Variability of suspended sediment yields within the Loire river basin (France)

Suspended sediment fluxes and their variability in time and space have received much attention over the past decades. Large databases compiling suspended sediment load (SL) data are often used to serve these purposes. Analyses of these databases have highlighted the following two major limitations: (i) the role of lowland areas in sediment production and transfer has been minimised, and studies on small-scale catchments (with a drainage area of ?102km2) are practically non-existent in the literature; and (ii) inhomogeneous data and calculation methods are used to estimate and compare the SL values. In this context, the present study aims to complete the existing studies by providing a reliable comparison of SL values for various catchments within lowland river basins. Therefore, we focused on the Loire and Brittany river basins (France). 111 small to large catchments covering 78% of this area and representative of the basins landscape diversity were chosen. We first present a large database of area-specific suspended sediment yields (SY) calculated from the suspended sediment concentration and flow discharge data over 7-40 yr of measurements at gauging stations. Two calculation methods are used, and the calculated loads are confined within a factor of 0.60-1.65 of the real values. Second, we analyse the temporal and spatial variability of the calculated SY values. Finally, using a nested catchment approach, we provide insight into sediment transport from upstream to downstream gauging stations and into the role of small- and medium- scale catchments in sediment production and transfers. The SL values at the outlet of the catchments range from 2.5 102to 8.6 105t yr-1, and the SY values range from 2.9 to 32.4 t km-2yr-1. A comparison with the limited values available in the literature for this region corroborates our estimations. Sediment exports from the Loire and Brittany river basins are very low compared with mountainous regions and European exports. However, a strong spatial variability within this territory exists. The expected results on the SY spatial pattern distribution and the correlation between SY values and basin sizes are not observed. An analysis of the SY values at different time steps shows a strong effect of the seasonal availability of detached particles to be transported with a high concentration of suspended sediments during the winter and lower values during the summer and autumn. Annual variations are also observed, with export values varying by a factor 2 to 10 between years for one catchment and the amplitude of the annual variations varying between catchments. The influence of rainfall in the sediment exports is predominant, but investigations on physical characteristics of each catchment (e.g., lithology, slope, land use) are required to better understand the production and transfer processes within a drainage basin. These annual variations imply that long-term data are required to provide mean SY values representative of the catchment functioning. From our calculations, 18 complete years of data are required to obtain a mean SY value with less than 10% of variation on average around the mean. From our results on nested catchments over a long-time scale (40 yr), it appears that most of the suspended sediment load entering the water system is transported downstream. Covariations of the annual-SY values are generally observed for two gauging stations located on the same river. The nested catchment approach is an interesting tool for the identification of active sediment sources within a large catchment and for the construction of detailed sediment budgets.