Impacts of environmental factors and freshwater discharges from multiple river outlets on estuarine salinity variation in rainy season: A case study of the Pearl River Estuary in China

Abstract

Estuarine salinity is one of the key factors affecting coastal ecosystems. However, due to impenetrable situation of mixing streamflow from river basins and seawater currents, our capability of evaluating salinity variation is far behind requirement in preventing coastal environment. Thus, this paper presents an integrated study on salinity variation in the Pearl River Estuary (PRE) in South China in the rainy season since the estuary would experience more salinity variation at the time. The PRE has a complicated water body with eight river outlets, four ones in the east and the others in the west. The paper develops a new research framework to evaluate the impacts of streamflow on the salinity variation, including adjusting diversion ratios of freshwater discharges to the estuary and using two numerical models for simulating basin hydrologic processes and estuary salinity variations. With the salinity observation from a field cruise in the PRE in July 2017 and the newly-developed framework, this study validates the numerical simulation results, detects the salinity variation under different scenarios of freshwater discharge, and discloses the environmental factors related to salinity variation. Specifically, the study shows the distinct characteristics of salinity variation in association with freshwater discharge and environmental factors, including seawater current velocity and topography in the east (a funnel-shaped bay consists of four narrow deep channels), and west (a sub-estuary of the Modaomen waterway) part of the estuary. The results indicate that salinity in the east part is more sensitive to streamflow variation with a farther freshwater-saltwater divide (about 25 km) away from the outlets than that in the west part (14 km). The estuary area affected by salinity variation due to different streamflow diversion ratios is more significant in the east (1,266 km2) which is 7.6 times larger than that in the west (166 km2). However, the values of salinity change due to different discharges are greater in the west (more than 2.0 psu in 34% of the affected area) than those in the east (all less than 2.0 psu). Moreover, the study shows that the seawater salinity variation in the east is greatly influenced by the distance to the outlet, freshwater discharge and seawater depth; while in the west, the salinity variation is largely determined by seawater current velocity.