Contrasting mountain and piedmont dynamics of sediment discharge associated with groundwater storage variation in the Biobio River

Supply and transport of sediment in catchments involve processes with fundamental consequences for river management, land use, and the prediction of climate-driven sediment fluxes. In the present study we addressed spatial variability in the water routes through the surface and subsurface of a catchment and the suspended sediment discharge (Q(s)) over a mountain-piedmont system. We analyzed daily suspended sediment concentration (C-s) and water discharge (Q) measurements at stations located in different topographic settings (mountain and piedmont) in the Biobio River basin (southern central Andes, 37-39 degrees S). In steep catchments, the Q versus Q(s) relationship has a marked seasonal hysteresis. In the piedmont, Q(s) is proportional to Q, with no seasonal hysteresis. The contrast in the hysteresis pattern between catchments with different topographies is explained by differences in the routing of rainfall-derived water. In the piedmont, most of the rainfall is converted into surface runoff because the water table is near the surface. In the mountains, groundwater storage results in large seasonal variations in the proportion of Q that flows at the surface and transports sediment from the hillslopes, producing hysteresis. By separating the total Q into two components (direct discharge, Q(d) and base flow, Q(b)), we observed the response of Q(s) to the fraction of water that quickly leaves the catchment after a rainfall event (Q(d)). Similar results between the mountain and piedmont and the absence of hysteresis simplify the behavior of Q(s) into a linear relationship with Q(d) over the entire catchment and lead us to propose that sediment mobilization to the river along the Biobio catchment is primarily controlled by overland flow. Our findings highlight the importance of an adequate hydrological model for understanding the erosion and transport processes of a catchment, and which can be applied to other natural and modeled mountain-piedmont systems.