Conselho Nacional de Desenvolvimento Científico e Tecnológico / Knowledge of the main diffuse sources of sediment can enhance efficiency in the use of public resources invested in soil conservation management strategies. Conventional fingerprinting methods based on geochemical composition are time-consuming and require critical preliminary sample preparation. In this context, spectroscopic methods can be less labor-intensive, cheap, and viable alternative for this purpose. The present study aimed to quantify the sediment sources supplied to rivers in agricultural catchments of Rio Grande do Sul State, Brazil, and to evaluate the potential use of spectroscopy measurements as a low cost and easy alternative to fingerprint sediment sources. Five study areas with increased size (Arvorezinha, Júlio de Castilhos 1, Júlio de Castilhos 2, Conceição, and Guaporé, with areas of 1.19, 0.80, 1.43, 804.3, and 2,027.2 km2, respectively) were evaluated. Sediment sources evaluated were crop fields, grasslands, unpaved roads, and stream channels. Sediment sampling strategies included time-integrated samplers, fine-bed sediments, and storm-event sediments. The total concentrations of several geochemical tracers were measured in both sediment and source samples. Spectral measurements were made for ultraviolet-visible, near-infrared, and mid-infrared ranges only for the Arvorezinha catchment. Source ascriptions obtained by alternative methods based on spectroscopy analysis were in agreement with ascriptions from classical fingerprinting method based on geochemical composition. Spectral information can provide as relevant information as the geochemical tracers. Furthermore, combining visible-based-colour with geochemical tracers was a rapid and cheap way to enhance discrimination between source types and to improve the precision of sediment sources apportionment. Results of sediment source apportionment demonstrate that other factors than proportion of land use, such as distribution of croplands, forests, and unpaved roads across the landscape play an important role in sediment production. Riparian forests seems to be a key factor to control stream channel erosion. The sediment yielded from unpaved roads seems to be strongly scale-related and it depends on the number of intersections between roads the stream network. Crop fields, even when cultivated with no-tillage, are still the main source of sediment to rivers in agricultural catchments in Southern Brazil. The amount of cropland specific sediment yield ranged from 0.06 to 3.95 ton ha 1 yr 1. These variations are partly attributed to the relief and slope, but land use and soil management are also important control factors. The cropland specific sediment yield remains too high for areas with low sensitivity to erosion where no-tillage is applied, as in Conceição catchment (1.30 ton ha 1 of cropland yr 1), indicating that additional efforts are necessary to further reduce soil erosion. Therefore, there is an urgent need to better plan land use and cover in these catchments, inasmuch as the soil management systems used by farmers proved to be inefficient to reduce runoff and erosion in cultivated areas of Southern Brazil. / Knowledge of the main diffuse sources of sediment can enhance efficiency in the use of public resources invested in soil conservation management strategies. Conventional fingerprinting methods based on geochemical composition are time-consuming and require critical preliminary sample preparation. In this context, spectroscopic methods can be less labor-intensive, cheap, and viable alternative for this purpose. The present study aimed to quantify the sediment sources supplied to rivers in agricultural catchments of Rio Grande do Sul State, Brazil, and to evaluate the potential use of spectroscopy measurements as a low cost and easy alternative to fingerprint sediment sources. Five study areas with increased size (Arvorezinha, Júlio de Castilhos 1, Júlio de Castilhos 2, Conceição, and Guaporé, with areas of 1.19, 0.80, 1.43, 804.3, and 2,027.2 km2, respectively) were evaluated. Sediment sources evaluated were crop fields, grasslands, unpaved roads, and stream channels. Sediment sampling strategies included time-integrated samplers, fine-bed sediments, and storm-event sediments. The total concentrations of several geochemical tracers were measured in both sediment and source samples. Spectral measurements were made for ultraviolet-visible, near-infrared, and mid-infrared ranges only for the Arvorezinha catchment. Source ascriptions obtained by alternative methods based on spectroscopy analysis were in agreement with ascriptions from classical fingerprinting method based on geochemical composition. Spectral information can provide as relevant information as the geochemical tracers. Furthermore, combining visible-based-colour with geochemical tracers was a rapid and cheap way to enhance discrimination between source types and to improve the precision of sediment sources apportionment. Results of sediment source apportionment demonstrate that other factors than proportion of land use, such as distribution of croplands, forests, and unpaved roads across the landscape play an important role in sediment production. Riparian forests seems to be a key factor to control stream channel erosion. The sediment yielded from unpaved roads seems to be strongly scale-related and it depends on the number of intersections between roads the stream network. Crop fields, even when cultivated with no-tillage, are still the main source of sediment to rivers in agricultural catchments in Southern Brazil. The amount of cropland specific sediment yield ranged from 0.06 to 3.95 ton ha 1 yr 1. These variations are partly attributed to the relief and slope, but land use and soil management are also important control factors. The cropland specific sediment yield remains too high for areas with low sensitivity to erosion where no-tillage is applied, as in Conceição catchment (1.30 ton ha 1 of cropland yr 1), indicating that additional efforts are necessary to further reduce soil erosion. Therefore, there is an urgent need to better plan land use and cover in these catchments, inasmuch as the soil management systems used by farmers proved to be inefficient to reduce runoff and erosion in cultivated areas of Southern Brazil.

