Evaporation from Porous Media in the Presence of a Water Table

Evaporation from porous media in the presence of a water table is influenced by the water table depth and the hydraulic characteristics of the medium, among other factors. For shallow water tables, the upward capillary liquid flow maintains continuous liquid pathways connecting the water table to the surface. For deeper water tables, however, the hydraulic continuity may be disrupted due to the opposing gravity and viscous forces. The main objective of this study was to demonstrate the limitation of the common assumption of liquid continuity through the unsaturated zone across large distances above a water table. The concept of the evaporation characteristic length was used to predict the maximum depth of the water table hydraulically connected to the surface. When the water table depth exceeded this characteristic length, the evaporative water loss was significantly suppressed due to the loss of liquid continuity. For model comparison, experimental data from the literature and data from laboratory evaporation experiments were used. Experimental results from cylindrical columns packed with sand with different particle size distributions connected to water tables fixed at various depths below the surface confirmed the existence of a finite characteristic length indicating the maximum depth of the water table hydraulically connected to the surface. The experimentally determined characteristic lengths were in a good agreement with the theoretical predictions. The obtained results also provided new insights regarding the solute transport and deposition patterns during evaporation in the presence of a water table.