Evapotranspiraton Partitioning Using Stable Water Isotopes in a Semi-Arid Evergreen Forest

Total evapotranspiration (ET) is the key process that links the land and the atmosphere via water, energy and carbon exchange. ET is a combination of evaporation and transpiration, which behave dynamically in very different ways. In this work we investigate the relative contribution of transpiration and soil evaporation to total ET in a semi-wooded, semi-arid forest in the Manitou Research Park northwest of Colorado Springs, CO. We use stable water isotopes measured at different levels within and outside the canopy, over a 30-day period (June 26 - July 26, 2010), using a field-deployable cavity ring-down spectrometer. The traditional "Keeling plot" analysis is used to partition the ET flux from moisture that comes from outside of the ecosystem, and then a simple model is used to partition the transpiration flux. In addition, we introduce a new alternative "multi-level" method to calculate the fraction of transpiration to total ET. Both the "Keeling plot" method and the "multi-level" method yield very similar fractions of transpiration to total ET, ranging from about 15% to about 85%. We compare both methodologies and discuss some of the corrections that must be made when measuring with high-frequency field-deployable instruments