Arid and hyper-arid deserts are characterized by having deep groundwater and negligible recharge by precipitation. Populations in these areas heavily rely on groundwater, so any loss of groundwater by evaporation could be critically important for sustaining their water supply. Evaporation in these environments is also important at the global scale, as hyper-arid deserts represent 10% or more of the terrestrial landscape; therefore, the water balance in these areas, and especially the evaporation component, may play an important role in the global hydrologic cycle. Hence, it is important to improve our ability to quantify evaporation under dry conditions for understanding the local and global impacts of evaporation on the hydrologic cycle. The subsurface of these deserts is characterized with deep groundwater and negligible recharge, whereby water flows from the water table to the surface and evaporates to the atmosphere.
For modeling evaporation under such conditions, we consider steady-state water flow, with liquid-phase flow from the water table and vapor-phase flow towards the surface, separated into two distinct regions of liquid- and vapor-phase flow by an evaporative front that is located within the subsurface. The driving forces for evaporation are pressure head gradients for Darcian liquid flow, and thermal and relative humidity gradients for Fickian diffusive vapor flow. The model accounts for water table depth, atmospheric conditions, and the hydraulic properties of the media. We demonstrate how evaporation predictions are affected by soil type, groundwater depth and atmospheric conditions. The partitioning between liquid and vapor phase flows is presented for the different evaporation rates and conditions. The effect of the geothermal gradient is evaluated and the extent and intensity of water vapor condensation in the soil profile is estimated. Results show that evaporation rates are enhanced as groundwater levels are closer to the surface, and as atmospheric temperatures increase and/or relative humidity values decrease. Evaporation rates drop significantly when the water table deepens from 50 to 200 m, but are almost constant for groundwater levels below ~500 m. The impact of soil type and other related uncertainties are mostly important for water tables with depths shallower than ~300 m. The contribution of condensation to the water regime in the profile, to the recharge of the deep aquifers, and to the hydrologic cycle of such systems, appeared to be very small, practically negligible.
