New isotope-based evapotranspiration partitioning method using the Keeling plot slope and direct measured parameters

Abstract. To better quantify water and energy cycles, numerous efforts to partition evapotranspiration (ET) into evaporation (E) and transpiration (T) have been made over the recent half century. Various methods such as direct measurements, analytical models and satellite-based estimations have been used to separate ET across the field scale to the global scale. One of the analytical methods, isotopic approach, has been often applied in terrestrial ecosystem ET partitioning. The isotopic composition of ET (δ_ET) is a crucial parameter in the traditional isotope-based ET partition model, which however has considerable uncertainty. Here we proposed a new method relying on Keeling plot slope (k), and relying on the direct measurements of atmospheric vapor concentration (C_v) and isotopic composition of atmospheric vapor (δ_v), to avoid the direct use of δ_ET. Mathematical derivation of the new method was provided, and field observations were used to evaluate the new method. The T/ET results based on the new method agreed well with those using the traditional isotopic method. The new method eliminates the high sensitivity contribution parameter δ_ET. In addition, the new method utilized directly measured values and regressive slope of Keeling plot instead of using the interpolated Keeling plot intercept. Our study shows an analytical framework to estimate T/ET based on the Keeling plot slope and direct-measured parameters. The new method potentially reduces the uncertainty of isotope-based ET partition approach.