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, 44 leading to the siltation of numerous man-made reservoirs and therefore a loss of water storage 45 capacity. The objective of this paper is to propose a methodology for estimating the relative 46 contributions of gully/channel bank erosion and surface topsoil erosion to the sediment 47 accumulated in small reservoirs. Materials and methods: We tested an approach based on the sediment fingerprinting technique 49 for sediments collected in a reservoir installed in 1994 at the outlet of a pilot catchment 50 (Kamech, 2.63 km²). Sampling efforts were concentrated on the soil surface (in both cropland 51 and grassland), gullies and channel banks. A total of 17 sediment cores were collected along a 52 longitudinal transect of the Kamech reservoir to investigate the sediment origin throughout 53 the reservoir. Radionuclides (particularly caesium-137) and nutrients (organic matter, total 54 phosphorous and total nitrogen) were analysed as potential tracers. 55 Results and discussion: The applications of the mixing model with caesium-137 alone or 56 caesium-137 and total organic carbon provided very similar results: the dominant source of 57 sediment was surface erosion, which was responsible for 80% of the total erosion within the 58 Kamech catchment. Additionally, we showed that the analysis of a single composite core 59 sample provided information on the sediment origin that was consistent with the analysis of 60 all successive sediment layers observed in the core. We demonstrated the importance of the 61 core sampling location within the reservoir for obtaining reliable information regarding 62 sediment sources and the dominant erosion processes. 63 Conclusions: The dominance of surface erosion processes indicates that conservation farming 64 practices are required to mitigate erosion in the Kamech agricultural catchment. Based on the 65 results from 17 sediment cores, guidelines regarding the number and location of sampling 66 cores to be collected for fingerprinting purposes are proposed. We showed that the collection 67 of two cores limited the sediment source apportionment uncertainty due to the core sampling 68 scheme to less than 10%.

Impact of farmland fragmentation on rainfed crop allocation in Mediterranean landscapes: A case study of the Lebna watershed in Cap Bon, Tunisia

“…The Lebna watershed is subject to soil erosion and subsequent reservoir siltation (IAO, 2002;Gaubi et al, 2016). The diffuse erosion rate, which results from agricultural practices, is four times greater than the concentrated erosion rate (Ben Slimane et al, 2013. One-third of the watershed area is covered by natural vegetation ( Table 1).…”

Evaluating sampling efficiency when estimating sediment source contributions to suspended sediment in rivers by fingerprinting

“…Collins et al, 2010a;Evrard et al, 2011;Owens et al, 2000;Franz et al, 2013). The method has been used in a wide range of objectives, scales, types and number of sources, and tracer variables (Ben Slimane et al, 2013;Tiecher et al, 2015;Zebracki et al, 2015). This has led to the development of robust methods capable of defining potential tracers and of estimating sediment contributions from different sources with greater accuracy in mathematical terms, together with measures of the associated uncertainties.…”

Sources and export of particle-borne organic matter during a monsoon flood in a catchment of northern Laos

“…Masiello and Druffel, 2001;Hilton et al, 2010;Smith et al, 2013) stable isotope fingerprinting methods may be applied to particulate material collected from hillslopes to rivers, either independently or in combination with fallout radionuclides to document variations in sediment sources and pathways across catchments (e.g. Ritchie and McCarty, 2003;Ellis et al, 2012;Schindler Wildhaber et al, 2012;Ben Slimane et al, 2013;Koiter et al, 2013). In addition, complementary information on sediment conveyed to the river by runoff and overland flow can also be inferred from water tracers such as 18 O natural abundance (for a review see Klaus and McDonnell, 2013).…”