Partitioning of evapotranspiration through high frequency water vapor isotopic measurement over a paddy field

Abstract:

Partitioning the ecosystem evapotranspiration (ET) into soil evaporation (E) and transpiration (T) is challenging and crucial to understand hydrological processes in land. In this study, we deployed a recently developed partitioning technique using high frequency in-situ water vapor isotopic measurement and surface/soil water sampling over a paddy field near Tokyo, Japan for a full growing season. We partitioned ET via estimating the isotopic signal of ET (δET) and its two components, i.e. T (δT) and E (δE). δET was determined through the use of Keeling plot. Estimates of δT was obtained by direct measurement of surface/soil isotopic measurement assuming a steady state of transpiration. δE was estimated using the Craig-Gordon model in combination with surface and vapor water isotopic measurement. The T/ET ranged approximately from 0.3 to 1, almost kept increasing during early growing season, then kept a relative constant value close to 1 after that. T/ET was strongly controlled by green leaf area index (LAI) in day to day time scale (y=0.14ln(x)+0.66; R-squared=0.62). We also investigated the uncertainties in estimating these three terms (δET, δT, and δE) and their effect on ET partitioning. The uncertainty analysis showed the uncertainty from δE had a minor impact on the ET partitioning but mainly controlled by uncertainty from δT and δET estimations. During early growing season, ET partitioning uncertainty was mainly due to uneven spatial distribution of surface water isotope (δT). As LAI increases, the impact of δET uncertainty became much larger than the other two terms. The results demonstrated the robustness of using isotope measurements for partitioning evapotranspiration.