Transit time tracing using wetness-adaptive StorAge Selection functions—application to a Mediterranean catchment

Not just the quantities but also the transit times are of crucial importance for understanding the transfer of water through a catchment. This information is essential, for example, for determining the risk of the transfer of contaminants from diffuse, agricultural sources toward surface water bodies. We created a hydrological age-tracking reservoir model of a meso-scale Mediterranean catchment that is prone to flash floods. We implemented StorAge Selection (SAS) functions that adapt to the catchment's wetness condition in order to represent changing flow processes. The goal is to evaluate the evolution of transit time distributions (TTDs) through the catchment (1 h time step, 1 h resolution), providing an example for a rural catchment under a Mediterranean climate. The focus is on flood events, and the results are interpreted with regard to the risk of contaminant transfer from diffuse sources. The dependence of water age preferences on catchment wetness is examined. We propose and test a multi-tracer approach to parametrize SAS functions on multiple reservoirs. The model was calibrated and validated against the streamflow discharge (Q), deuterium isotope signature (δ2H) and dissolved silica concentration (Si) at the outlet. While δ2H acted as a direct tracer of rain-water, Si was used as a tracer of the contact time with the soil and rock minerals. The model revealed high event water fractions during flood events (with up to 63\% of water younger than one day) and a dominance of water older than one year most of the remaining time. This suggests an elevated risk of the transfer of agricultural contaminants toward streams during flood events. The results also indicate an inverse storage effect (ISE), with young water preferences increasing under wet conditions.