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.
Highlights
A pivotal parameter in landscape hydrology and ecology is the transpiration (T ) to evapotranspiration (ET ) ratio (T /ET )
The central aim of this study is to identify from the literature the challenges the ensemble of isotopic methods currently face and how they should progress in the future
Two options are found in the literature (Fig. 2i) for determining the isotopic composition of the E flux, δE: i. by solving one of either mass balance equations (Eqs. 7 or 11; see Sect. 3.1) in combination with dynamically purged closed bare-soil chambers (15 % of the reviewed studies, e.g. Dubbert et al, 2013, 2014b) or ii. by solving the so-called “Craig and Gordon equation” (Eq 18 below), which is derived from the atmospheric part of a transport model of water stable isotopologues, based on an analogy to Ohm’s law (Craig and Gordon, 1965) (69 % of the studies)
Summary
A pivotal parameter in landscape hydrology and ecology is the transpiration (T ) to evapotranspiration (ET ) ratio (T /ET ) (see the reviews of Kool et al, 2014; Anderson et al, 2017; Stoy et al, 2019). The uncertainty of the T /ET estimate remains high; it has been estimated to range from 13 % to 90 %, depending on the source and type of data (e.g. satellite- or isotopic-based) and method (modelling or data reanalysis) (Lawrence et al, 2007; Alton et al, 2009; Jasechko et al, 2013; Wang et al, 2014; Wei et al, 2017). Rothfuss et al.: Novel isotopic insights into evapotranspiration partitioning surface models to provide sensitivity of the overall ET flux to changes in precipitation and land cover (Wang and Dickinson, 2012)
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