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Water and sediment discharges can change rapidly, and low-frequency measurement devices might not be sufficient to elucidate existing dynamics. As such, above-water radiometry might enhance monitoring of suspended particulate matter (SPM) dynamics in inland waters. However, it has been barely applied for continuous monitoring, especially under partially cloudy sky conditions. In this study, an in situ, high-frequency (30 s timestep), above-water radiometric dataset, collected over 18 days in a tropical reservoir, is analyzed for the purpose of continuous monitoring of SPM concentration. Different modalities to retrieve reflectance spectra, as well as SPM inversion algorithms, were applied and evaluated. We propose a sequence of processing that achieved an average unsigned percent difference (UPD) of 10.4% during cloudy conditions and 4.6% during clear-sky conditions for Rrs (665 nm), compared to the respective UPD values of 88.23% and 13.17% when using a simple calculation approach. SPM retrieval methods were also evaluated and, depending on the methods used, we show that the coefficient of variation (CV) of the SPM concentration varied from 69.5% down to 2.7% when using a semi-analytical approach. As such, the proposed processing approach is effective at reducing unwanted variability in the resulting SPM concentration assessed from above-water radiometry, and our work paves the way towards the use of this noninvasive technique for high-frequency monitoring of SPM concentrations in streams and lakes.

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The debris flood created by the 5 November 2015 failure of the Fundão dam in Brazil injected a large amount of fine sediment in the Gualaxo do Norte River system. Although coarse and sand-sized sediment injections in rivers have been studied in relative detail, little is known about river response to overloading of mud-sized sediments (consisting of clay and silt). This paper presents an assessment of suspended sediment transport occurring in and along the Gualaxo do Norte River after the Fundão dam failure to contribute to the general understanding of how rivers recover following large inputs of mud-sized sediments (consisting of clay and silt). The average total sediment removal estimated based on the last 2 rainy periods is 54 466 tonnes, ranging from 37 385 to 71 546 tonnes according to the uncertainty analysis. The sediment transport analysis suggests that the Gualaxo do Norte River is returning to its pre-event morphological character in terms of sediment transport. However, the morphologic recovery of the system has been constrained in recent years by decreased stream power, the result of moderate wet seasons and limited large flood events. We anticipate that future larger flood events will transport most of the remaining available in-channel tailings, speeding up the physical morphologic recovery of the Gualaxo do Norte River, which is a key component of improving water quality and eventually the river ecology. Although the proposed approach for the sediment budget is simplified and has limitations and uncertainties, it provides a scientific basis to explain the natural fluvial processes that have been occurring in the river system. The approach used for the sediment budget presented in this paper could be applied to similar cases with limited data. Integr Environ Assess Manag 2020;16:572-582. © 2020 SETAC.

A passagem da onda de rejeitos criada pelo rompimento da barragem de Fundão, em 5 de novembro de 2015, despejou uma grande quantidade de sedimentos finos no rio Gualaxo do Norte. Embora a inserção de sedimentos grossos e do tamanho de areia em rios tenham sido estudada em maiores detalhes, pouco se sabe sobre a resposta de rios à uma sobrecarga de rejeitos do tipo lama (sedimentos do tamanho de argila-silte). Este artigo apresenta uma avaliação do transporte de sedimentos em suspensão que tem ocorrido no rio Gualaxo do Norte depois do rompimento da barragem de Fundão, com o objetivo de contribuir para o entendimento geral de como sistemas fluviais se recuperam após sobrecargas de sedimentos finos (argila-silte). A remoção média total de sedimentos estimada com base nos dois últimos períodos chuvosos é de 54.466 toneladas, variando de 37.385 a 71.546 toneladas de acordo com a análise de incerteza. A análise do transporte de sedimentos sugere que o rio Gualaxo do Norte está retornando a sua condição morfológica pré-evento em termos de transporte de sedimentos. No entanto, a recuperação física do sistema tem sido limitada pelo comportamento hidráulico do rio nos últimos anos, com estações de chuvosas moderadas e limitados eventos de cheia de grande magnitude. É muito provável que futuros eventos de cheia de grande magnitude irão transportar a maior parte dos rejeitos restantes disponíveis no canal, acelerando a recuperação morfológica física do rio Gualaxo do Norte. Esse processo é um componente essencial na recuperação da qualidade da água e da ecologia do rio. Embora a abordagem proposta para o balanço de massa de sedimentos seja simplificada e tenha limitações e incertezas, ela fornece uma base científica para explicar os processos fluviais naturais que vêm ocorrendo no sistema fluvial. A abordagem apresentada para o balanço de massa de sedimentos pode ser aplicada a casos semelhantes com dados limitados. Integr Environ Assess Manag 2020;16:572-582.

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In Brazil, the cultivation of bioenergy crops is expanding at an accelerated rate. Most of this expansion has occurred over low-intensity pasture and is considered sustainable because it does not involve deforestation of natural vegetation. However, the impacts on the water quality of headwater streams are poorly understood, especially with regard to the influence of land use patterns in the watershed. In this study, we investigated the effects of land-use conversion on the water quality of streams draining sugarcane fields and examined whether the preservation of forested areas at the top of the headwaters would help mitigate the negative impacts of intensive agriculture. Water samples were collected in two paired catchments in southeastern Brazil, which is one of the largest sugarcane production regions in the world. Our results show significant differences in the water quality of streams predominantly draining the pasture or the sugarcane field. Several parameters commonly used to indicate water quality, such as the concentrations of nitrate and suspended solids, were significantly higher in the sugarcane than in the pasture stream. Differences in water quality between the streams draining predominantly pasture or sugarcane fields were accentuated during the wet season. The preservation of forests surrounding the headwater streams was associated with overall better water quality conditions, such as lower nitrate concentrations and temperature of the stream water. We concluded that forest conservation in the headwater agricultural catchments is an important factor preventing water quality degradation in tropical streams. Therefore, we strongly recommend the preservation of robust riparian forests in the headwaters of tropical watersheds with intensive agriculture. More studies on the effects of best agricultural practices in bioenergy crops can greatly improve our capacity to prevent the degradation of water quality in the tropical waterways as intensive agriculture continues to expand in this region of the world.

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