Experimental and numerical studies of dynamic variation of liquid water and water vapor flow in the vadose zone

Abstract

In arid and semi-arid regions, the depth of water table has a significant effect on evaporation. The soil in these regions is unsaturated, and the transport of liquid water and water vapor is accompanied by variation of the water table depth. In this study, we designed a laboratory experimental apparatus to conduct evaporation experiments of soil under shallow and deep water table conditions through continuous monitoring of water content and temperature variation in the vadose zone. A numerical model was used to simulate the coupled transport of liquid water, water vapor and heat in the vadose zone under two different water table elevations. The experimental and numerical results were in good agreement. Liquid water and water vapor fluxes driven by pressure head and temperature gradient were calculated and analyzed to illustrate the distribution of the water content. Both isothermal liquid flux and thermal vapor flux dominates the variation of water content. By determining the location of the evaporation front and analyzing its dynamic variation, we found that the evaporation front ranged from the surface to a depth of 2.5 cm under a shallow water table. For a deep water table, the evaporation front was nearly at a constant depth of 15 cm.