Investigation of Kinetic Isotopic Fractionation of Water During Bare Soil Evaporation

The kinetic fractionation factor (alpha(K)) controls to a large extent the isotopic enrichment of surface waters during evaporation (E). In contrast to the well-known vapor-to-liquid isotopic equilibrium fractionation factor, alpha(K) has still not yet been properly characterized for soil water evaporation. In this study, we investigated the alpha(K) daily dynamics during a series of three laboratory experiments differing in soil water availability and aerodynamic conditions. For this, we applied a commonly used isotopic evaporation model and tested it in two different approaches. First, a two-end-member mixing model (Keeling plot) was fitted to the measured isotopic composition of the laboratory air water vapor to obtain alpha(K). In a second approach, alpha(K) was obtained from the slope of the evaporation line in a dual isotopic coordinate system. For both methods, the isotopic composition of the soil water was determined nondestructively and online by sampling the soil water vapor with gas-permeable microporous tubing. Results highlighted the limitation of the first approach, as the determination of the isotopic composition of E with the Keeling plot was challenging with the laboratory setup. The second approach provided alpha(K) values within the range (alpha(2H)(K) = 1.0132 +/- 0.0013; alpha(18o)(K) = 1.0149 +/- 0.0012) reported in the literature and pointed to the prevalence of turbulent water vapor transport under water-saturated soil conditions but also at soil water content significantly lower than the saturated value. In a third experiment, temporal dynamics of the atmospheric water vapor intrusion in the topmost soil layer could be observed during an isotopic labeling pulse.