Stefanie Kiemle

Abstract

Abstract 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.

Abstract

Abstract The atmosphere-soil system forms a highly coupled system, which makes key processes such as evaporation complex to analyze as the mass, energy, and momentum transfer is influenced by both domains. To enhance the understanding of evaporation processes from soils, stable water isotopologues are suitable tools to trace water movement within these systems as heavier isotopologues enrich in the residual liquid phase. Due to the complex coupled processes involved in simulating soil-water evaporation accurately, quantifying fractionation during flow and transport processes at the soil-atmosphere interface remains an open research area. In this work, we present a multi-phase multi-component transport model that resolves flow through the near-surface atmosphere and the soil, and models transport and fractionation of the stable water isotopologues using the numerical simulation environment DuMux. Using this coupled model, we simulate transport and fractionation processes of stable water isotopologues in soils and the atmosphere by solving compositional flow equations and by using suitable coupling conditions at the soil-atmosphere interface instead of commonly used parameterization. In a series of examples of evaporation from bare soil, the transport and distribution of stable water isotopologues are evaluated numerically with varied conditions and assumptions, including different atmospheric conditions (turbulent/laminar flow, wind speed) and their impact on the spatial and temporal distribution of the isotopic composition. Building on these results, we observed how the enrichment of the isotopologues in soil is linked with the different stages of the evaporation process. A qualitative study is conducted to verify single fractionation processes in our approach.