Partitioning Water Vapor Fluxes by the Use of Their Water Stable Isotopologues: From the Lab to the Field

Abstract

Water stable isotopes are powerful tracers for partitioning of the terrestrial ecosystem water vapor fluxes into process-based components, i.e. evapotranspiration (ET) into soil evaporation (E) and plant transpiration (T). The isotopic methodology for ET artitioning is based on the fact that E and T have distinct water stable isotopic compositions, which in turn are due to each flux being differently affected by isotopic kinetic effects. To use stable isotopologues of water in ET partitioning studies, a good knowledge of the isotopic (equilibrium and kinetic) fractionation effects is crucial. While the temperature-dependent equilibrium fractionation factor is well characterized (Majoube 1971), the kinetic fractionation factor (αK), relevant, e.g., during soil evaporation, needs further investigation. In order to address this knowledge gap, we conducted a series of three different long-term bare soil evaporation experiments (differing in soil-water availability and aerodynamic conditions) to obtain αK values from the collected isotopic data and the inversion of a well-known resistance-totransfer model (i.e., the Craig and Gordon (1965) model). The isotopic composition of the soil water (δs) vapor was monitored non-destructively by using gas-permeable tubing (Rothfuss et al. 2013).The Craig and Gordon (1965) model was used in two different approaches. The first approach uses the Keeling (1958) plot to obtain values for the isotopic composition of the evaporation (δE). The second approach uses the slope of the linear regression between δs 2H and δs 18O. Results showed that the largest source uncertainty in the computation of αK stemmed from the uncertainty associated with the δE values modeled with the Keeling (1958) plot method. In the second approach αK values werewithin the theoretical range proposed by Dongmann et al. (1974) and Mathieu and Bariac (1996), which pointed to the prevalence of the turbulent transport of water vapor under saturated and unsaturated soil conditions