Contributions of on-land sediment sources and transfer to the overall Loire river basin sediment budget

Sediment fluxes within continental areas play a major role in the global biogeochemical cycles and are often at the source of soil surface degradation as well as water and ecosystems pollution. In a context of significant land use and climate change, it appears important to be able to carry out sediment budgets to assess potential future impacts induced by such changes. The purpose of this study is to examine the source-to-sink dynamic of the sediment cycle for 123 small to medium catchments (10 to 10^4 km²) distributed over the Loire River basin (France). To this aim, three different steps will be carried out. First, estimations of mean annual specific sediment load for the 123 catchments are estimated from suspended sediment concentration and water discharge measurements at basin outlets over the past four decades. Secondly, catchment global parameters (topography, landuse, drainage density...) and simple indices, combining these parameters, will be calculated to study the relationship between hillslope production and sediment load at basins outlet. Finally, we will focus on the development of a distributed modelling approach of on-land sediment transfer processes. Special attention will be given to hillslope runoff and sediment connectivity by taking into account landscape spatial organisation and linear pathways as determinant features for sediment transport. The results from the first step on mean annual specific sediment loads, show that catchments contribute from 30 to 4*10^5 t.yr-1 to the overall Loire river sediment exports (which equals 10^11 t.yr-1). Investigations on catchments global characteristics should then allow the identification of dominant processes in sediment redistribution, help to draw local and then regional distributed sediment budgets and bridge the gap between the different spatial scales. Contribution of hillslopes to the overall catchment budget should finally help to assess in-stream contributions and redistribution processes. Contributions of on-land sediment sources and transfer to the overall catchment budget should finally help to assess in-stream contributions and redistribution processes