Measuring and modelling soil erosion and sediment yields in a large cultivated catchment under no-till of Southern Brazil
Résumé
Erosion processes can be exacerbated when inappropriate soil conservation practices are implemented. In Brazil, very few measurements are available to quantify the impact of conservation practices on erosion processes in agricultural catchments. The objective of this study is to quantify the impact of different conservation measures on soil erosion and sediment dynamics in an agricultural catchment under no-till of southern Brazil, and to simulate conservation scenarios using a model calibrated with sediment data measured at the catchment outlet. Monitoring was carried out in a large agricultural catchment (800 km²) of southern Brazil affected by extensive soil erosion and runoff despite the widespread use of no-till. Rainfall, river water discharges and suspended sediment concentrations were monitored during a five-year period (2011-2015). The WaTEM/SEDEM model was then calibrated. Then, validation, four scenarios including a Business-As-Usual (BAU) scenario and the implementation of alternative conservation strategies were simulated, and their impact on erosion, sediment deposition and sediment yield was quantified. All four scenarios were simulated twice, using either rainfall measured during a dry year or during a humid year. All the scenarios including alternative conservation measures drastically reduced erosion and sediment yields, with reductions reaching up to 400% when compared to the BAU scenario. The implementation of mechanical conservation measures such as crop levelling and terracing had the highest impact on soil erosion, and the most effective scenario included the implementation of crop rotation, crop levelling, terracing and the creation of forest protected areas. Model simulations indicated that no-till alone has a low impact on erosion processes and that additional measures increasing the vegetation cover/density of the soil are necessary to significantly reduce sediment transfers in these agricultural areas. The simulations also demonstrate that during wet years, erosion processes increase on average by 5133.9% for all scenarios. This study demonstrates that soil losses due to erosion processes remain significant and unsustainable in agricultural catchments of southern Brazil. Soil erosion is exacerbated by the lack of information provided to the farmers and the use of isolated conservation measures without coordination at the catchment scale. Farmers' and local communities' awareness should be raised to reduce soil degradation and sediment transfer to river systems.
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