A multi-layer soil model is constructed taking into consideration the diffusion of water vapor and vaporization in the soil pores. The resistance to vaporization in the pores is introduced as a function of the volumetric soil water content. The soil layer is treated as a ‘porous canopy’ and the vapor density profile in the soil is calculated by an equation for vapor diffusion. The model simulates the evaporation rate, the soil temperature, and the soil water content profile rather well when compared to measurements from field observations and pan experiments.
Recent numerical models usually assume that the water vapor found in the soil pores is in equilibrium with the soil water at each depth, with the equilibrium humidity expressed by thermodynamical theory (Philip, 1957). However, the present model calculations reveal that the relative humidity in the pores close to the ground surface is different from that at equilibrium, since the water vapor is continuously transported to the atmosphere during the drying stage.