Revealing the origin and age of tree water uptake along a forested hillslope

crouvenaz@student.ethz.ch

floriancic@ifu.baug.ethz.ch

 

The forest water cycle is dominated by vegetation-mediated processes, such as interception, infiltration, and transpiration, that greatly affect the redistribution of water between the atmosphere and subsurface. Yet, subsurface water transport and storage are poorly understood, complicating comprehensive analyses of tree water uptake.

Here we explore the performance and sensitivity of the model EcH₂O-iso with a novel isotope tracer dataset from the WaldLab experimental forest site, a small catchment located in a mixed beech and spruce forest in Zürich, Switzerland. Five years ago, we began measuring water fluxes and stable water isotopes in precipitation, soils of various depths, groundwater, streams and xylem. The model EcH₂O-iso is a process-based, spatially distributed ecohydrological model which allows to use water isotopic tracers (²H and ¹⁸O) for age tracking. Each grid cell is locally coupled with energy balance, hydrological transfer, vegetation growth and dynamics.

After setting up and parametrizing the model we validated model outputs with the measured isotope timeseries in different depths of the soil, groundwater, streamflow and xylem water along the sampled hillslope. We also tested to what extent input precipitation isotopes measured outside the forest are a reliable input to the model, by rerunning simulations with inputs from i) measured throughfall isotopes and ii) isotopic values obtained from drainage from the forest-floor litter layer. We performed multiple sensitivity analyses to better understand the sensitivity of certain model parameters and assessed which parameters need to be calibrated more precisely for future use of the model for this site.