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

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 (F_yw^obs), 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 (${\delta }^{18}$O) 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 F_yw (F_yw^sim). We compared F_yw^sim with F_yw^obs, obtained from ${\delta }^{18}$O 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 F_yw^obs to constrain the model effectively reduced parameter equifinality and simulation uncertainty. However, the value of F_yw^obs on reducing model uncertainty varied across sites, with larger values (F_yw^obs$\ge$0.10) leading to simulations with a narrower uncertainty band and higher model efficiency, whilst smaller values (F_yw^obs$\le$0.05) had limited influence on reducing model output uncertainty. We discussed the potential and limitations of combining SAS functions with F_yw^obs, and considered broader implications of this approach for enhancing our understanding of catchment functioning and water quality status.