Physical Interpretation of Time-Varying StorAge Selection Functions in a Bench-Scale Hillslope Experiment via Geophysical Imaging of Ages of Water

Understanding transit times (TT) and residence times (RT) distributions of water in catchments has recently received a great deal of attention in hydrologic research since it can inform about important processes relevant to the quality of water delivered by streams and landscape resilience to anthropogenic inputs. The theory of transit time distributions (TTD) is a practical framework for understanding TT of water in natural landscapes but, due to its lumped nature, it can only hint at the possible internal processes taking place in the subsurface. While allowing for the direct observation of water movement, Electrical Resistivity Imaging (ERI) can be leveraged to better understand the internal variability of water ages within the subsurface, thus enabling the investigation of the physical processes controlling the time-variability of TTD. In this study, we estimated time-variable TTD of a bench-scale bare-soil sloping soil lysimeter through the StorAge Selection (SAS) framework, a traditional lumped-systems method, based on sampling of output tracer concentrations, as well as through an ERI SAS one, based on spatially distributed images of water ages. We compared the ERI-based SAS results with the output-based estimates to discuss the viability of ERI at laboratory experiments for understanding TTD. The ERI-derived images of the internal evolution of water ages were able to elucidate the internal mechanisms driving the time-variability of ages of water being discharged by the system, which was characterized by a delayed discharge of younger water starting at the highest storage level and continuing throughout the water table recession.