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 (2013–2017). 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 2073 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 (~1035.5 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.