The partitioning of water fluxes in the critical zone is of great interest due to the implications for understanding water cycling and quantifying water availability for various ecosystem services. We used the tracer-aided ecohydrological model EcH2O-iso to use stable water isotopes to help evaluate water, energy, and biomass dynamics at an intensively monitored study plot under two willow trees, a riparian species, in Berlin, Germany. Importantly, we assessed the value of in situ soil and plant water isotope data in helping to quantify xylem water sources and transit times, with coupled estimates of the temporal dynamics and ages of soil and root uptake water. The willows showed high water use through evapotranspiration, with limited percolation of summer precipitation to deeper soil layers due to the dominance of shallow root uptake (>80% in the upper 10cm, 70%-78% transpiration/evapotranspiration). Lower evapotranspiration under grass (52%-55% transpiration/evapotranspiration) resulted in higher soil moisture storage, greater soil evaporation, and more percolation of soil water. Biomass allocation was predominantly foliage growth (57% in grass and 78% in willow). Shallow soil water age under grass was estimated to be similar to under willows (15-17d). Considering potential xylem transit times showed a substantial improvement in the model's capability to simulate xylem isotopic composition and water ages and demonstrates the potential value of using in situ data to aid ecohydrological modelling. Root water uptake was predominately derived from summer precipitation events (56%) and had an average age of 35d, with xylem transport times taking at least 6.2-8.1d. By evaluating isotope mass balances along with water partitioning, energy budgets, and biomass allocation, the EcH2O-iso model proved a useful tool for assessing water cycling within the critical zone at high temporal resolution, particularly xylem water sources and transport, which are all necessary for short- and long-term assessment of water availability for plant growth.
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Acknowledgements. The in situ data could be measured through equipment funded by funding by the Bundesministerium für Bildung und Forschung. Funding for Chris Soulsby was through the project “Modelling surface and groundwater with isotopes in urban catchments” (MOSAIC) provided by the Einstein Foundation Berlin. Contributions from Chris Soulsby were also funded by the Leverhulme Trust’s ISOLAND project. We acknowledge the IGB and Leibniz Association’s Open Access Publication Fund. The authors acknowledge the assistance of David Dubbert, Lukas Kleine, and Jonas Freymüller in isotope analysis and study site set-up and Marco Maneta for discussions on EcH2O-iso modelling. The authors thank the associate editor, Nicolas Brüggemann, and the reviewers (Matthias Beyer and one anonymous reviewer) for their invaluable comments.
This research has been supported by the Bundesministerium für Bildung und Forschung (grant no. 033W034A), the Einstein Stiftung Berlin (award no. EVF-2018-425), and the ISOLAND project of the Leverhulme Trust (grant no. RPG-2018-425).
The publication of this article was funded by the Open Access Fund of the Leibniz Association.