A Parameterization of Evaporation from Bare Soil Surfaces

A simple model of evaporation from a bare soil surface is developed. This model combines two processes of water vapor transport: one is the vapor transport in air expressed by the bulk formula, and the other is molecular diffusion of vapor in the surface soil pore with the vapor being carried from the interior of the soil pore to the land surface. The resistance to the vapor diffusion in the soil pore is expressed using a new parameter, estimated by experimental means. General formulation of the so-called “surface moisture availability” is expressed with this model. The formulation shows that the “surface moisture availability” depends not only on volumetric soil moisture, but also on wind velocity, and on the ratio of the specific humidity of the air to that of the saturation value at the soil surface temperature. This dependence agrees with experiments performed with loam and sand under various conditions. In the evaporation parameterization used in current numerical simulations, the humidity of the air adjacent to the water in the soil pore, which is determined thermodynamically, is often substituted for the land surface humidity. The present study suggests, however, that such a parameterization is invalid except for saturated conditions.