Testing a conceptual model of runoff generation processes for a small pre-alpine catchment with tracer data
Authors: Zuecco Giulia , Marchina Chiara , Gelmini Ylenia , Penna Daniele , Borga Marco , van Meerveld Ilja 
Published: February 2023

Conceptual models of catchment hydrological functioning are crucial to understand and predict runoff and the tracer responses during rainfall events, and thus for sound water resources management and pollution mitigation measures. In this study, we use hydrometric and tracer data (stable isotopes, major ions and electrical conductivity (EC)) collected in the Ressi catchment, a 2-ha watershed in the Italian pre-Alps, to test our existing conceptual model of runoff generation mechanisms. This model was based on previous hydrometric measurements and isotope data for selected events and highlights the importance of precipitation and antecedent conditions for hillslope-riparian zone-stream connectivity. More specifically, we determined if the temporal variability in the concentration-discharge relations can be explained by event characteristics, i.e., rainfall event size, intensity, and antecedent moisture conditions.The Ressi catchment is characterized by high seasonality in runoff response, due to the seasonality in rainfall (high in fall) and evapotranspiration (high in summer). Discharge and rainfall have been measured continuously since August 2012. Stream water, precipitation, shallow groundwater and soil water samples were collected for tracer analyses during 20 rainfall-runoff events between September 2015 and August 2018. All samples were analyzed for EC, isotopic composition (2H and 18O) and major ion concentrations. To investigate the possible controls on the concentration-discharge relations, we determined the main event characteristics (e.g., total event rainfall, rainfall intensity, antecedent soil moisture and depth to water table, runoff coefficient) for each event.Based on previous applications of isotope- and EC-based hydrograph separation in the Ressi catchment, we expected different dynamics of the major ions in stream water, depending on the magnitude of the rainfall-runoff events. For all major ions, we hypothesized a dilution effect, and a more marked response for large, long duration events with wet antecedent conditions. The temporal dynamics of calcium, magnesium, sodium and sulfate concentrations confirmed our hypotheses. On the contrary, nitrate, potassium and chloride concentrations sometimes increased at the onset of the event, before a later dilution. These temporal dynamics led to complex hysteretic relations with discharge that could not be explained by the event characteristics. We attribute the rapid increase in the concentrations of these solutes to a quick flushing from the dry parts of the stream channel and the near surface-soil layers of the riparian zone at the onset of the event. The revised conceptual model for the geochemical response of this catchment should, therefore, include a rapid flow pathway that leads to the mobilization of nitrate, chloride and potassium ions, and describe the rapid establishment of hydrological connectivity along the streambed and the near-channel zones.&#160;Keywords: concentration-discharge relation; major ions; electrical conductivity; stable isotopes; hysteresis; forested catchment.