sink terms? CLM3, in its simulation, uses saturation and capillary pressure profiles, calculated by TOUGH2, as state variables. This new model, CLMT2, preserves the best aspects of both submodels: the state of the art modeling capability of surface energy and hydrologic processes from CLM3 and the more realistic physical process based modeling capability of subsurface hydrologic processes from TOUGH2. The preliminary simulation results show that the coupled model greatly improves the predictions of the water table, evapotranspiration, surface temperature, and moisture in the top 20 cm of soil at a real watershed, as evaluated from 18 years of observed data. The new model is also ready to be coupled with an atmospheric simulation model, representing one of the first models that are capable to simulate hydraulic processes from top of the atmosphere to deep ground.