Toward understanding transport and chloride dynamics at the catchment scale by combining StorAge Selection functions and a hydrological model

An in-depth understanding of the transport and fate of solutes, nutrients and pollutants within a catchment is crucial to address issues related to water quality and quantity, including the protection and management of water resources. Transit Time Distributions (TTDs) of streamflow can provide useful descriptors of catchment hydrological functioning and can inform on solute transport mechanisms. They can be estimated using StorAge Selection (SAS) functions, which is a time-varying approach that describes how catchments selectively release water of different ages from storage through discharge, thereby regulating the streamflow TTDs and solute composition. For instance, the proportion of young water tends to increase during wet conditions and decrease during dry periods.

In this study, we explore how the dynamics of SAS functions can be linked to the different fluxes and state variables of a conceptual hydrological model. To achieve this, we tested various coupling strategies over the French Orgeval catchment (104 km²) in France, using chloride concentrations as a conservative tracer and the GR6J hydrological model (internal state variables). The modelling results showed that incorporating dynamic (time-varying) SAS functions is essential for accurately capturing the temporal variability observed in the chloride concentration time series. Furthermore, the results showed that the signal of inter-catchment groundwater flow (IGF), conceptually defined as the groundwater inflows and outflows across the topographic boundaries of a catchment, is the best variable for driving the dynamic of the age of the river flow.