Observations on Factors Influencing the Evaporation of Soil Moisture

Important variables which influence the rate of evaporation of soil moisture are (1) the vapor pressure difference between the layer from which water is evaporating and that of the turbulent atmosphere and (2) the resistance to vapor flow of the intervening layer. The vapor pressure difference is associated with the temperature of the layer from which evaporation is taking place and the temperature and relative humidity (vapor pressure) of the turbulent air. While capillarity moves water fast enough to keep the surface moist, the principal resistance to vapor flow is caused by the thin layer of nonturbulent air adjacent to the surface. Resistance increases rapidly as the surface dries so that water vapor moves through a thickening layer of dry soil. Adsorption of moisture by the dry layer and re‐evaporation associated with the diurnal temperature cycle complicate the process. Mositure movement in the upper layers of soil during the evaporation cycle was studied by periodic weighing of soil in vertically stacked porous bottomed containers and through the use of tensiometers. The data show the highly dynamic nature of the moisture regime in the surface layers of soil associated with diurnal temperature changes and the evaporation cycle.