Hysteresis in streamflow-water table relation provides a new classification system of rainfall-runoff events

Rainfall-runoff event types can be used to better understand the hydrological functioning of catchments. In this study, we propose a framework to characterize and classify runoff events relying on the hysteresis analysis of the relation between streamflow (independent variable) and depth to water table (dependent variable). We evaluated hysteresis for 112 rainfall-runoff events occurred in the period 2012-2016 in a small forested catchment in the Italian Pre-Alps. Three main groups of rainfall-runoff events were identified, each associated with specific hydro-meteorological characteristics of the events. A first group, identified by a faster response of streamflow compared to the groundwater level (which led to a clockwise hysteretic loop), was characterized by dry antecedent conditions, short rainfall events, low streamflow peaks and small runoff coefficients. A second group had characteristics similar to the first group, that is, faster response of streamflow compared to the groundwater level (clockwise hysteretic loop), but on average displayed a narrower hysteretic loop. A third group, identified by a faster response of the groundwater level compared to streamflow (which led to an anti-clockwise loop), was characterized by wet antecedent conditions, long rainfall events, high streamflow peaks and large runoff coefficients. Results showed statistically significant differences among the groups, corroborated by an analysis based on environmental tracers (H-2 and electrical conductivity). This study shows how the analysis of the hysteretic relation between streamflow and depth to water table can be used for grouping rainfall-runoff events and to better identify the catchment's hydrological response to rainfall inputs.