Can the young water fraction reduce predictive uncertainty in water transit time estimations?

Arianna Borriero*, Tam V. Nguyen, Stefanie R. Lutz, Jan H. Fleckenstein, Andreas Musolff, Rohini Kumar

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

Transit time distributions (TTDs) of streamflow are informative descriptors of catchment hydrological functioning and solute transport mechanisms. Conventional methods for estimating TTDs generally require model calibration against extensive tracer data time series, which are often limited to well-studied experimental catchments. We challenge this limitation and propose an alternative approach that uses the young water fraction (Fywobs), an increasingly used water age metric which represents the proportion of streamflow with a transit time younger than 2–3 months, and that can be robustly estimated with sparsely measured tracer data. To this end, we conducted a proof of concept study by modeling TTDs using StorAge Selection (SAS) functions with oxygen isotopes (δ18O) measurements for 23 diverse catchments in Germany. In a Monte-Carlo approach, we computed the (averaged) marginal TTDs of a prior parameter distribution and derived a model-based Fyw (Fywsim). We compared Fywsim with Fywobs, obtained from δ18O measurements, and constrained the prior SAS parameters distribution. Subsequently, we derived a posterior distribution of parameters and resulting model simulations. Our findings showed that using Fywobs to constrain the model effectively reduced parameter equifinality and simulation uncertainty. However, the value of Fywobs on reducing model uncertainty varied across sites, with larger values (Fywobs≥0.10) leading to simulations with a narrower uncertainty band and higher model efficiency, whilst smaller values (Fywobs≤0.05) had limited influence on reducing model output uncertainty. We discussed the potential and limitations of combining SAS functions with Fywobs, and considered broader implications of this approach for enhancing our understanding of catchment functioning and water quality status.

Original languageEnglish
Article number132238
Number of pages13
JournalJournal of Hydrology
Volume645
Issue numberPart B
DOIs
Publication statusPublished - Dec 2024

Bibliographical note

Publisher Copyright:
© 2024 The Authors

Keywords

  • SAS functions
  • Transit time distribution
  • Uncertainty
  • Young water fraction

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