Influence of groundwater seasonality on suspended sediment dynamics, in a 20 km ² Mediterranean mountainous catchment

This study investigates the interplay between groundwater and surface runoff in controlling suspended sediment dynamics within the Galabre catchment, a 20 km ² Mediterranean mountainous catchment characterized by extensive badland areas. Using an End-Member Mixing Analysis framework that accounts for event-specific variability in end-member chemistry, the study quantifies surface runoff and groundwater flows during 26 rainfall–runoff events from three hydrological years. Hydrological analyses reveal that groundwater flow is the major contributor to streamflow, accounting for approximately 75 % of the event flow on average. Groundwater response varies substantially between seasons, with mean peak flows increasing from 2.5 ± 2.1 m ³  s ⁻¹ in spring/summer (May–September) to 5.9 ± 5.4 m ³  s ⁻¹ in autumn/winter. While both surface runoff and groundwater flows increase with event rainfall accumulation, groundwater flow shows a stronger dependence on rainfall accumulated over the preceding 15 d, emphasizing the influence of antecedent catchment wetness and long-term hydrological connectivity. Hydrosedimentary analyses further demonstrate that suspended sediment concentrations (SSC) correlate more strongly with surface runoff flow rate than with flow rate ( Q ), emphasizing the dominant role of surface runoff in sediment detachment and transport on hillslopes. Marked seasonal differences in hydrosedimentary processes were observed. Spring/summer events, driven by short and intense rainfall, produce low-flow responses with high suspended sediment concentration and exhibit anticlockwise SSC– Q hysteresis loops. This suggests that these fine sediments could be mobilized from sources located relatively far from the outlet. The high suspended sediment concentrations and moderate flow rate in the river network likely lead to sediment deposition in the river network. Conversely, autumn/winter events, driven by prolonged low-intensity rainfall, and characterized by enhanced groundwater flow, produce high flow responses, with low suspended sediment concentration, and clockwise SSC– Q hysteresis loops. This hysteresis shape indicates close sediment sources that are likely associated with riverbed sediment remobilization. These findings reveal a fundamental seasonal shift from primary (i.e. hillslope mobilization) to secondary (i.e. riverbed remobilization) erosion processes, controlled by the seasonal variations of groundwater flow throughout the year.