Validity of Assuming Equilibrium Between Liquid Water and Vapor for Simulating Evaporation

Abstract

AbstractThe standard assumption used in evaporation theory that at any location in a soil water vapor density is equal to the value in thermodynamic equilibrium with the liquid water has recently been questioned. This paper defines precise criteria for nonequilibrium to occur during evaporation in soils. In doing so a physically based model of the pore transfer coefficient critical to quantifying the degree of nonequilibrium is developed. The likelihood of departure from the equilibrium assumption for various soil textures and temperature conditions, including the extreme case of surface fire, is presented. Furthermore the pore transfer coefficient is incorporated into steady‐state nonequilibrium simulations of evaporation from a coarse sand under field conditions and compared with the corresponding equilibrium simulation. Outside of the narrow subsurface evaporation zone there were no significant differences between the nonequilibrium and equilibrium simulations. Since nonequilibrium effects were most likely to be seen with the coarse sand abandoning the standard assumption in future work is unnecessary, and all previous work based on it need not be questioned.