Mechanisms of evaporation from soil with a dry surface

Abstract

Evaporation from bare soil proceeds in three stages as the soil surface dries. The formation of a dry surface layer (DSL), where water moves only in the vapour phase, is the most remarkable and symbolic feature of the last stage, and the following phenomena, observed recently, seem to be closely related to it: (a) humidity inversion across the soil surface. Water vapour density increases sharply from just under the dry surface to just above it in the daytime on fine days; (b) humidity inversion in the surface boundary layer. The mixing ratio of water vapour increases with height, at least, in the 1 to 20 m layer in desert areas on fine days; (c) counter-gradient flow of water vapour. Evaporation occurs from soil with a dry surface under the humidity inversion conditions, so that water vapour is transported against its density gradient near the surface in the daytime on fine days. A hypothesis is proposed to explain these phenomena, such that hot, dry soil surfaces produce dry air, or the humidity inversion and make the water vapour moving upwards in the DSL go through the surface against its density gradient by thermal diffusion. The hot, dry air which is made and trapped in the uppermost soil pores can stay there under moist boundary air layers with lower temperatures. When instability occurs, however, the trapped hot, dry air bursts out like jets and the humidity inversion across the surface is transported upwards by convection, by which means humidity inversions are made and kept in the surface boundary layer. 1998 John Wiley & Sons, Ltd.