Description of the Biosphere-Atmosphere Transfer Scheme (BATS) for the Soil Moisture Workshop and evaluation of its performance

Abstract

This paper has two major emphases. First, a concise description of the framework of the Biosphere-Atmosphere Transfer Scheme (BATS) is given to help better understand the results of research including BATS. Focus is put on the formulations of drag coefficients, soil surface evaporation, infiltration, surface runoff, drainage, soil internal water fluxes, canopy interception loss and transpiration, and soil water budgets. Secondly, the simulations from BATS using the default parameters are compared with those using the parameters provided during the workshop, as well as with observations. The simulations of soil moisture content from these two sets of parameters are similar, and are both in agreement with observations. In particular, the model results are discussed in the context of the applicability of the demand-supply approach. We have examined the model's sensitivity to the parameters in the formulations of runoff components. The excessive surface runoff can be fixed using a modified scheme for surface runoff. When the averaged soil moisture threshold is close to field capacity, the modified surface runoff scheme produces results essentially identical to the default scheme. With the modified scheme, in which the averaged soil moisture threshold is set to unity, the best simulation of annual evaporation and runoff components has to be obtained at the expense of the soil moisture content in the bottom layer.