Remote sensing of large reservoir in the drought years: Implications on surface water change and turbidity variability of Sobradinho reservoir (Northeast Brazil)

Abstract

Sobradinho reservoir has been suffering a severe water loss caused by multi-year drought in the Northeast Brazil. This reservoir contributes to the socio-economic development of the semi-arid region, and the monitoring of water shortage is crucial for people living in this climate-vulnerable region. In this study, we evaluate the surface water change and turbidity variability of Sobradinho reservoir during recent drought years (2013)(2014)(2015)(2016)(2017). A time-series dataset was created using 109 Landsat-8 OLI images for mapping the water extent in the reservoir. A non-linear regression between measured turbidity and surface reflectance (red band) was developed and applied for turbidity retrievals. Additionally, we performed a long-term precipitation analysis (17-year) to assess the rainfall deficit over the catchment area. Our results show that the annual precipitation regimes are below the long-term average during 2012 -2017 period, except 2013. We also found that negative anomalies occur during 26 out of 36 months between 2014 and 2016, mostly in the rainy season. Since the rainfall regimes and river discharges are the major drivers for water recharge, these drought years have a critical impact on the reservoir level. According to our results, the water surface receded about 2,073 km² (out of total 3303 km²) during September 2017; this represents a reduction of 62.8 % in the total water extent. The surface water change is spatially distinct across the reservoir. For instance, the upper section of the reservoir was almost totally dried during September 2017, and the water coverage was ~ 8% (91.25 km2 out of 1128 km²). Although other sections had a relatively low water change (reduction of ~ 40%), the losses are significant in terms of area (~1,035.5 km²). The receding of water extent affects the people living near to the reservoir, and local communities are more distant from water (up to 13 km). We also observed that the turbidity is seasonally dependent, and water clarity presents a strong variability between rainy and dry seasons. In general, the turbidity levels vary from clear water (0 -20 NTU) during the dry season to turbid condition (> 50 NTU) during the rainy season. A lack of access to clean and safe drinking water in some periods might be harmful to humans, livestock and domestic animals. Finally, this research contributes to the assessment of drought-related impacts in the Sobradinho, the largest reservoir in the Northeast Brazil. The water shortage is a recurring concern in the semi-arid region, and the remote sensing techniques provide spatially explicit information to enhance the livelihood resilience during drought years.

Respostas da qualidade da água e fitoplânctons à redução de vazão e recepção de cargas de sedimentos no reservatório de Xingó/AL

“…Na série histórica de 2010 a 2015, o ano mais seco da bacia do rio São Francisco foi o ano de 2015 com a precipitação média de 563,4 mm, assim, a precipitação do ano de 2015 foi equivalente à metade (50%) da precipitação média de longa duração (1.083 mm). Nesse contexto, Martins et al (2019) destacam o ano de 2012 como o marco do início consecutivo de chuvas abaixo da média, tanto nas regiões hidrográficas, quanto na bacia do rio São Francisco, exceto em 2013. É possível observar na Figura 4 que a região do submédio é a que apresenta mais baixa pluviometria, haja vista a sua inserção no semiárido do Nordeste Brasileiro, tal região hidrográfica destaca-se por exercer fortes influências na qualidade da água do baixo São Francisco (Medeiros et al, 2016), onde se localiza o reservatório de Xingó.…”

“…Na redução de vazão-Qred, a mediana da transparência de P1 teve um acréscimo de 1,1m, ou seja, os níveis aumentaram devido à diminuição de entrada de sedimentos através do escoamento superficial na região semiárida da bacia hidrográfica (Medeiros et al, 2016;Mosley et al, 2012). Já as medianas de P2 e P3 reduziram (Figura 6c), segundo Martins et al (2019), a contínua redução de vazão do rio São Francisco também conduziu à concentração de sedimentos no principal canal do reservatório de Sobradinho, à montante de Xingó, acarretando a desconexão com as regiões próximas à barragem. Essa falta de ligação impediu a renovação das águas na parte ocidental, afetando as condições estéticas e o acesso à água potável.…”

“…Na bacia do rio São Francisco, a redução de vazão foi responsável pelo aumento da salinidade em sua foz (Agência Nacional de Águas, 2019), interrompendo o abastecimento público por diversas vezes na cidade de Piaçabuçu/AL. Segundo Martins et al (2019) a extrema redução de vazão afetou o balanço hídrico de Sobradinho com aumento da temperatura e taxas de evaporação, nessa mesma lógica, a redução do fluxo das águas alterou a dinâmica dos sedimentos, promovendo a degradação das águas na regiões médias e superiores. Esse autor cita que a escassez das águas no reservatório de Sobradinho se agrava na região do semiárido brasileiro, porque os reservatórios a jusante, como Paulo Afonso e Xingó, são dependentes da vazão sazonal de Sobradinho.…”

Analysis of Degradation Processes in Reservoirs Based on Remote Sensing Data

“…Unfortunately, this method provides information limited to point-based representation and does not give a spatial overview of the reservoir's condition. Currently, remote-sensing techniques have a unique advantage in monitoring and mapping seasonal dynamics of processes in reservoirs from space (Klein et al, 2017;Martins et al, 2019;Pekel et al, 2016). Satellite imagery has numerous operational and economic advantages compared to in situ measurements.…”

Analysis of Degradation Processes in Reservoirs Based on Remote Sensing Data

“…Unfortunately, this method provides information limited to point-based representation and does not give a spatial overview of the reservoir's condition. Currently, remote-sensing techniques have a unique advantage in monitoring and mapping seasonal dynamics of processes in reservoirs from space (Klein et al, 2017;Martins et al, 2019;Pekel et al, 2016). Satellite imagery has numerous operational and economic advantages compared to in situ measurements.…”