Water erosion affects nearly 3 million hectares of agricultural soil in Tunisia, and presents a threat to the sustainability of reservoirs to store surface waters. The high density of gully systems in Tunisia, like in many Mediterranean regions, raises the question of their role in the siltation of reservoirs. In this context, the objective of this thesis is to propose a methodology for evaluating the relative contribution of gully erosion (gullies/channel) compared to surface erosion (rill and interrill) in sediment fluxes at the outlet of small catchments (0.1-10 km ²), for pluri-annual periods (about 15 years). The proposed approach is based on the method of fingerprinting sediment sources developed by Walling et al. (1999), and takes advantage of the stable nature of certain elements to discriminate the origin of sediment trapped in reservoirs. The different steps of this approach consist in i) the identification of potential tracers adapted to the objectives of the study, ii) the quantification of these tracers in sources samples and in sediment cores, iii) the application of a mixing model combined with a Monte Carlo model. The results of this fingerprinting approach primarily tested on the Kamech experimental catchment were subsequently validated using eroded soil volumes estimated by two independent approaches. The results for this catchment show the dominance of surface erosion which represents nearly 80% of the 15 t/ha/year of total erosion. In addition, guidelines for method application have been proposed, including the implementation of a core sampling strategy which limits the level of uncertainty to less than 10%. The method was then applied to four other catchments representative of the diversity of the Tunisian Dorsal and Cape Bon landscapes. The comparison of the results for the five studied catchments shows a wide variability of erosion behaviour. Surface erosion is the dominant process in three catchments which require the establishment of protective cultivation of the soil resource. The two other catchments are dominated by gully erosion and require the priority use of structural measures such as improvements of gullies and bank channel protection. These differences between catchments have been related to the characteristics of the sites in order to identify predictive factors.