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 policymaking. 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 an 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. One partitioning method consists of determining the water stable isotopic compositions of ET, E, and T (δET, δE, and δE, respectively) from the water retrieved from the atmosphere, the soil, and the plant vascular tissues. The present work emphasizes 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 δE 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 subdaily dynamics of T/ET is by 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 δE at high vertical resolution with field-deployable lift systems.

Estimation of evaporation and transpiration rates under varying water availability for improving crop management of soybeans using oxygen isotope ratios of pore water

“…It is based on the coherent and simultaneous course of carbon uptake and water loss during T, both of which depend on stomata regulation. The second measurement-based approach used to determine the ratios of the ET fractions is a stable isotope-based partitioning technique using different water isotopologues (Rothfuss et al, 2021;Xiao et al, 2018). Isotope partitioning techniques are based on isotope fractionation processes during phase change.…”

“…In general terms, the stable isotope analysis of water is an appropriate tool used to investigate both fractions of ET and further aspects of water use efficiency. The studies using various stable water isotope monitoring methods in combination with other measurements to partition ET to E and T on a field or ecosystem scale were reviewed in detail by Rothfuss et al (2021). With a focus on crop water use and intrinsic water use efficiency, stable isotopes have been used to evaluate the effect of water stress, e.g.…”

“…As a consequence, a stable isotope-based ET partitioning technique based on δ 18 O measurements in water and soil-water samples was chosen to investigate the E and T rates. Isotope-based techniques have become established in ET research (Rothfuss et al, 2021) but are still less often applied in agricultural investigations than alternative techniques (Kool et al, 2014). Therefore, a water and stable isotope mass balance method from the laboratory was used (Sutanto et al, 2012;Wenninger et al, 2010), which was adapted and tested in field conditions (Liebhard et al, 2022) for the first time, according to our review of the literature, for the analysis of E and T rates under the conditions of varying soil water availability.…”

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

“…(e.g., Aron et al, 2020; Dubbert et al, 2013, 2014; Iraheta 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 − δ E )/( δ T − δ E )) (after Rothfuss et al, 2020; Yakir & Sternberg, 2000).…”