ET partitioning method comparison for a winter wheat stand at the Land-Atmosphere-Feedback Observatory, Hohenheim, Germany

The partitioning of evapotranspiration (ET) into crop transpiration (T) and soil evaporation (E) is crucial for accurate modelling of land-atmosphere processes and for assessing climate sensitivity in agricultural systems, yet it remains methodologically challenging. Here, we quantify diurnal dynamics of ET and its components in winter wheat using high-resolution in-situ water isotope flux chamber measurements in comparison to estimates from micro-lysimeters, sap flow measurements and eddy-covariance (EC) measurements.

Field campaigns were conducted on 2-5 consecutive days per month during the 2025 growing season at an agricultural experimental site “The Land-Atmosphere-Feedback Observatory” (University of Hohenheim, Germany), spanning key crop phenological stages of winter wheat. Water isotope-based chamber measurements were performed in the vicinity of the EC tower from sunrise, after morning dew evaporation, until sunset. E, T and ET chambers were measured consecutively, resulting in 8-12 measurements per flux type per day. Isotopic compositions of E, T and ET used for ET partitioning were estimated using the Keeling plot method. Independent estimates of E and T were derived from five micro-lysimeters installed in a star-like pattern around the EC tower, and five sap flow micro sensors installed on five individual plants, and compared to the water isotopic partitioning results.

Our method comparison focuses on a measurement period in June 2025, whereas chamber-based water isotope measurements are presented for the entire growing season.

In June, ET derived from EC measurements ranged from 0 to 350 W m^-2, peaking around midday, while E obtained from micro-lysimeters was always less than 120 W m⁻². Sap flow measurements often led to reasonable values only in the afternoon, showing a decreasing trend of similar magnitude as ET. Daily patterns varied depending on meteorological conditions. Chamber-based E, T and ET estimates were close to EC tower/micro-lysimeter/sap flow flux-based measurements, but showed larger scattering, likely due to spatial heterogeneity. Across measurement approaches, T/ET ratios predominantly ranged between 0.5 and 1, indicating that T dominated ET. The T/ET ratio showed a “U-shaped” diurnal pattern when derived from micro-lysimeter and EC tower measurements but is decreasing over the day when sap flow and lysimeter data were considered. T/ET derived from chamber fluxes did not show a clear diurnal pattern. Isotope-based ET partitioning results showed large scattering mainly due to weak isotopic contrast between T and E, but also due to spatial heterogeneity and temporal variability in isotope flux signatures. In general, hydrogen isotope-based partitioning showed better agreement with flux-based estimates than oxygen isotopes, likely due to stronger isotopic fractionation signal between fluxes.

The dominant controls on isotopic variability and flux dynamics of E, T and ET will be discussed in relation to meteorological conditions, plant physiological parameters and soil water availability. Measurement uncertainty across approaches will be evaluated and a best-estimate ET and its partitioning will be derived. These findings will help to evaluate model representation of T/ET and reduce uncertainties associated with T/ET estimates.