La connaissance des principales sources diffuses de sédiments permettrait d'améliorer l'utilisation des ressources publiques investies dans les stratégies de gestion des sols. Les méthodes de traçage (fingerprinting) conventionnelles basées sur la composition géochimique sont laborieuses et nécessitent une préparation importante des échantillons. Cette étude visait à rechercher les sources de sédiments dans des bassins versants agricoles du Rio Grande do Sul (sud du Brésil) et d'évaluer l'utilisation des outils spectroscopiques comme technique alternative. La superficie des bassins versants étudiés est comprise entre 0,80 et 2027 km². Les sources de sédiments correspondent aux terres cultivées, aux prairies, aux chemins agricoles et aux berges des cours d'eau. L'échantillonnage des sédiments est basé sur des préleveurs automatiques pour un suivi temporel, des prélèvements de sédiments fins du lit de la rivière, et lors d'événements de pluvieux. La concentration totale des traceurs géochimiques a été mesurée dans les échantillons de sédiments et de sols. Les mesures spectrales ont été faites dans la gamme ultraviolet-visible, proche infrarouge et infrarouge moyen pour le bassin versant d'Arvorezinha. Les deux méthodes, classiques et spectroscopiques, permettent de discriminer et de fournir la contribution des sources de sédiments. L'information spectrale peut être aussi précise que les traceurs géochimiques. En outre, la combinaison de paramètres de couleur dérivés du spectre dans le visible avec traceurs géochimiques était une façon rapide et peu coûteuse pour améliorer la discrimination entre les sources et la précision des prédictions. La contribution des sources de sédiments démontrent que d'autres facteurs que proportion de l'utilisation des terres, comme la distribution de terres agricoles, les forêts et les chemins agricoles dans le paysage, jouent un rôle important dans la production de sédiments. Forêt riparienne semble être un facteur clé de l'érosion des berges des cours d'eau. L'érosion due aux chemins agricoles semble être fortement liée échelle d'observation et dépend du nombre de points ou les routes traversent le réseau hydrographique. Les terres cultivées, même lorsque cultivées sans labour (semis direct), sont encore la principale source de sédiments dans les bassins versants agricoles dans le sud du Brésil. La quantité de sédiment produite par les terres cultivées et par unité de surface qui atteint efficacement le réseau de drainage variait de 0,06 à 3,95 tonnes ha-1 an-1. Ces variations sont partiellement liées au relief et à la pente, mais elles sont essentiellement influencées par l'utilisation des terres et la gestion des sols. La quantité de sédiments provenant des terres cultivées est encore trop élevée pour des zones de faible érosivité et cultivées sans labour du sol, comme le bassin versant de Conceição (1, tonnes ha-1 an-1). Ceci, indique que des efforts supplémentaires sont encore nécessaires pour réduire l'érosion du sol. Par conséquent, il est urgent de mieux planifier l'utilisation et l'occupation des terres dans ces bassins versants, dans la mesure où les systèmes de gestion des sols utilisés par les agriculteurs sont encore inefficaces pour réduire le ruissellement et l'érosion dans les zones cultivées dans le sud du Brésil. / The knowledge of the main diffuse sources of sediment production can enhance efficiency in the use of public resources invested in management strategies that seek to mitigate sediment transfer. Conventional fingerprinting methods based on geochemical composition are timeconsuming and require critical preliminary sample preparation. In this sense, spectroscopic methods can be less labor-intensive, cheap, and viable alternative for this purpose. The present study aimed to evaluate the sediment sources in agricultural catchments from Rio Grande do Sul State, Brazil, and to evaluate the potential use of spectroscopy measurements as a low cost and easy alternative to fingerprinting sediment sources. The total area of the study includes five catchments as Arvorezinha, Júlio de Castilhos 1, Júlio de Castilhos 2, Conceição, and Guaporé with areas of 1.19, 0.80, 1.43, 804.3, and 2,027.2 km², respectively. Sediment sources evaluated were crop fields, grasslands, unpaved roads, and stream channels. Sediment sampling strategy included time-integrated samplers, fine-bed sediments, and storm-event sediments. The total concentrations of several geochemical tracers were estimated in sediment and source samples. Spectral measurements were made for ultraviolet-visible, near-infrared, and mid-infrared ranges only for the Arvorezinha catchment. Source ascriptions obtained by alternative methods based on spectroscopy analysis were in agreement with ascriptions from classical fingerprinting method