Combining Sediment Fingerprinting, Numerical Modeling and Analyses of Structural Connectivity to Understand Temporal Suspended Sediment Flux Variability in Mesoscale Mediterranean Catchments

Identification of sediment sources in catchments is crucial for applying erosion control measures and best management practices as well as for scientific understanding of hydro-sedimentary processes in the critical zone. As suspended sediment fluxes are highly variable in time, knowledge on sediment provenance at high temporal resolution is necessary. This information can be obtained with sediment fingerprinting and numerical modeling. Both approaches are prone to errors and uncertainties but also offer different opportunities each, so past studies have already shown the advantages of combining them.

For this study, a sediment fingerprinting protocol based on low-cost tracers was applied to a high number of sediment samples taken during flood events in two mesoscale Mediterranean catchments belonging to the French network of critical zone observatories OZCAR. Numerical simulations of soil erosion and sediment transfer were performed with a distributed, physically based hydro-sedimentary model in Iber in order to assess to which extent structural connectivity of sediment sources to the river network and finally to the outlet have an impact on the simulated sediment flux variability.

Our results show that the contributions of different sources to suspended sediment at the outlet vary substantially within and between flood events. The simulated sediment fluxes were found to be very sensitive to choices made during model set-up that define how structural connectivity is represented in the model. These include the definition of the river network based on a threshold of contributing area and the parameterization of Manning's roughness coefficient and the coefficient of erodibility. Simulated sediment fluxes are further determined by the location of the potential sediment sources because the distance to the outlet and the distance to the stream influence the travel time from the sources to the outlet.

Sediment fingerprinting has revealed the suspended sediment sources and the temporal variability of sediment fluxes at the outlet and numerical modeling has proven to be a valuable tool for testing hypotheses of the reasons for this variability. Thus, the two approaches are found to complement each other optimally and their combined application has a high potential to understand sediment flux dynamics.