The use of stable isotopes to partition evapotranspiration fluxes into evaporation and transpiration

It is crucial to partition evapotranspiration (ET) into evaporation (E) and transpiration (T) components for better understanding eco-hydrological processes and their underlying mechanisms, and improving the establishment and validation of hydrological models at the ecosystem scale. Traditional eddy covariance technique serves as a useful tool to estimate ET, but it encounters difficulties in quantifying the relative contribution of E and T to ET. Combining with eddy covariance technique, it is possible to partition ET based on the measurements of stable oxygen and hydrogen isotopes in liquid and vapor phases of water in the Soil–Plant–Atmosphere Continuum (SPAC) system. The key challenge is to precisely determine the oxygen-18 and deuterium isotopic compositions of ET (δ_ET), E (δ_E) and T (δ_T). δ_E can be estimated based on the Craig–Gordon model. δ_T is usually approximated by the δ¹⁸O and δD of water in xylem or twig (δ_x), assuming δ_T equals δ_x under isotopic steady state (SSA). However, the SSA is only likely satisfied during midday in field conditions. The diurnal variations of δ_T is affected by isotopic composition of atmospheric water vapor (δ_v) and leaf water at the evaporating sites (δ_L,e), and relative humidity, resulting in the non-steady-state behavior of δ_T at the sub-daily cycles. δ_ET can be estimated using the flux-gradient approach or the Keeling plot by measuring the vapor mixing ratio and δ_v at different heights in the surface layer. However, δ_v observations by the traditional cold trap/mass spectrometer method are limited to a coarse time resolution, leading to discrete time series of δ_ET. It is now possible to make in situ and high time resolution measurements of δ_v and to analyze a large number of plant and soil samples due to technical and instrumental advances in recent years. It provides an opportunity to improve the model prediction of δ_L,e, and more importantly, to calculate δ_T from δ_L,e without invoking the SSA. Combining with the flux-gradient approach or the Keeling plot technique, continuous δ_ET measurements can be made. It offers us a premise for accurate ET partitioning on diurnal time scale. In this review we introduced the recent advances, foci and challenges for studies on ET partitioning using the stable isotopes technique.