Analysis of experimental and simulation data of evaporation-driven isotopic fractionation in unsaturated porous media

Stable water isotopologs can add valuable information to the understanding of evaporation processes. The identification of the evaporation front from isotopolog concentration depth profiles under very dry soil conditions is of particular interest. We compared two different models that describe isotopolog transport in a drying unsaturated porous medium: SiSPAT-Isotope and DuMux. In DuMux, the medium can dry out completely whereas in SiSPAT-Isotope, drying is limited to the residual water saturation. We evaluated the impact of residual water saturation on simulated isotopic concentration. For a low residual water saturation, both models simulated similar isotopolog concentrations. For high residual water saturation, SiSPAT-Isotope simulated considerably lower concentrations than DuMux. This is attributed to the buffering of changes in isotopolog concentrations by the residual water in SiSPAT-Isotope and an additional enrichment due to evaporation of residual water in DuMux. Additionally, we present a comparison between high-frequency experimental data and model simulations. We found that diffusive transport processes in the laminar boundary layer and in the dried-out surface soil layer need to be represented correctly to reproduce the observed downward movement of the evaporation front and the associated peak of isotopolog enrichment. Artificially increasing the boundary layer thickness to reproduce a decrease in evaporation rate leads to incorrect simulation of the location of the evaporation front and isotopolog concentration profile. Isotopolog fractionation in the unsaturated zone was studied. A comparison between high-frequency experimental data of isotope enrichment and model simulations was carried out. Effects of drying below residual water content and of boundary conditions on isotope concentrations were analyzed. The evaporation front depth was estimated by using the depth of the concentration gradient maximum.