Using stable isotope hydrology to partition evapotranspiration in the sagebrush steppe

Stable isotope hydrology offers a method to partition evapotranspiration (ET) into its components evaporation (E) and transpiration (T). Our objective is to test a mass balance equation, presented by Wenninger et al. (2010), to evaluate the relative contributions of E and T to ET retrospectively based on stable isotope profile signatures in soil. We tested the approach under controlled laboratory conditions and in a heterogeneous, sagebrush-steppe ecosystem where microsites vary in contributions of T to vapor efflux. This presentation describes the theoretical and experimental approach to this study, along with an assessment of the approach under a simplified, purely evaporative condition in the lab, in which we asked whether the equation would correctly confirm that ET was entirely attributable to E.

Independent soil microcosms were created such that only the upward movement of water was allowed. Under constant radiation and wind, the soil water contents and corresponding isotopic composition of water were evaluated after wetting to field capacity and after a period of E. Under these conditions, the equation can be simplified to the evaporated fraction being equal to the initial liquid product minus the final, divided by the composition of the vapor. Water isotopes were measured using the liquid-vapor equilibration method and laser spectroscopy. Mean soil water across the profiles increased in d¹⁸O by 2‰, and the effluxing vapor was 5‰ depleted. Soil water decreased to 70% of field capacity. From these preliminary data the equation verified all (99%) vapor efflux was E. Using a similar approach in the field setting, we have observed greater variability in soil profiles. Factors affecting the performance and testing of this approach in the field will be discussed.