08 Mar 2022
08 Mar 2022
Status: a revised version of this preprint is currently under review for the journal HESS.

Precipitation fate and transport in a Mediterranean catchment through models calibrated on plant and stream water isotope data

Matthias Sprenger1,2,a, Pilar Llorens1, Francesc Gallart1, Paolo Benettin3, Scott T. Allen4, and Jérôme Latron1 Matthias Sprenger et al.
  • 1Institute of Environmental Assessment and Water Research (IDAEA-CSIC), Barcelona, Spain
  • 2Ecohydrology & Watershed Science group, North Carolina State University, Raleigh, USA
  • 3Laboratory of Ecohydrology ENAC/IIE/ECHO, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
  • 4Dept. of Natural Resources and Environmental Science, University of Nevada, Reno, USA
  • anow at: Earth & Environmental Sciences Area, Lawrence Berkeley National Laboratory, Berkeley, USA

Abstract. To predict hydrologic responses to inputs and perturbations, it is important to understand how precipitation is stored in catchments, released back to the atmosphere via evapotranspiration (ET), or transported to 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 of measurements, comprising > 550 rainfall and > 980 stream water samples, capturing intra-event variations. 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 model-fitting approach was not able to constrain the model parameters that represented the age of water supplying ET. Consequently, the ET isotope ratios simulated by the conventionally calibrated model failed to adequately simulate the observed xylem isotope ratios. However, the SAS model was capable of adequately simulating both observed stream water and xylem water isotope ratios, if those xylem water isotope observations were used in calibration (i.e., the multi-objective approach). The 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 median ages of water supplying evapotranspiration is much older (150–300 days) than what was suggested by the former (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.

Matthias Sprenger et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2022-93', Anonymous Referee #1, 07 Apr 2022
    • AC1: 'Reply on RC1', Matthias Sprenger, 20 May 2022
  • RC2: 'Comment on hess-2022-93', Anonymous Referee #2, 20 Apr 2022
    • AC2: 'Reply on RC2', Matthias Sprenger, 20 May 2022

Matthias Sprenger et al.

Matthias Sprenger et al.


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Short summary
Our catchment scale transit time modeling study shows that including stable isotope data on evapotranspiration in addition to the commonly used stream water isotopes helps constraining the model parametrization and reveals that the water taken up by plants has resided longer in the catchment storage than the water leaving the catchment as stream discharge. This finding is important for our understanding how water is stored and released, which impacts the water availability for plants and humans.