Abstract:
Soil moisture movement in the vadose zone is a multi-filed coupling process largely controlled by the depth of water table. The purpose of this study is to investigate the mechanisms of moisture dynamics in the vadose zone under the combined influence of groundwater levels and evaporation. This study investigated coupled moisture and heat transport in the vadose zone under evaporative conditions through laboratory experiments and numerical simulation. Based on the height of capillary water rise, two distinct water table depths, shallow and deep, were examined to explore differences in soil moisture behavior. Results show that under a shallow water table, strong hydraulic connectivity is established between the soil surface and water table, and water content significantly decreases due to evaporation. In contrast, under a deep water table, a near-surface zone of low and weakly variable water content develops, where upward liquid water fluxes and downward water vapor fluxes achieve equilibrium. Isothermal liquid water fluxes and thermal water vapor fluxes are the main forms of soil moisture movement. Matric potential significantly influences water vapor movement only within 0.3 cm of the soil surface, while the influence of temperature on liquid water movement can be ignored. Water vapor movement under deep water table is more intense than that under shallow water table. The moisture movement in the vadose is governed by both evaporation and groundwater levels. This study can provide reference for analyzing the water movement in the vadose zone using fully coupled simulation methods, which is helpful to moisture conservation and environmental protection in the arid and semi-arid areas.