Effects of thermal gradients on the evaporation from a dry soil surface

Abstract

The role of thermal gradients played in the evaporation process from bare soil after the dry surface layer (DSL) forms is discussed. Water moves only in the vapor phase within the DSL. There exist two mechanisms by which water vapor transfer is influenced by temperature gradients ; that is, thermal diffusion and thermal dependence of saturation vapor density. The former can be ignored within the DSL, but the latter cannot be neglected when the daily amount of evaporation is much smaller than 0.1 mm day-1. On the other hand, in the thin air layer next to the ground surface, very large temperature gradients can exist during the day, when the water vapor is forced upward through the layer by thermal diffusion. Consequently, even if water vapor inversion occurs in the thin air layer, water vapor can move upward through the layer against the gradient.