Quantifying land use contributions to suspended sediment in a large cultivated catchment of Southern Brazil (Guaporé River, Rio Grande do Sul)

Abstract

Cropland cultivated under no-tillage has dramatically increased in Brazil during the last three decades. However, soil degradation by water erosion remains excessive in the country. Furthermore, the impact of inappropriate agricultural practices on soil erosion is often neglected by farmers. Therefore, the contribution of potential sources of sediment should be quantified in order to raise awareness among rural communities on the need to improve land management for protecting soil and water resources. This study quantified the contribution of potential sources to sediment transiting the Guaporé River, draining a 2032-km2 cultivated catchment, in Southern Brazil. Potential sediment source types were surface of cropland soils (n=159), unpaved roads (n=58), and stream channel banks (n=46). A total of 175 suspended sediment samples were collected following different sampling strategies (including sampling of river water during floods, installation of time-integrated suspended sediment samplers, and collection of bed sediment) in 10 sub-catchments, from January 2011 to March 2014. Discriminant properties were selected among the concentrations in 22 geochemical elements and total organic carbon. Results showed that sediment source contributions were similar for the different sediment sampling strategies. Although, the contributions of sediment sources varied in space across Guaporé catchment, they were dominated by cropland (91±15%), while stream channels (5±2%) and unpaved roads (4±10%) were sources of minor importance. Cropland contribution increased with the drained cropland surface area. The unambiguous dominance of cropland as the main source supplying sediment to the Guaporé River justifies the urgent need to better plan land use and to promote the adoption of appropriate conservation farming practices in similar areas of Southern Brazil. Soil losses in cropland could be reduced by implementing crop rotation, mechanical runoff control strategies, and by avoiding the construction of unpaved roads in thalwegs.