Isothermal and nonisothermal evaporation from four sandy soils of different water repellency

Soil water evaporation is an important component of the surface energy balance. Evaporation also affects plant available water content of soil. Soil wettability is known to affect water-holding capacity and water How, but the impact of soil water repellency on evaporation has not been thoroughly investigated. The objective of the study was to measure and analyze isothermal and nonisothermal evaporation rates for four sandy soils showing different degrees of water repellency. Eight vertical soil columns (two of each soil) were exposed to isothermal conditions (20 +/- 1 degreesC), and another eight soil columns were exposed to nonisothermal conditions. During the nonisothermal experiment, the top boundary temperature was held constant at 21 degreesC, and the bottom boundary temperature was constant at 55 degreesC, thus maintaining a constant thermal gradient. After 195 d of evaporation, each column was sectioned to determine the soil water content distribution along its axis. A numerical model based on the Philip-de Vries theory was used to predict soil water flow. Deviations between predicted and measured values increased with increasing contact angle. Isothermal cumulative evaporation was 25% lower for water repellent soil than for the most wettable soil. Nonisothermal cumulative evaporation was 75% larger than isothermal evaporation for the wettable soil but only 14% larger than isothermal evaporation of the most water repellent soil. Evaporation and residual water contents, especially in the surface layer, were found to be strongly affected by soil wettability.