Inferring the dynamics of StorAge Selection functions from GR4J

Models based on StorAge Selection (SAS) functions are useful tools for understanding of factors controlling transit time distribution (TTD) and catchment-scale solute export. SAS functions describe the sampling of different ages in catchment storage that produces river discharge. This sampling may vary over time, for instance according to soil moisture:  the young water fraction is generally higher in wet conditions and lower in dry conditions. 

In this work, we investigated how the dynamic of this sampling could be related to the different fluxes and model states of the hydrological model GR4J. Different coupling strategies are tested over the French Orgeval catchment (ORACLE observatory, 104 km²) using chloride concentrations as a conservative tracer. The modelling results allowed to verify that the groundwater contribution, and in particular that outside the topographic catchment (intercatchment groundwater flow), strongly influences the age of the river flow. This study thus opens perspectives to better constrain the modelling of groundwater contribution to the river flow within the GR4J model.