The Drying of Soil: Thermal Regimes and Ambient Pressures

Evaporation from a loam soil was studied in the laboratory during the falling-rate stage of drying. The primary variables were temperatures regimes and ambient pressures. The data suggested that: (a) there is a significant relation between the soil temperature and the vapor pressure of water in the air above the surface; (b) the water vapor transfer through the dry layer at the surface is accomplished in part by viscous air flow; (c) diurnal thermal gradients may cause a significant movement of water near the soil surface during the falling-rate stage of drying. Consequently, the conclusion follows that the isothermal "soil-moisture diffusion theory" cannot be expected to describe drying during time intervals of less than 24 hours. Because of the periodic nature of the thermally induced transfer phenomena, the "diffusion theory" will give curves which approximate cumulative drying over long time periods, though such relations must be considered as semi-empirical