Evaporation beneath the soil surface: some observational evidence and numerical experiments

In order to re-examine the process of bare soil evaporation, field observations and numerical experiments were carried out. Vertical distributions of water content, temperature and humidity were measured with a lysimeter in which fine sand was uniformly packed under field conditions. Numerical experiments were conducted by using a high resolution model of coupled water and heat flow for three example soils under constant or diurnally varying atmospheric conditions. Comparison between the results of the field observation and the numerical experiments revealed that evaporation can actually take place beneath the soil surface, mainly at the bottom boundary of the dry surface layer (DSL). In addition, the results of numerical experiments under different experimental conditions indicated that the thickness of the evaporation zone located at the bottom boundary of the DSL depends on the hydraulic properties of the soil, and that evaporation can also take place transiently within the DSL under varying atmospheric conditions.