Reservoir siltation because of water erosion is an important environmental issue in Mediterranean countries where storage of clear surface water is crucial for their economic and agricultural development. The high density of gully systems observed in Mediterranean regions raises the question of their contribution to reservoir siltation. In this context, this study quantified the absolute and relative contributions of rill/interrill and gully/channel erosion in sediment accumulation at the outlet of small Tunisian catchments (0·1–10 km2) during the last 15 years (1995–2010). To this end, a fingerprinting method based on measurements of caesium‐137 and total organic carbon combined with long‐term field monitoring of catchment sediment yield was applied to five catchments in order to cover the diversity of environmental conditions found along the Tunisian Ridge and in the Cape Bon region. Results showed the very large variability of erosion processes among the selected catchments, with rill/interrill erosion contributions to sediment accumulated in outlet reservoirs ranging from 20 to 80%. Overall, rill/interrill erosion was the dominant process controlling reservoir siltation in three catchments whereas gully/channel erosion dominated in the other two catchments. We identified the presence of marly gypsum substrates and the proportion of catchment surface covered by soil management/conservation measures as the main drivers of erosion process variability at the catchment scale. These results provided a sound basis to propose guidelines for erosion mitigation in these Mediterranean environments and suggested to apply models simulating both rill/interrill and gully/channel erosion in catchments of the region. Copyright © 2015 John Wiley & Sons, Ltd.

Mapping and monitoring linear erosion features (LEFs) over large areas is fundamental for a better understanding of the main erosion processes and for planning suitable protection measures. The advent of very high‐resolution satellite imagery has expanded the range of satellite LEF identification to moderate‐size elements. After determining the relationship between satellite imagery resolution and the ability to detect LEFs, we discuss a highly automated method for extracting such LEFs from a very high spatial resolution image (0.61 m resolution). The method is based on a two‐stage strategy: (1) extraction of all linear features visible on the satellite image using filters and photo‐interpretation; (2) filtering these linear features according to geometric criteria (e.g. orientation relative to slope, sinuosity, position in landscape, etc.) so as to retain only those relative to linear erosion. A series of three images with increasing spatial resolution (10.5 and 0.61 m) was prepared for an area on the Cap Bon peninsula (Tunisia). This predominantly agricultural area has a high density of LEFs with very varied geometric characteristics. The area's problems are both onsite for the agriculture itself, and offsite with the silting up of hillside reservoirs. Respectively 22 per cent, 37 per cent and 73 per cent of the site's LEFs, with respective average widths of 2.8, 3.0 and 2.2 m, are visible on the 10, 5 and 0.61 m resolution images. Gully identification should help to identify the most threatened areas to help land use planning and management or to validate erosion models whether at regional or local (drainage basin) scale. Copyright © 2011 John Wiley & Sons, Ltd.

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%.

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Abstract. Monitoring agricultural areas threatened by soil erosion often requires decimetre topographic information over areas of several square kilometres. Airborne lidar and remotely piloted aircraft system (RPAS) imagery have the ability to provide repeated decimetre-resolution and -accuracy digital elevation models (DEMs) covering these extents, which is unrealistic with ground surveys. However, various factors hamper the dissemination of these technologies in a wide range of situations, including local regulations for RPAS and the cost for airborne laser systems and medium-format RPAS imagery. The goal of this study is to investigate the ability of low-tech kite aerial photography to obtain DEMs with decimetre resolution and accuracy that permit 3-D descriptions of active gullying in cultivated areas of several square kilometres. To this end, we developed and assessed a two-step workflow. First, we used both heuristic experimental approaches in field and numerical simulations to determine the conditions that make a photogrammetric flight possible and effective over several square kilometres with a kite and a consumer-grade camera. Second, we mapped and characterised the entire gully system of a test catchment in 3-D. We showed numerically and experimentally that using a thin and light line for the kite is key for a complete 3-D coverage over several square kilometres. We thus obtained a decimetre-resolution DEM covering 3.18 km2 with a mean error and standard deviation of the error of +7 and 22 cm respectively, hence achieving decimetre accuracy. With this data set, we showed that high-resolution topographic data permit both the detection and characterisation of an entire gully system with a high level of detail and an overall accuracy of 74 % compared to an independent field survey. Kite aerial photography with simple but appropriate equipment is hence an alternative tool that has been proven to be valuable for surveying gullies with sub-metric details in a square-kilometre-scale catchment. This case study suggests that access to high-resolution topographic data on these scales can be given to the community, which may help facilitate a better understanding of gullying processes within a broader spectrum of conditions.

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Water scarcity and salinization of arable land in Tunisia are forcing the largest water user, the irrigation sector, to modernize and increase its efficiency. This study evaluated the impact of deficit irrigation with saline water in a sandy loam soil cultivated with potatoes in northern Tunisia. The evaluation considered soil salinization and irrigation water productivity in a changing climate by feeding a calibrated model in HYDRUS‐2D/3D with season‐long weather scenarios from one current and two future climate periods. Future weather scenarios were produced by statistically downscaling results from a regional climate model run with two greenhouse gas emission scenarios (RCP2.6 and RCP8.5). Results showed an increase in salt accumulation over the growing season and a potential decrease in crop yield for RCP8.5. RCP2.6 did not, however, result in any significant changes in salt accumulation or crop yield. Even with saline irrigation water, the use of drip technology resulted in a general leaching of salts from the root zone. This was slightly less apparent during deficit irrigation. However, irrigation water productivity was still higher during deficit irrigation than during full irrigation. Deficit irrigation could thus be profitable but long‐term effects should be addressed before making final recommendations. © 2018 John Wiley & Sons, Ltd.