Tracer-aided assessment of catchment groundwater dynamics and residence time

Sustainable groundwater management is becoming increasingly important due to intensifying water deficits around the world, which highlights the necessities and challenges to explore the dynamics of groundwater at the catchment-scale. In this paper, a tracer-aided approach has been developed and applied to quantify the relationships between groundwater storage and discharge, upon which the dynamics of groundwater storage and its residence time are assessed. Robust nonlinear storage-discharge relationships are found for the eight studied catchments located in a headwater region of the Murray-Darling Basin, Australia. The dynamics of groundwater storage for all the studied catchments largely show synchronization with regional climate. The estimated highest and active groundwater storages in a year are more responsive to precipitation than the lowest storages. For all the studied catchments, during dry seasons without recharge, the depletion rate of groundwater is found to decrease with the decline in groundwater storage, which can be ascribed to the vertical heterogeneity of their unconfined aquifers. The mean residence time of groundwater represented by MRT75 (i.e. time taken for groundwater storage to diminish from its mean level to the 75th percentile) varies between 57 and 111 days across the studied catchments, suggesting considerable hydrogeological differences among the studied catchments. The groundwater dynamics observed in this research could contribute to improving hydrogeological model performance in simulating the connection between groundwater and surface water at the catchment-scale. Moreover, the findings could serve to develop guidelines for groundwater extraction when considering the constraints of environmental flow requirements.