Reviews and syntheses: Gaining insights into evapotranspiration partitioning with novel isotopic monitoring methods

Abstract

Abstract. Disentangling ecosystem evapotranspiration (ET) into evaporation (E) and transpiration (T) is of high relevance for a wide range of applications, from land surface modelling to policy making. Identifying and analysing the determinants of the ratio of T to ET (T / ET) for various land covers and uses, especially in view of climate change with increased frequency of extreme events (e.g., heatwaves and floods), is prerequisite for forecasting the hydroclimate of the future and tackling present issues, such as agricultural and irrigation practices. A powerful partitioning method consists in determining the water stable isotopic compositions of ET, E, and T (δET, δE, and δT, respectively) from the water retrieved from the atmosphere, the soil, and the plant vascular tissues. The present work emphasises the challenges this particular method faces (e.g., the spatial and temporal representativeness of the T / ET estimates, the limitations of the models used and the sensitivities to their driving parameters) and the progress that needs to be made in light of the recent methodological developments. As our review is intended for a broader audience beyond the isotopic ecohydrological and micrometeorological communities, it also attempts to provide a thorough review of the ensemble of techniques used for determining δET, δE, and δT, and solving the partitioning equation for T / ET. From the current state of research, we conclude that the most promising way forward to ET partitioning and capturing the sub-daily dynamics of T / ET is in making use of non-destructive online monitoring techniques of the stable isotopic composition of soil and xylem water. Effort should continue towards the application of the eddy covariance technique for high-frequency determination of δET at the field scale as well as the concomitant determination of δET, δE, and δT at high vertical resolution with field-deployable lift systems.

Reply on RC2

“…A: As stated L514-518, all authors simply assume steady state between both liquid and vapour phases. Consequently, only temperature is needed to convert δ_soilˆvap into δ_soilˆliq values, e.g., using the equations of Majoube (1971) and Horita and Wesolowski (1994). We further note in the text that only Rothfuss et al 2013provided evidence of near-isotopic equilibrium conditions between liquid and vapour in the soil pore space.…”

Evapotranspiration partitioning based on field‐stable oxygen isotope observations for an urban locust forest land

“…(e.g., Aron et al, 2020;Dubbert et al, 2013Dubbert et al, , 2014Iraheta et al, 2021;Rothfuss et al, 2010;Wang et al, 2010;Williams et al, 2004;Wu et al, 2017;Wu et al, 2021). Soil evaporation is more fractionated in comparison to vegetation transpiration at equal initial isotopic boundary conditions (Rothfuss et al, 2020;Wu et al, 2017). As a consequence, leaf water may reach steady-state (SS) conditions at a faster rate than the soil water, which results in different isotope composition of ET, E, and T (δ ET , δ E , and δ T ) (Liu et al, 2015) (the isotope variables are given as the ratio of heavy [ 18 O (H 2 O)] to light [ 16 O (H 2 O)] isotopologues and are relative to the normalized delta notation (δ); after Wei et al, 2015).…”

“…As a consequence, leaf water may reach steady-state (SS) conditions at a faster rate than the soil water, which results in different isotope composition of ET, E, and T (δ ET , δ E , and δ T ) (Liu et al, 2015) (the isotope variables are given as the ratio of heavy [ 18 O (H 2 O)] to light [ 16 O (H 2 O)] isotopologues and are relative to the normalized delta notation (δ); after Wei et al, 2015). The different isotope compositions of ET, E, and T can be used for the partition of ET (FT = (δ ET À δ E )/(δ T À δ E )) (after Rothfuss et al, 2020;Yakir & Sternberg, 2000).…”

“…For example, Williams et al (2004) indicated that the FT measured by the sap flow method was 15% lower than the stable water isotope method. The primary uncertainty in the stable isotope method comes from the determination of three isotope composition end-members, namely, δ ET , δ E , and δ T (Rothfuss et al, 2020;Sun et al, 2021;Wei et al, 2018;Xiao et al, 2018). Among them, the isotope composition of vegetation transpiration (δ T ) is typically estimated at SS condition.…”