Using Stable Isotope Hydrology to Partition Evapotranspiration in the Sagebrush Steppe

Evapotranspiration (ET) is a major part of ecosystem water loss. This thesis aims to develop methods that partition soil water loss into evaporation (E) and transpiration (T). Water balance methods may improve with relative contributions of evaporation (E) and transpiration (T). Isotopic fractionation distinguishes soil water loss due to evaporation from that of plant uptake. This provides a means to assess E and T in retrospect rather than only measuring ET fluxes. To measure the isotopic composition of soil water, we used a liquid-vapor equilibration method following Wassenaar (2008). Experimental trials of different soil amounts and equilibration times were performed to adapt the liquid-vapor equilibration method for dry desert soils. We tested a silty loam soil with 10%, 5%, and 2% gravimetric water content (GWC) and found time-to-equilibration was 3, 4, and 5 days, respectively. Second, we tested the ability of a simplified isotope mass balance model to predict 100% E following Wenninger et al. (2010), under controlled conditions with no plants available to remove soil water for T. The simplified model resulted in 99% E (+/- 4.3%). This was the first experiment to test this model under controlled conditions. Third, we used the simplified model to assess changes in E and T across microsites, at a sagebrush steppe field site in southern Idaho. We expected the proportion of E:ET to change with time and vegetation type. Soil water loss and changes in isotopic composition from 0-10 cm were measured in a 72-hour time series. We evaluated ratios of E:ET in sagebrush, bunchgrass, and bare ground microsites in June and September 2014. In September, sagebrush used 18% of soil water from the upper 10 cm for transpiration. Bunchgrass microsite type did not use the near-surface water for transpiration. This method appears to be successful in measuring E:ET ratios in retrospect and may be used to further understand water losses in the sagebrush steppe and improve water balance methods