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Intensification of water erosion by agricultural activities is one of the main causes of soil degradation in subtropical regions. In addition to the on-site negative impacts observed in cropland, the excess of surface runoff increases the transfer of sediments and contaminants to water bodies, resulting in environmental, economic and social deleterious effects. The objective of this thesis is to develop and apply original sediment source fingerprinting techniques in two tributaries of the Uruguay River Basin with contrasting conditions in terms of geology, land use, management, and soil types, representative of those found in the drainage area (266,132 km²). The Conceição River catchment (804 km²) is located in upper parts of the Uruguay River Basin. This catchment is representative of the basaltic plateau, where deep Oxisols rich in clay and iron oxides predominate, and are cultivated for soybean, corn and cereals production or covered with pastures for dairy farming. In contrast, the Ibirapuitã River catchment (5,943 km²) is representative of the sandstone plateau. With a predominance of shallow Regosols and sandy Acrisols more sensitive to degradation, it is mainly occupied by native grasslands of the Brazilian Pampa Biome with extensive cattle ranching, although irrigated rice fields are also found in lowlands. Moreover, a rapid increase in the surface area cultivated with soybean in this catchment has been observed in the last twenty years. Soil samples were collected to characterize land use-based sediment sources and suspended sediment was sampled at the outlet of these catchments using different strategies. All samples were analyzed for a panel of properties. Their geochemical composition, diffuse reflectance spectroscopy properties, radionuclide activities and magnetic susceptibility were determined to provide a set of potential tracing parameters. Magnetic and diffuse reflectance spectroscopy analyses were also carried out on sediment samples collected on the main stream of the Uruguay River, downstream of its confluence with both representative tributaries. Results of sediment source contribution are consistent with field observations. However, as catchment area and source number increase, more uncertainty is observed. Surface sources (pasture and cropland) were indicated as the main source of suspended sediment in the Conceição catchment, contributing with approximately 50%. Stream banks provided the second main source of fine bed sediment, contributing with approximately 35%. Cropland was also found to provide the main source in the Ibirapuitã catchment, contributing 32% of the sediment despite occupying only 9.5% of the total catchment area, followed by subsurface sources (stream bank and gully) with 29%. Unpaved roads and native grassland provided lower contributions (24 and 15%, respectively) in Ibirapuitã catchment. These results indicate that the soil conservation practices used in these catchments are not enough to prevent soil erosion in cropland. Moreover, agricultural activities may enhance erosion and sediment delivery to the river systems, causing soil impoverishment and contamination of water resources. Finally, through the comparison of sediment properties of both tributary catchments, this thesis provided – to our knowledge – the first insights into the contribution of both contrasting geomorphologic regions to the main Uruguay River. The main river sediment samples had similar characteristics to those observed in the Ibirapuitã River, indicating the Pampa region as the main likely source. Despite the associated uncertainties, these results further increase the current concerns regarding the ongoing land use changes observed in the Pampa biome. Appropriate soil conservation practices should therefore be urgently applied in agricultural areas of this region to reduce soil erosion and sediment delivery to the river systems., L'intensification de l'érosion hydrique par les activités agricoles constitue l'une des principales causes de dégradation des sols dans les régions subtropicales. En plus des impacts négatifs observés à l’amont, l'excès de ruissellement accroît le transfert de sédiments et de contaminants vers les masses d'eau. L'objectif de cette étude est de développer des techniques originales de traçage des sources de sédiments dans deux affluents-type du fleuve Uruguay présentant des conditions contrastées en termes de géologie, d'utilisation des terres, de gestion et de types de sols, représentatifs de ceux que l'on trouve dans la zone de drainage (266 132 km²). Le bassin de la rivière Conceição (804 km²) est représentatif du plateau basaltique où prédominent les Oxisols profonds riches en argile et en oxydes de fer, où sont cultivés le soja, le maïs, les céréales et où on trouve des pâturages pour l'élevage laitier. Au contraire, le bassin de la rivière Ibirapuitã (5 943 km²) est représentatif du plateau gréseux où prédominent les Regosols et Acrisols plus sensibles à la dégradation, les prairies indigènes utilisées pour l’élevage bovin extensif prédominent, bien que l'on trouve également des rizières irriguées dans les plaines alluviales. Des échantillons de sol ont été prélevés pour caractériser les sources de sédiments liées à l'utilisation des terres. Des échantillons de sédiments en suspension ont été collectés à l’exutoire de ces bassins versants en utilisant différentes stratégies. Tous ces échantillons ont été analysés avec plusieurs techniques. Leur composition géochimique, leurs propriétés de spectroscopie de réflectance diffuse, leurs activités en radionucléides et la susceptibilité magnétique ont été déterminées afin de fournir un ensemble de paramètres de traçage potentiels. Des analyses magnétiques et de spectroscopie ont également été réalisées sur des échantillons de sédiments collectés sur le cours principal du fleuve Uruguay, en aval de la confluence avec les deux affluents représentatifs. Les résultats de la contribution des sources de sédiments sont cohérents avec les observations sur le terrain. Cependant, plus la superficie du bassin versant et le nombre de sources augmentent, plus les incertitudes sont importantes. Les sources superficielles (pâturages et terres cultivées) ont été indiquées comme la principale source de sédiments dans le bassin de Conceição, avec environ 50% d’apports. Les berges ont fourni, quant à elles, la seconde source de sédiments, avec une contribution d'environ 35%. Les terres cultivées se sont également avérées être la principale source de sédiments dans le bassin d'Ibirapuitã, fournissant 32% des sédiments bien qu'elles n'occupent que 9,5% de la surface totale du bassin, suivies par les sources de subsurface (berges et ravines) avec 29%. Les routes et les prairies indigènes ont fourni, au contraire, des contributions bien plus faibles (24 et 15%, respectivement). Ces résultats indiquent que les activités agricoles peuvent augmenter l'érosion et l'apport de sédiments dans les systèmes fluviaux, provoquant l'appauvrissement des sols et la contamination des ressources en eau. Enfin, en comparant les propriétés des sédiments des deux affluents-type, cette thèse a fourni - à notre connaissance – la première estimation de la contribution de ces deux régions géomorphologiques contrastées aux sédiments du fleuve Uruguay. Les échantillons de sédiments du fleuve principal présentent en effet des caractéristiques similaires à celles observées dans l’affluent Ibirapuitã, indiquant que la Pampa en constitue la source principale. Malgré les incertitudes associées, ces résultats renforcent les préoccupations actuelles en lien avec les changements d'utilisation des terres observés dans la Pampa. Il est donc urgent de mettre en œuvre des stratégies de conservation des sols dans les zones agricoles de cette région afin de réduire l'érosion des sols et l'apport de sédiments aux systèmes fluviaux.

