Utilizing High Temporal Resolution Stable Isotope, Sap-Flux, and Eddy Covariance Measurements to Partition Daily Evapotranspiration in an Oak Woodland

The fractional contribution of transpiration (T) to total evapotranspiration (ET), fT, is an important indicator of water use efficiency in forests and other ecosystems, and an improved understanding of fT is necessary for refined water resource management. Recent advancements in cavity ringdown spectrometers have made the collection of high temporal resolution water isotope data possible, yet this technology has not been fully utilized for determining fT and partitioning ET. The primary objective of this study was to effectively partition ET in an oak woodland using stable isotopes, sap-flux, and eddy covariance techniques on days where soil and twig sampling provided real values for δe and δt. We then wanted to model δe and δt on days when samples were not collected, and to compare the efficacy of using δ2H versus δ18O within the stable isotope method. Transpiration was determined by fitting eleven oak (Quercus stellata and Quercus marilandica) trees with thermal dissipation sap flow sensors and averaging 30-sec measurements over 30-min intervals. Isotope turbulent mixing relationships (Keeling Plots) were used to determine δET, while δE was calculated using a combination of the Craig-Gordon model and monthly soil and twig samples to verify δe and δt. It was determined that average daytime vapor pressure deficit (VPD) and soil moisture were the best predictors of δe and δt, respectively. These models were significant at the 95% confidence level for both stable isotopes for twigs (δt), but only δ18O for soil (δe). There was a 41 - 49% overestimation of fT when utilizing the stable isotope technique compared to T/ETEC or T/ET0. When normalizing fT for either δ18O and δ2H, the overestimation was reduced to 4 - 12%, suggesting there may be a systematic bias to the CGM leading to overestimation of fT in natural systems. When comparing δ18O and δ2H within the stable isotope method, there was much agreement between the two, which suggests that higher resolution data can lead to a greater utility of δ18O in stable isotope studies. However, δe models were insignificant during the dormant season, suggesting that the CGM may not perform under these conditions in natural systems