Cluster A - A03

A03

Parameterization of evapotranspiration partitioning function in land-surface models using water stable isotopes

Prof. Dr. Youri Rothfuss
Forschungszentrum Jülich  |    +49-2461-96925  |   This email address is being protected from spambots. You need JavaScript enabled to view it.

Prof. Dr. Nicolas Brüggemann
Forschungszentrum Jülich  |    +49-2461-618643  |   This email address is being protected from spambots. You need JavaScript enabled to view it.

Summary

In project A03, we analyze water component fluxes of the terrestrial water cycle by partitioning evapotranspiration (ET) into its components evaporation (E) and transpiration (T). The method is based on the non-destructive analysis of the temporal dynamics of water stable isotopologues, ¹H²H¹⁶O and ¹H₂¹⁸O, in atmospheric, soil and plant water. These analysis will be carried out in the ecosystems with four specific plant functional types (PFTs) defined in CLM 5.0: managed irrigated crops, C₃ grass, temperate climate needleleaf evergreen trees, and Mediterranean managed rain-fed un-irrigated crops. Using the gathered data, the isotope-enabled soil-vegetation-atmosphere transfer model SiSPAT-Isotope will be parametrized for field-scale simulations of T/ET for the analyzed PFTs and different climatic conditions. These modeling results will be compared to estimations of E and T using the CLM 5.0 and improve the PFT-specific modeling parametrization.

Graphical summary

Figure 1: Possibilities and complexity of using the ¹⁸O isotopic composition of water for partitioning evapotranspiration (Rothfuss et al., 2021).

Contribution to the CRC

The project contributes to the CRC’s modelling system CLM 5.0 by providing essential information of T/ET time series for the analyzed field sites using the SiSPAT-Isotope model. These modelling results will be compared to estimations of T/ET ratios using CLM 5.0 with the goal to statistically determine systematic errors in the CLM 5.0 estimations and to update the parameters used for T/ET calculation for agricultural fields, grasslands and forest sites. Finally, simulations with the improved CLM 5.0 will provide new values for PFT-specific characteristics used in the computation of E and T fluxes.

Approach

The isotopic partitioning of ET is based on a two-end member linear mixing model (assuming complete mixing of evaporated and transpired water in the atmosphere):

Measurements of the isotopic composition of the water in each of the three components E, T and ET therefore enable the assessment of the ratio of T/ET in environment over time. Multiple field campaigns will be carried out in the different PFT environments at different times of the year over the course of the project to account for seasonal variations of the T/ET ratio and to isotopically characterize T/ET of the different ecosystems during all seasons.

Main results in 2022

Two field campaigns have already been carried out in 2022 in the C₃-grassland and Mediterranean environments. The work focus until the next campaign is the improvement of the mobile measurement station to account for measurement uncertainties, and the preparation of the code used in the data analysis to enable efficient processing of data gathered during future campaigns.

Figure 2: Field campaign in summer 2022 in the PFT ‘Mediterranean managed rain-fed unirrigated crops’, Auradé, FR.