International audience

International audience; An important step in the sediment source fingerprinting approach is the selection of the appropriate tracing parameters to maximise source discrimination. In order to reduce uncertainties and increase discrimination between sources it may be necessary to use multiple tracing parameters. Accordingly, this study investigates the discrimination and apportionment of sediment sources in a rural agricultural catchment obtained by combining colour, magnetic, and geochemical fingerprinting approaches. The Conceição River catchment (804 km 2) has predominantly deep and highly weathered Ferralsols with land-use consisting of croplands (73%), pastures (18%), forests (8%) and other uses (1%). A total of 189 samples were taken from the main sediment sources, including: Crop-lands (CR, n=78), pastures (P, n=24), unpaved roads (UR, n=38), gullies (G, n=15) and stream bank (SB, n=34). Sediment samples were taken from the surface bed (n=10) of the river and with time integrated samplers (n=4). Twenty-two geochemical tracers, 6 magnetic properties and 24 colour parameters were analyzed. Tracers were selected following a three step procedure, including: (i) a conservative range test (95% confident interval), (ii) a Kruskal-Wallis H test, and (iii) discriminant function analysis (DFA). The DFA was performed using four different sets of variables: (i) geochemical variables only (G); (ii) geochemical+magnetic+colour (GMC); (iii) geochemi-cal+colour (GC); (iv) geochemical+magnetic (GM). The selected tracers were introduced into a modified version of the classical Solver-based mixing model that in order to determine the relative contribution of different sources to in-stream sediment through simultaneously minimizing mixing model difference. The G and GC DFAs both resulted 69% of samples correct classified (SCC) as no colour parameters were selected by the DFA. The GMC and GM approaches improved the discrimination, both resulting in 76% of SCC. For the G and GC approaches, the average source contribution for the 12 sediment samples were P 47%, SB 29%, CR 19%, UR 5% and G 0%. For the GMC and GM approaches, the contribution of each source was P 41%, SB 37%, CR 14%, UR 8% and G 0%. These results are counterintuitive to field observations where cropland is anticipated to contribute more sediment than pastures. Future research should use artificial mixtures to validate these results. Both magnetic and colour parameters hold potential to improve discrimination between sources, particularly magnetic parameters in catchments with high weathered soils.

International audience; The hydrological dynamics of a small rural river catchment in southern Brazil have been used to understand the impact of agricultural activity on soil and water resource degradation. The monitoring and modeling strategies employed to better understand the integrated phenomena focus on surface flow dynamics and the associated processes, such as erosion, sediment yield (SY), and chemical element transfer. In this context, monitoring and modeling techniques are combined in order to include the main hydrological processes involved in soil and water resource degradation. The project began in 2002 and over 500 rainfall-runoff-sediment events and suspended sediment concentration make up the hydrological database under different land use and soil management conditions. The 1.2 km2 catchment of the study is characterized by the presence of shallow soils, high slope, intense agricultural activity, and high SY (∼150 t.km-2.y-1). Techniques that enable the identification of the origin and redistribution of sediments (e.g fingerprinting approach and Cs137) and hydrograph analysis are being used to further understand the dynamics of SY in the catchment. The comprehension of the factors involved in the SY dynamics has allowed the improvement and application of different mathematical models (e.g., LISEM and WATERSHED) to simulate surface flow and SY. These models are used to represent the influence of soil and water conservation practices while considering the interaction between the different landscape components (crops, roads, drainage network, etc.). From this set of data and tools, the capacity of the agricultural production system, drainage network, wetlands and riparian forests in accelerating or retarding water flow, sediment, and chemical elements in the landscape are evaluated. All of these studies have been used to improve soil and water conservation practices at catchment scale, meeting the expectations of farmers in erosion control and water storage as well as of the society interested in maintaining the hydrological functions of soils in catchment scale.