Quantifying land use contributions to suspended sediment in a large cultivated catchment of Southern Brazil (Guaporé River, Rio Grande do Sul)
Résumé
18 Cropland cultivated under no-tillage has dramatically increased in Brazil during the last three 19 decades. However, soil degradation by water erosion remains excessive in the country. 20 Furthermore, the impact of inappropriate agricultural practices on soil erosion is often neglected 21 by farmers. Therefore, the contribution of potential sources of sediment should be quantified in 22 order to raise awareness among rural communities on the need to improve land management for 23 protecting soil and water resources. This study quantified the contribution of potential sources to 24 sediment transiting the Guaporé River, draining a 2032-km² cultivated catchment, in Southern 25 Brazil. Potential sediment source types were surface of cropland soils (n = 159), unpaved roads 26 (n = 58), and stream channel banks (n = 46). A total of 175 suspended sediment samples were 27 collected following different sampling strategies (including sampling of river water during 28 floods, installation of time-integrated suspended sediment samplers, and collection of bed 29 sediment) in 10 sub-catchments, from January 2011 to March 2014. Discriminant properties 30 were selected among the concentrations in 22 geochemical elements and total organic carbon. 31 Results showed that sediment source contributions were similar for the different sediment 32 sampling strategies. Although, the contributions of sediment sources varied in space across 33 Guaporé catchment, they were dominated by cropland (91±15%), while stream channels (5±2%) 34 and unpaved roads (4±10%) were sources of minor importance. Cropland contribution increased 35 with the drained cropland surface area. The unambiguous dominance of cropland as the main 36 source supplying sediment to the Guaporé River justifies the urgent need to better plan land use 37 and to promote the adoption of appropriate conservation farming practices in similar areas of 38 Southern Brazil. Soil losses in cropland could be reduced by implementing crop rotation, 39 mechanical runoff control strategies, and by avoiding the construction of unpaved roads in 40 thalwegs. 41 42
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