Contrasting drivers of riparian–stream hydrological dynamics during rainfall events across forest headwater ecoregions

Abstract. Examining how hydrological responses of streams and riparian groundwater interact during rainfall events can help understand runoff generation mechanisms and their links to biogeochemistry. We analysed rainfall–runoff events in four forest headwater catchments spanning semiarid Mediterranean, subhumid Mediterranean, temperate, and boreal ecoregions. Using sub-daily data, we evaluated how hydroclimatic drivers shaped stream and riparian groundwater. We additionally focussed on the slope (slopeLin) and hysteresis (h) of the riparian groundwater–stream relationship, describing coupled responses and connectivity patterns. Rainfall amount and antecedent wetness primarily controlled individual stream and riparian groundwater hydrological response across sites, while the vertical activation of riparian layers, defined by slopeLin, and timing, defined by h, were site-specific. At the Mediterranean sites, deep, low-conductivity layers dominated the lateral connectivity unless sufficient rainfall and wetness allowed activation of shallower flow pathways. At the temperate and boreal sites, shallow, conductive layers were typically active, with temporary disconnection during warm periods at the temperate site and snowmelt-induced activation of upslope sources at the boreal. Clockwise hysteresis, indicating faster riparian than stream responses, prevailed at the temperate and boreal sites. The Mediterranean sites showed weaker or anticlockwise patterns, suggesting activation of other water sources, likely ephemeral tributaries during intense rainfall. Our results show that the activation and hysteresis of riparian–stream connectivity differs across ecoregions, which has implications for solute mobilization. This framework provides a physically grounded perspective for interpreting both rainfall–runoff and concentration–discharge relationships, which can help predict shifts in catchment functioning under changing hydroclimatic regimes.