Evaporation From Unsaturated Soils as a Function of the Air and Soil Sides of the Land Surface

Abstract

A theoretical framework for computing the evaporation from unsaturated soils is presented and validated based on laboratory experiments that were conducted in an uncontrolled environment where forcing variables vary in time according to diurnal meteorological cycles. This theory introduces a dimensionless number that controls the ratio between the actual and potential evaporation from unsaturated soils. The dimensionless number depends on the transfer velocity, which characterizes evaporation into the atmosphere, the diffusion coefficient of water vapor in the soil, and the water table depth. We show that depending on the value of the dimensionless number, evaporation can be limited by either the air-side or the soil-side of the land surface. For large transfer velocity values, evaporation is controlled by water vapor diffusion in the soil, while for shallow water tables, evaporation is controlled by water vapor transport from the land surface into the atmosphere. Despite the good agreement between the shape of the observed and predicted evaporation rates, a fitted dimensionless parameter is required to match the predicted evaporation rates. Possible explanations for this disagreement are given in the discussion.