Water vapor transfer beneath bare soil where evaporation is influenced by a turbulent surface wind

Abstract

A laboratory experiment on evaporation in various porous material layers was undertaken by applying a turbulent air flow to the exposed surface, to investigate the mechanism of the water vapor transport beneath a bare ground surface influenced by a turbulent surface wind. Variation of air pressure within these layers was also measured. Based on a physical model of water vapor transfer, an observed function decribing bulk resistance for water vapor transfer with evaporating front depth was investigated, using an observed power spectrum of the pore-air pressure. The model is based on the so-called turbulent mixing theory. The following conclusions were reached. When porous material layers are of low permeability such as fine sand, water vapor transfer under the exposed surface occurs by molecular diffusion as well as by turbulent diffusion caused by variation of the pore-air pressure due to the turbulent surface wind (P-type). The surface turbulence significantly affects the transport just beneath the surface. Turbulent diffusivity near the surface is determined by horizontal and vertical fluctuating components of the pore-air pressure, but with increasing depth is driven only by the vertical component. When the porous material layer is more permeable, water vapor transfer in this layer is analogous to that within a vegetation canopy (F-type), so that turbulent diffusivity decays exponentially with depth. The turbulent diffusivity is a function of the mixing length. The mixing length for F-type was of the order of the particle size. For P-type the mixing length ranged from the particle size to ten times larger.