Fingerprinting sediment sources in the outlet reservoir of a hilly cultivated catchment in Tunisia

Abstract : Purpose: Approximately 74% of Tunisian agricultural soils are affected by water erosion, leading to the siltation of numerous man-made reservoirs and therefore a loss of water storage capacity. The objective of this paper is to propose a methodology for estimating the relative contributions of gully/channel bank erosion and surface topsoil erosion to the sediment accumulated in small reservoirs. Materials and methods: We tested an approach based on the sediment fingerprinting technique for sediments collected in a reservoir installed in 1994 at the outlet of a pilot catchment (Kamech, 2.63 km²). Sampling efforts were concentrated on the soil surface (in both cropland and grassland), gullies and channel banks. A total of 17 sediment cores were collected along a longitudinal transect of the Kamech reservoir to investigate the sediment origin throughout the reservoir. Radionuclides (particularly caesium-137) and nutrients (organic matter, total phosphorous and total nitrogen) were analysed as potential tracers. Results and discussion: The applications of the mixing model with caesium-137 alone or caesium-137 and total organic carbon provided very similar results: the dominant source of sediment was surface erosion, which was responsible for 80% of the total erosion within the Kamech catchment. Additionally, we showed that the analysis of a single composite core sample provided information on the sediment origin that was consistent with the analysis of all successive sediment layers observed in the core. We demonstrated the importance of the core sampling location within the reservoir for obtaining reliable information regarding sediment sources and the dominant erosion processes. Conclusions: The dominance of surface erosion processes indicates that conservation farming practices are required to mitigate erosion in the Kamech agricultural catchment. Based on the results from 17 sediment cores, guidelines regarding the number and location of sampling cores to be collected for fingerprinting purposes are proposed. We showed that the collection of two cores limited the sediment source apportionment uncertainty due to the core sampling scheme to less than 10%.