Perda de solo e aporte de sedimentos na Bacia Hidrográfica do Rio Camaquã – Passo do Mendonça

Abstract

The soil loss by water erosion represents a type of environmental degradation that must be measured and managed to avoid or to reduce the negative effects for the environment, such as silting of water bodies, loss of the productive layer of soils and loss of nutrients and agricultural pesticides. Therefore, it is important to measure soil loss and sediment yield, with the purpose to understand the dynamics of sediment production and transport in the study area. The objective of this study was to evaluate the feasibility of using sediment rating curves, adjusted based on different data entry scenarios, to estimate the sediment yield in the control section of the Camaquã River Basin upstream of the Passo do Mendonça fluviometric station (BHRC-PM); to estimate soil loss due to water erosion using the Revised Universal Soil Loss Equation model (RUSLE) and to verify the sediment transport capacity in the area through the Sediment Delivery Ratio (SDR), for the period 1997 to 2019. Four scenarios of manipulation of the input data were tested for the adjustment of the sediment rating curves in power function, which were: complete data, subdivided into time periods, streamflow intervals and seasonal intervals. The evaluation of soil loss by water erosion was performed with the aid of geoprocessing tools, with the RUSLE equation factors modeled for the study area and based on bibliographic research. The relationship between soil loss due to water erosion and sediment yield in the watercourse was also verified through the SDR. The obtained results showed that the use of complete data for the period provided the best fit for the sediment rating curves, as well revealed that subdivision of input data into smaller intervals did not represent an improvement in the predictive performance of the sediment rating curves. It was found that there was a worsening of soil losses in the study area from 1997 to 2019, in addition to the Neosols, Argisols and Planosols were the soil classes that suffered the most degradation and presented greater erosion risks. Different methodologies to obtaining the R and K factors of RUSLE model influenced the average soil losses so that those that resulted in lower values for the factors, reduced the soil losses in more critical classes of soil loss and increased losses in less critical classes, while those that resulted in higher values for the factors implied the growth of soil areas classified with losses of greater proportions. The sediment delivery ratio in the BHRC-PM went from 8% in 1997 to 5,2% in 2019, indicating that there was greater deposition of sediments in the watercourse over the years, while the use of methodologies based on the physiographic features of the area to obtain the SDR overestimated the sediment transport capacity in the watershed outlet.