Dynamics of the evaporation zone in dry sandy soils

Abstract

To evaluate the effect of soil moisture deficit on the restriction of evaporation from bare soil surfaces, the process of evaporation in dry soil was investigated from field observations at a fallow site and a sand dune site. Analysis of the surface energy balance showed that soil drying affects not only the inter-diurnal variation of energy fluxes, but also their diurnal variation. In addition, the relationship between the surface resistance ( r s) and soil water content ( θ) of surface soil was found to be different between inter-diurnal and diurnal time scales. Stable isotopic analysis of soil water showed that liquid water transport from deeper soil layers stops at the bottom boundary of the dry surface layer (DSL) and that vapor water transport is dominant in the DSL. In addition, diurnal variations in water content and soil air humidity suggest that evaporation and condensation occurs alternatively within the DSL with diurnal cycle due to cyclic change in solar radiation and soil temperature. From these observational results, a conceptual model of the dynamics of the evaporation zone in dry sandy soils is presented. In the model, the evaporation zone consists of two sub-zones: a narrow sub-zone around the bottom boundary of the DSL, and a relatively wide sub-zone within the DSL. The former is active through a day and moves vertically with the development/reduction of the DSL at both diurnal and inter-diurnal time scales. The latter acts as evaporation zone only in the morning and acts as condensation zone in the late afternoon and at night. The dynamic behavior of the evaporation zone and those related to the form of water content profile can explain the difference in the r s– θ relationship between inter-diurnal and diurnal time scales.