A multi-objective parameterization of StorAge Selection functions based on stable isotopes sampled at the catchment outlet and in trees to constrain water age estimations for runoff and evapotranspiration fluxes

It is important to understand how precipitation is stored in catchments, released via evapotranspiration (ET), or recharges aquifers and streams. We investigated this partitioning of precipitation using stable isotopes of water (²H and ¹⁸O) at the Can Vila catchment in the Spanish Pyrenees mountains. The isotope data covered four years, comprising >550 rainfall and >980 stream water samples. They were complemented by fortnightly plant-water-isotope data sampled over eight months. The isotope data were used to quantify how long it takes for water to become evapotranspiration or discharged as streamflow, using StorAge Selection (SAS) functions. We calibrated the SAS functions using a conventional approach, fitting the model solely to stream water isotope data, as well as a multi-objective calibration approach, in which the model was simultaneously fitted to tree-xylem-water isotope data.

Our results showed that the conventional calibration approach was not able to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed streamwater and xylem water isotope ratios, if those xylem water isotope observations were used in calibration. This multi-objective-calibration approach led to a more constrained parameter space, facilitating parameter value identification. The model was tested on a segment of data reserved for validation, showing a Kling-Gupta Efficiency of 0.72, compared to the 0.83 observed during in the calibration period.

The water age dynamics inferred from the model calibrated using the conventional approach differed substantially from those inferred from the multi-objective-calibration model. The latter suggested that the water supplying evapotranspiration is much older (median age 150-300 days) than what was suggested by the former (median age 50-200 days). Regardless, the modeling results support recent findings in ecohydrological field studies that highlighted both subsurface heterogeneity of water storage and fluxes and the use of relatively old water by trees. We contextualized the SAS-derived water ages by also using young-water-fraction and endmember-splitting approaches, which respectively also showed the contribution of young water to streamflow was variable but sensitive to runoff rates, and that ET was largely sourced by winter precipitation, that must have resided in the subsurface across seasons.