Water Flux Tracking With a Distributed Hydrological Model to Quantify Controls on the Spatio‐temporal Variability of Transit Time Distributions

Water transit times and flow pathways are crucial elements in characterizing catchment hydrologic response. Understanding their variability in space and time sheds light on the link between discharge formation and water quality at the catchment scale. Here, we introduce a novel modeling framework to explore water transport mechanisms using the Hafren catchment in Wales (UK) as a case study. We show that a fully distributed hydrological model coupled with a transport component for conservative tracers is useful in analyzing how hydrometeorological conditions and spatial heterogeneity may affect water transit times and age distributions in a real catchment. We use the model to track the paths of water parcels that entered the catchment as rainfall over 2 years, labeling each day of rain individually. There is a reasonable agreement between tracer simulations and observations, suggesting that dynamic transit time distributions (TTDs) both forward and backward in time can be approximated using a high spatial and temporal resolution hydrochemical model, without assuming a priori any transit and storage selection functions at the catchment scale. TTDs are quantified for the modeled internal dynamics of the study catchment. TTDs conditional on a given rainfall time are mostly correlated to the season in which the rain event occurs, whereas TTDs conditional on a given exit time are mostly affected by catchment wetness. When TTDs for individual rainfall events are re‐scaled as functions of cumulative discharge, they collapse around a single common distribution, suggesting a potential characteristic catchment function.