Response of land surface fluxes and precipitation to different soil bottom hydrological conditions in a general circulation model
Very different approaches exist in land surface models (LSMs) to describe the water fluxes at the soil bottom, from free drainage to zero flux, and even upward fluxes if the soil is coupled to a water table. To explore the influence of these conditions on the water cycle in a unified framework, we introduce new boundary conditions in the ORCHIDEE LSM, which is coupled to the atmospheric general circulation model LMDZ. We use a zoomed and nudged configuration centered over France to reproduce the observed regional weather. Soil moisture and evapotranspiration increase ranging from free drainage to impermeable bottom, then by prescribing saturation closer and closer to the surface. The corresponding response patterns can be related to both climate regimes and soil texture. When confronted to observations from the SIRTA observatory 25 km south of Paris, which exhibits a shallow water table, the best simulations are the ones with prescribed saturation. The local precipitation, however, is only increased if the new bottom boundary conditions are applied globally. The magnitude of this increase depends on the evaporation and on the relative weight of local versus remote sources of moisture for precipitation between Western and Eastern Europe. This suggests that the summer warm/dry bias of many climate models in this region might be alleviated by including a sufficiently realistic ground water description.