Estratégias de amostragem para estimar a concentração de sedimentos em suspensão para a calibração do turbidímetro

The influx of sediments carried by rivers can be measured continuously using a turbidimeter through the indirect relationship between turbidity and suspended sediment concentration (SSC). However, this process is dependent on obtaining water samples with suspended sediments to calibrate the turbidimeter. Sampling can be carried out through different methods, resulting in errors and uncertainties in estimating the SSC. Four different possibilities for the composition of the calibration curve were tested in order to evaluate the effectiveness of the turbidimeter calibration for different sampling strategies. The study was conducted in 2012 and 2013 in two catchments in southern Brazil that had a strong sediment yield impact. The results indicate that sampling during rainfall events generate a significant and representative SSC for the turbidimeter calibration. The use of an integrating sampler for synthetic samples provides an effective calibration, but overestimates SSC compared to calibration from rainfall event sampling. The samples collected at fixed intervals - weekly and daily - generated an insufficient amount of data and are not representative of real sediment concentrations of the catchments.O fluxo de sedimentos em rios pode ser medido continuamente com o uso de turbidímetro, pela relação indireta entre turbidez da água e concentração de sedimentos em suspensão (CSS). Entretanto, isso é dependente da obtenção de amostras com sedimentos em suspensão para a calibração do turbidímetro, a qual pode ser realizada de diferentes formas, originando erros e incertezas na estimativa da CSS. No intuito de avaliar a eficácia de diferentes estratégias de amostragem para a calibração do turbidímetro foram testadas quatro diferentes possibilidades para a composição da curva de calibração. O trabalho foi realizado em duas bacias hidrográficas no sul do Brasil com forte impacto na produção de sedimentos entre 2012 e 2013. Os resultados indicam que a amostragem durante eventos gera uma quantidade significativa e representativa de valores de CSS para a calibração. O uso do amostrador integrador no tempo para a obtenção de amostras sintéticas permite uma boa estratégia de calibração, mas superestima os valores de CSS em relação à calibração com amostras de eventos. As amostras coletadas em intervalos fixos (semanais e diários) geram uma quantidade insuficiente de dados e não representativos das concentrações de sedimentos que ocorrem nas bacias

Measuring and modelling soil erosion and sediment yields in a large cultivated catchment under no-till of Southern Brazil

International audienceErosion 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

How to model the effect of mechanical erosion control practices at a catchment scale?

Conservation agriculture practices are a crucial factor in mitigating and controlling erosion by water. To address water erosion estimates, most environmental models use the USLE, RUSLE, and MUSLE models. Management practices that affect soil erosion by modifying the flow pattern, such as contour farming, strip farming, or terracing, are represented within these models as a support practice (P) factor. However, due to the difficulty in accurately mapping the P-factor, many studies choose to ignore it, using only the default value P-factor 1which represents the absence of sowing at the level or cultivation in strips. This study proposes a methodology that evaluates the current P-factor based on the angle between the crop row orientation and the elevation contour lines. The method was tested in four areas under soybean crop fields in southern Brazil, totaling 25 km2. The reason for choosing four areas is to select different characteristics of rural properties and topographic conditions. The ideal values of the P-factor are expected to be between 0.5 and 0.6; however, in our case, a P-factor greater than 0.8 was obtained in 60% of the area, indicating the low occurrence of contour farming reduces erosion rates. The results show that policymakers could potentially use this methodology (angle between the crop rows and contour lines) to run soil-erosion risk scenarios for a broader application of contour farming. This allows the P-factor to be quantified via a thematic map instead of assigning uniform P-factor values. With a detailed study of the P-factor on the slopes, there is a better understanding of where to target support practices to reduce erosion

Mobilization and transport of pesticides with runoff and suspended sediment during flooding events in an agricultural catchment of Southern Brazil

International audienceBrazil is one of the largest consumers of pesticides in the world, and these chemicals present a high contamination risk for the country’s water bodies. The mechanisms of mobilization and transport of pesticides from cropland to river systems are controlled by runoff and erosion processes occurring at the catchment scale. In addition to the excessive use of pesticides, the transport processes of these substances are also accelerated by inadequate soil management and the absence of soil conservation measures at the catchment scale. The current research relied on hydrological monitoring to investigate the transport and persistence of pesticides in response to hydrological dynamics. The study was conducted in the Conceição River watershed where runoff and suspended sediment fluxes are continuously monitored at the outlet. This study area is representative of the grain production system in southern Brazil including the application of large amounts of pesticides combined with extensive runoff and erosion problems. Sample collection in the river for pesticide analysis included the analysis of both water and suspended sediment. The sediment deposit analysis was performed in a single location at 4 depths. Results demonstrate the occurrence of pesticides including simazine, 2,4-D, carbendazim, imidacloprid, tebuconazole, propiconazole, tetraconazole, and trifloxystrobin in water, while glyphosate and AMPA were detected in suspended sediments, and AMPA and carbendazim were found in sediment deposits. The study demonstrated the strong dependence of the mechanisms of pesticide mobilization and transport in the catchment with the intra- and interevent variability of hydro-sedimentary processes. Pesticide detections can be related to several factors, including the magnitude of the rainfall event, the period of pesticide application, or the transport of suspended sediment. All these factors are correlated, and the mechanisms of transportation play an important role in the connections between sink and sources. The results suggest that pesticide monitoring should take into account the runoff and erosion pathways in each particular catchment, and it should especially include the monitoring of major rainfall events for identifying and quantifying the occurrence of pesticides in the environment. The transport of pesticides indicates to be potentiated by intensive pesticide use, the magnitude of rainfall-runoff events, and the absence of runoff control measures (e.g., terracing). These results demonstrate that water and soil conservation techniques should be planned and coordinated at the watershed scale to reduce the connectivity of water and sediment flows from agricultural areas to river systems with the implementation of effective runoff control practices. This will control the mobilization agents (runoff), as well as limit the connection between the sources and the water bodies

