Quantifying sediment fluxes and sources in eucalyptus and grassland catchments in the brazilian Pampa biome

Abstract

Brazil is one of the largest producers of forests planted with eucalyptus in the world. In the southern region of the country, the effects of silviculture with this genus in hydro-sedimentological processes are still not well known in the basin scale. Knowledge of sediment sources and their spatial and temporal variations is a prerequisite for designing effective management measures for land use and management. The objective of this study was to evaluate the hydro-sedimentological responses in two paired river basins located in the “Campanha” region in the state of Rio Grande do Sul, and to evaluate the relative contribution of sediment sources and the dominant erosion process. The study areas are located in the municipality of São Gabriel, RS, Brazil, with predominant soil use, Eucalyptus saligna (EC-0.83 km2) and pasture with extensive livestock (GC-1.10 km2), respectively. In order to quantify the impacts of different land uses, the variables flow, turbidity and precipitation were monitored and recorded every 10 minutes of interval during the period between September 2013 and March 2017. In addition to the automation of the data, water and sediment were collected during rainfall events in the outlet of each catchment. In order to identify the origin of the sediment as a function of soil use, tracers from the conventional approach (radionuclides, stable and geochemical isotopes) and (visible) spectroscopy were combined to obtain the best source discrimination for the fine fraction of sediments suspension, <0.063 mm). For the coarse fraction (bottom sediment, 0.063-2 mm) only the geochemical elements were evaluated. The results evidenced a higher water loss due to surface runoff and, consequently, higher sediment yield and bed load to GC in relation to EC, with values higher up to 12 times. In relation to the contribution of soil use to the origin of fine sediment production, the following average magnitude was observed for GC: pasture with oats (49%) > channel (26%) ≥ degraded natural field (25%) with error <15%, the spectroscopic method being the best for its discrimination. For EC, the mean magnitude corresponded to: channel (81%) > eucalyptus (16%) > forest roads (3%) being the combination of different tracers similar for discrimination. Considering the fine fraction of sediments, the largest contribution of this fraction occurred through subsurface erosive processes in the two studied catchments, being characterized in the present study by the canal and road areas. For the coarse fraction of sediment, the discrimination of the erosive processes was not possible only with the analysis of the geochemical elements. The use of different sediment tracers demonstrates the divergence of tracer selection for discriminating the contribution of each soil use to sediment production, even though the areas present equal soil classes. The results of monitoring and identification of sediment sources contribute to a better understanding of the effect of land use on sediment production at the basin scale, which are useful for guiding the management of natural resources. In addition, it shows that, due to hydro- sedimentological monitoring and sediment tracing, the silviculture activity with eucalyptus in this region presents less contribution of sediment compared to field use with extensive livestock that is a representative use of the region.