| Summary: | Partitioning evapotranspiration (<i>ET</i>) into its constituent fluxes (transpiration (<i>T</i>) and evaporation (<i>E</i>)) 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 <i>ET</i> 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 <i>ET</i> 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 <i>δ<sup>2</sup></i>H versus <i>δ<sup>18</sup></i>O within the stable isotope method for partitioning <i>ET.</i> We determined that average daytime vapor pressure deficit and soil moisture could successfully predict the relative isotopic compositions of soil (<i>δ</i><sub>e</sub>) and xylem (<i>δ</i><sub>t</sub>) water, respectively. Contrary to past studies, <i>δ<sup>2</sup></i>H and <i>δ<sup>18</sup></i>O performed similarly, indicating CRDS can increase the utility of <i>δ<sup>18</sup></i>O in stable isotope studies. However, we found a 41–49% overestimation of the contribution of <i>T </i>to <i>ET</i> (<i>f<sub>T</sub></i>) 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.
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