Quantifying the impact of no-tillage on soil redistribution in a cultivated catchment of Southern Brazil (1964–2016) with 137^{137}Cs inventory measurements

International audienceNo-tillage is a soil management practice that results in reduced soil losses when compared to conventional tillage systems. However, when this practice is overly simplified, it may lead, over the years, to higher levels of soil loss than expected. In this context, this study sought to compare the rates of long-term soil redistribution on three hillslopes used for grain production under different soil management on deep weathered soils (Ferralsols) in southern Brazil. Soil samples were collected along three transects in different hillslopes characterized by either no-tillage or conventional tillage. Cs-137 inventories were used to estimate the soil redistribution rates based on Mass Balance Model - 2. The results indicate that along the three slopes and during the last five decades, changes in soil management impacted the patterns of soil erosion in the landscape, showing the occurrence of significant soil loss in the upper and backslope segments, and deposition in the lower parts of the three hillslopes studied. Even with no-tillage, erosion has continued to occur, although at lower rates when compared to conventional tillage. The use of the $^{137}$Cs marker associated with the Mass Balance Model - 2 (MBM - 2) conversion model provided an effective tool for estimating soil redistribution rates under different management systems. Although the introduction of no-tillage in the last 28 years has reduced erosion rates, these processes remain significant and the implementation of additional runoff and/or erosion control practices is recommended in order to keep erosion rates at sustainable levels

Soil management practices adopted by farmers and how they perceive conservation agriculture

Texto completoIn some rural properties of Brazil, soil and water conservation practices are partially or inappropriately implemented, intensifying soil erosion, even in no-tillage (NT) areas. This study aimed to check the farmer’s understanding of conservation agriculture (CA) and assess whether they are using practices appropriate to soil conservation. A basin in the state of Paraná, in Southern Brazil, was selected. Surveys were conducted based on a structured questionnaire and in-person interviews of 234 farmers. Among these farmers, 67 % do not understand CA pillars adequately, and 68 % stated they have been using NT for over 12 years. However, 58 % stated that they carried out some kind of soil preparation. Furthermore, some of the farmers only partially implemented CA, adopting a low level of crop diversification. The main problems pointed out by the farmers to justify simplifying the CA approach were soil compaction, low production of plant biomass and resistance of weeds to herbicides. Most farmers in the study do not have enough knowledge of the pillars of CA. As a result, there is little diversification in crop rotation, and the soil is continually disturbed with chiseling. Furthermore, although most farmers use terracing, they drive sprayers crossing terraces and following the slope, a practice that can impair the capacity of terraces to control surface water runoff. These practices do not contribute to soil and water conservation and endanger the sustainability of agricultural production systems

Estratégias de amostragem para estimar a concentração de sedimentos em suspensão para a calibração do turbidímetro

The influx of sediments carried by rivers can be measured continuously using a turbidimeter through the indirect relationship between turbidity and suspended sediment concentration (SSC). However, this process is dependent on obtaining water samples with suspended sediments to calibrate the turbidimeter. Sampling can be carried out through different methods, resulting in errors and uncertainties in estimating the SSC. Four different possibilities for the composition of the calibration curve were tested in order to evaluate the effectiveness of the turbidimeter calibration for different sampling strategies. The study was conducted in 2012 and 2013 in two catchments in southern Brazil that had a strong sediment yield impact. The results indicate that sampling during rainfall events generate a significant and representative SSC for the turbidimeter calibration. The use of an integrating sampler for synthetic samples provides an effective calibration, but overestimates SSC compared to calibration from rainfall event sampling. The samples collected at fixed intervals - weekly and daily - generated an insufficient amount of data and are not representative of real sediment concentrations of the catchments.O fluxo de sedimentos em rios pode ser medido continuamente com o uso de turbidímetro, pela relação indireta entre turbidez da água e concentração de sedimentos em suspensão (CSS). Entretanto, isso é dependente da obtenção de amostras com sedimentos em suspensão para a calibração do turbidímetro, a qual pode ser realizada de diferentes formas, originando erros e incertezas na estimativa da CSS. No intuito de avaliar a eficácia de diferentes estratégias de amostragem para a calibração do turbidímetro foram testadas quatro diferentes possibilidades para a composição da curva de calibração. O trabalho foi realizado em duas bacias hidrográficas no sul do Brasil com forte impacto na produção de sedimentos entre 2012 e 2013. Os resultados indicam que a amostragem durante eventos gera uma quantidade significativa e representativa de valores de CSS para a calibração. O uso do amostrador integrador no tempo para a obtenção de amostras sintéticas permite uma boa estratégia de calibração, mas superestima os valores de CSS em relação à calibração com amostras de eventos. As amostras coletadas em intervalos fixos (semanais e diários) geram uma quantidade insuficiente de dados e não representativos das concentrações de sedimentos que ocorrem nas bacias