Temporal streamflow reduction and impact on the salt dynamics of the São Francisco River Estuary and adjacent coastal zone (NE/Brazil)

Abstract

During the past decades, the São Francisco River basin encountered substantial changes due to intensive human activities such as river regularization and damming, and more recently, climate change. As a result, a decrease of streamflow in the lower São Francisco River sector has received considerable attention due to its drastic geomorphological and hydrodynamic changes. This study presents a detailed investigation on the changes of streamflow and its effect on spatiotemporal migration of estuarine plume along the São Francisco coastal zone from 1994 to 2017 based on salinity and water temperature extracted from HYCOM numerical modeling.The progressive saltwater intrusion from in situ observation for 2005 and 2017, is analyzed. The results show two distinct periods with substantial inter-annual variability of streamflow during the period from 1994 to 2011 and accentuated decreasing trends from the second semester of 2012 onwards, and discontinuous periodicities from later 2012 due to the extreme hydrological events of droughts. The spatiotemporal variability in salinity and water temperature on the coastal zone revealed a severe retraction of the estuarine plume due to the low streamflow rates, which started after the second semester of 2012, and the presence of Tropical Surface Waters (TSW, S > 35.9 g/kg) closer to the river mouth, shifting the previous estuarine mixing zone from the coastal region to inward areas of the estuary. The continuous penetration of saltwater towards inner areas of the São Francisco estuary (∼10 km upstream from estuary mouth) is attributed to the decline of water flow supply from the river upper reaches due to the extreme drought events and water conservation measures. These changes may lead to potential ecological and human problems due to persistent high salinity above the limit for human consumption (S > 0.5 g/kg) from the estuary mouth to the limits of local cities along the river.