How can we improve our hydrological models and their predicted travel times? Lessons learned from a vegetated lysimeter experiment.

Abstract:

Accurate travel time estimation by hydrological models is of significant importance as it is key to understand biogeochemical processes in the subsurface. In particular, our understanding of how plants rely on water from previous precipitation events remains poor and mainly based on streamflow and soil water data. In this study, we quantify the age of evapotranspiration fluxes by coupling transport models to tracer data from a high-resolution lysimeter experiment.

We conducted a 6-week experiment by injecting a doubly labeled water in a (1.1 m² x 2 m) lysimeter planted with a willow tree (Salix viminalis). We measured the tracer breakthrough in the plant's xylem, in the bottom drainage and in the bulk soil at 10, 25, 50, 80, and 150 cm depths on average twice a week. Moreover, in addition to hydrological fluxes, pressure and water content data at 25, 75, 125, and 175 cm depths were gathered on a daily basis. At the end of the experiment, root distribution was empirically quantified by depth profiling. Then, we used this data to calibrate a space-explicit (HYDRUS1D) and a space-implicit (tran-SAS) transport model and to compute residence times in the water storage and all the outfluxes.

The results show that the discharge isotopic concentration is more reflective of the average behavior of storage soil. Still, the measured plant isotopic concentration had an evident impact on calibration results and on betterment of our understanding of the underlying transport mechanisms in the soil column. In this study, we observed a prolonged breakthrough curve in transpiration which was facilitated by the root distribution of the willow tree and reflected in our model(s) by an equal affinity for all different ages at transpiration flux. These results are a basis to move on to larger scales.