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

Abstract

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.

Water Vapor Transport Across an Arid Sand Surface—Non‐Linear Thermal Coupling, Wind‐Driven Pore Advection, Subsurface Waves, and Exchange With the Atmospheric Boundary Layer

“…Its effects are captured by BC (40) at the bottom of our near‐surface domain without liquid water. This BC can also be used to match our model of the near‐subsurface to solutions for the deeper, moister domain below, for example, those of Kamai and Assouline (2018), Assouline and Kamai (2019), and Shao et al. (2021), or our own integration of Richards' equation (Louge et al., 2013; Richards, 1931).…”

“…The instantaneous evaporation flux exchanged with the ABL from the vadose zone is a crucial BC for understanding the hydrology (Assouline & Kamai, 2019; Shao et al., 2021) and microbiology (Kidron & Starinsky, 2019) of hyper‐arid regions. Because it is important to distinguish evaporation from transpiration when vegetation is involved (R. G. Anderson et al., 2017), flux techniques deployed in the ABL, such as eddy‐covariance, are supplemented by carbon dioxide measurements (Scanlon & Kustas, 2010), sometimes involving stable isotopes (Griffis, 2013).…”

“…Finally, the data set provides a unique "ground truth" for the moisture flux through a hyper-arid surface into the atmospheric boundary layer (ABL). As such, it has the potential to complement models of deeper moisture exchange in deserts, such as those of Kamai and Assouline (2018), Assouline and Kamai (2019), Shao et al (2021) and, when the near-surface holds no liquid water, to revisit the classical formulations of Philip and De Vries (1957), Jury and Letey (1979), Parlange (1980), Brutsaert (1982Brutsaert ( , 1986, and Cahill and Parlange (1998) for water transport in wetter soils.…”

Modeling Transient Evaporation From Porous Media as a Succession of Steady‐State Steps

“…Steady-state implies that water flow rate is constant throughout the profile; thus, E(t) is equal to the liquid and vapor water flow rates in the profile at every moment (Kamai & Assouline, 2018). At the EF, we impose continuity between h c in the liquid zone and c in the vapor zone, assuming equilibrium between the phases, and compute the relative humidity RH accordingly (Assouline & Kamai, 2019;Campbell & Norman, 1998)…”

Compaction effects on evaporation and salt precipitation in drying porous media

“…Evaporation plays a central role in the hydrologic cycle and surface energy balance (Bergstad et al, 2018) as it is the main process of soil-water transfer to the atmosphere (Hillel, 1980;85 Brutsaert, 2005). The evaporation in porous media is affected by and involves complex and highly dynamic interactions between boundary conditions, liquid flow and vapor diffusion (Lehmann et al, 2008;Or et al, 2013;Assouline et al, 2014;Kamai and Assouline, 2018;Assouline and Kamai, 2019). The evaporation process from bare soils consists of two stages: stage 1 (S1); and stage 2 (S2).…”