based on geochemical composition. Spectral information can be as precise as the geochemical tracers. Besides, combining visible-based-colour to geochemical tracers was a rapid and inexpensive way to enhance discrimination between source types and to improve precision of sediment sources apportionment. Findings of sediment source apportionment demonstrate that other factors than proportion of land use, such as distribution of croplands, forests, and unpaved roads in the landscape; play an important role in sediment production. Riparian forests seems to be a key factor for stream channel erosion. Unpaved roads seems to be strongly scale-related and dependent upon the number of points were roads across directly the stream network. The crop fields, even when cultivated with no-tillage, are still the main source of sediment in agricultural catchments in southern Brazil. The amount of sediment yielded from crop fields per unit of area that actually reaches the stream outlet ranged from 0.06 to 3.95 ton ha-1 yr-1. These variations are attributed partly to the relief and slope, but land use and soil management mostly influences them. The amount of sediment originated from crop fields are still too high for low susceptible areas with no-till, as Conceição catchment (1.30 ton ha-1 yr-1), indicating that further efforts are still necessary to further reduce soil erosion. Therefore, there is an urgent need to better plan land use and occupation in these catchments, inasmuch as the soil management systems used by farmers are still inefficient to reduce runoff and erosion in areas with crops in southern Brazil. / O conhecimento das principais fontes difusas de sedimentos pode aumentar a eficiência de utilização dos recursos investidos em estratégias de gestão que visem mitigar a transferência de sedimentos aos cursos d'água. Métodos convencionais baseados na composição geoquímica ainda tem alto custo, são onerosos e demandam preparação preliminar crítica das amostras. Dessa forma, métodos espectroscópicos podem ser uma alternativa menos trabalhosa, mais rápida e viável para esse propósito. Objetivou-se com o presente trabalho avaliar a contribuição das fontes de sedimentos em bacias hidrográficas agrícolas do estado do Rio Grande do Sul, e avaliar o potencial uso da espectroscopia como uma alternativa para traçar a origem dos sedimentos. As áreas de estudo são representativas dos principais impactos da agricultura nos recursos hídricos no estado. A área total das bacias hidrográficas de Arvorezinha, Júlio de Castilhos 1, Júlio de Castilhos 2, Conceição e Guaporé é de 1,19, 0,80, 1,43, 804,3 e 2.031 km², respectivamente. As fontes de sedimento avaliadas foram lavouras, pastagens, estradas e canais da rede de drenagem. A estratégia de amostragem de sedimentos incluiu coleta com amostradores do tipo integrador no tempo, sedimento de fundo e sedimento coletado durante eventos pluviométricos. A concentração de vários traçadores geoquímicos foram estimados nas amostras de sedimento e das fontes. Análises espectroscópicas foram realizadas na região do ultravioleta-visível, infravermelho próximo e infravermelho médio na bacia hidrográfica de Arvorezinha. A contribuição das fontes estimadas pelos métodos espectroscópicos foram similares às obtidas com traçadores geoquímicos. Além disso, a combinação de parâmetros de cor derivados da faixa espectral do visível pode ser uma alternativa rápida e de baixo custo para melhorar a discriminação das fontes e aumentar a precisão das predições. Os resultados demonstram que outros fatores além da proporção do uso do solo são importantes na produção do sedimento, como a distribuição das lavouras, florestas e estradas na paisagem. As florestas ripárias exercem um fator chave na erosão dos canais de drenagem. As estradas parecem ser fortemente dependentes da escala e do número de pontos em que cruzam os canais de drenagem. As lavouras, mesmo cultivadas sob plantio direto, ainda são as principais fontes de sedimentos nas bacias hidrográficas rurais do sul do Brasil. A quantidade de sedimento produzida nas lavouras que atinge a rede fluvial variou de 0.06 a 3.95 ton ha-1 ano-1. Essa variação é devida em parte às condições naturais intrínsecas de relevo e fortemente influenciada pelo uso e manejo do solo. A quantidade de sedimento erodida das lavouras ainda são muito elevadas para áreas de baixa susceptibilidade à erosão manejadas sob plantio direto, como na bacia hidrográfica do Conceição (1.30 ton ha-1 ano-1), indicando que maiores esforços ainda são necessários para reduzir a erosão do solo. Existe uma necessidade urgente de planejar a utilização e ocupação da terra nessas bacias, uma vez que os sistemas de manejo do solo utilizados pelos agricultores ainda são ineficientes para reduzir a erosão nas lavouras no sul do Brasil.