Liquid and Vapor Water in Vadose Zone Profiles Above Deep Aquifers in Hyper‐Arid Environments

Water vapor is a key element of the water regime in unsaturated profiles above deep aquifers in hyper‐arid regions. However, the interactions between water phases and the resulting evaporation and condensation are poorly understood under such conditions. The main driver for vapor condensation in deep vadose zone profiles is the geothermal gradient, displaying a decrease in temperatures toward the soil surface, thereby promoting condensation. We have analyzed the water regime in deep unsaturated profiles, with and without the geothermal gradient, and considered two types of hydrological scenarios: (1) assuming hydraulic continuity of liquid water over the entire profile and (2) assuming the presence of an evaporative front in the profile above which water flows to the surface in the vapor phase. We considered homogeneous profiles of two soil types, investigating the distribution with depth of the different state variables: temperature, relative humidity, water potential, and vapor pressure and concentration. We found that during evaporation, only extreme conditions of high relative humidity near the surface could lead to condensation. In addition, even when hydraulic continuity of liquid water is assumed over the entire soil profile, potential condensation amounts are very small, practically negligible. For the case of a water table at 200‐m depth, condensation occurs only when the relative humidity at the surface is above 95% and is less than 1.5% of the amount of water in the vapor phase in the profile. Key Points Water vapor and its potential condensation are analyzed in unsaturated profiles above deep water table in hyper‐arid environments Conditions of no flow (hydrostatic), downward flow (recharge), and upward flow (evaporation) are considered The potential amount of water from condensation is very small, practically negligible