Using high-frequency water vapor isotopic measurements as a novel method to partition daily evapotranspiration in an oak woodland

Partitioning evapotranspiration (ET) into its constituent fluxes (transpiration (T) and evaporation (E)) is important for understanding water use efficiency in forests and other ecosystems. Recent advancements in cavity ringdown spectrometers (CRDS) have made collecting high-resolution water isotope data possible in remote locations, but this technology has rarely been utilized for partitioning ET in forests and other natural systems. To understand how the CRDS can be integrated with more traditional techniques, we combined stable isotope, eddy covariance, and sap flux techniques to partition ET in an oak woodland using continuous water vapor CRDS measurements and monthly soil and twig samples processed using isotope ratio mass spectrometry (IRMS). Furthermore, we wanted to compare the efficacy of δ²H versus δ¹⁸O within the stable isotope method for partitioning ET. We determined that average daytime vapor pressure deficit and soil moisture could successfully predict the relative isotopic compositions of soil (δe) and xylem (δt) water, respectively. Contrary to past studies, δ²H and δ¹⁸O performed similarly, indicating CRDS can increase the utility of δ¹⁸O in stable isotope studies. However, we found a 41–49% overestimation of the contribution of T to ET (fT) when utilizing the stable isotope technique compared to traditional techniques (reduced to 4–12% when corrected for bias), suggesting there may be a systematic bias to the Craig-Gordon Model in natural systems.