The kinetic fractionation factor (αK) 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, αK has still not yet been properly characterized for soil water evaporation. In this study, we investigated the αK 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 αK. In a second approach, αK 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 non-destructively 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 αK values within the range 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.
