Season-specific evapotranspiration partitioning using dual water isotopes in a Pinus yunnanensis ecosystem, southwest China

Quantifying the regional evapotranspiration is critical in terrestrial water balance and global water cycle, while partitioning of evapotranspiration is challenging but fundamental to predict the fate of terrestrial ecosystems under climate changes. Here, we performed in-situ measurements of water isotopes in atmospheric vapor, plant tissues and soil pools in a Pinus yunnanensis forest ecosystem in southwest China, aiming to partitioning ET by estimating the stable isotopic compositions of ET and that of its two components, i.e., plant transpiration and soil water evaporation. We used combined high-frequency laser spectroscopy and chamber methods, to constrain the estimates of T/ET. We first found that the estimated daily T/ET ratio ranges from 0.59 to 0.81, with an apparent increasing shift in the early growing season and maintaining a plateau level of over 0.75 during the peak growing season. This higher averaged T/ET of 0.73 +/- 0.06 indicates that plant transpiration is the main component of evapotranspiration. The estimated dE and dT are in agreement with result from customized chamber method. We also found that in monsoon season (in June-September), soil water content is the main control of T/ET variations, with leaf area index playing only a partial role. Our study confirms the critical impact of soil water on the seasonal change of T/ET in Pinus yunnanensis ecosystem such as the SW China. However, we are also aware the sensitivity of controls on estimated T/ET at different time scale interested. Our results here provide insight into the regional hydrological cycle in alpine forest ecosystem and potentially benefits many applications from forest ecosystem protection to paleo isotope archives.