Evaporation From Deep Aquifers in Arid Regions: Analytical Model for Combined Liquid and Vapor Water Fluxes

Evaporation is a significant part of the water cycle in hyper-arid environments. The subsurface of these deserts is characterized by deep groundwater with negligible recharge, whereby water flows from the water table to the surface and evaporates. We propose an analytical model to predict the evaporation rate and the position of the evaporative front. The model accounts for water table depth, atmospheric conditions, and soil hydraulic properties. We consider steady state flow, with two distinct regions separated by an evaporative front, liquid-phase flow from the water table to the front and vapor-phase flow from the front toward the surface. The driving forces are pressure head gradients for Darcian liquid flow, and thermal and relative humidity gradients for Fickian diffusive vapor flow. Evaporation rates are predicted for different soil types. The impact of constitutive models applied for characterizing these soils, groundwater depth, and atmospheric conditions are evaluated. Evaporation increases as groundwater levels are shallower, and as atmospheric temperatures increase and/or relative humidity values decrease. Evaporation decreases exponentially with groundwater depth, approaching a constant value of about 0.02 mm per year under typical atmospheric conditions and water table depths below 500 m. The impact of soil type and other related uncertainties are important when groundwater is shallower than 300 m. The relative portion of the liquid phase region increases compared to that of the vapor one as evaporation rates increase. The actual size of the liquid phase flow region, however, reaches its maximum when the water flux approaches zero at hydrostatic conditions.