Connectivity between hillslopes and rivers for the main French hydrographic basins
Résumé
Soil erosion is one of the major drivers of landscape evolution in Western Europe. However, depending on the geological and topographical settings, the miscellaneous forms of erosion may lead to very diversified morphological evolution. A key element is the link between the hillslope and the river. In this context, the objective of this study is to investigate to what extent eroded soil particles contribute to river sediment exports for the French major basins. The hillslope erosion rates are deduced from a distributed pan-European assessment of hillslope, rill, and interrill erosion by water, which was quantified interpolating erosion plot data as a function of topographical, land use, and pedological parameters. Sediment fluxes were mainly assessed from the French river quality database, using an improved rating curve approach from daily discharge data (IRCA), which allows the estimation of mean annual sediment load. The resulting mean annual suspended sediment loads show that French rivers export ca. 50 Mt of sediments per year to the seas among which ca. 17 Mt as particulate matter corresponding to a specific sediment yield of ca. 0.4 ton/ha/yr. The analysis of sediment budgets at different spatial and temporal scales permits to identify some of the dominant processes in the different contexts. Concerning upland physical soil surface erosion rates, we find that the average travel distance of eroded particles may be limited, implying a strong decrease in physical erosion rates when moving from the local scale (m²) to the river basin scale (> 103 km²). Chemical erosion rates are less sensitive to scale and can either decrease or increase with increasing area in function of lithology, anthropogenic land management and topography. The results also highlight the predominant role of surface connectivity to characterize the fraction of sediment exported out of river drainage areas by physical soil surface erosion.