Investigation of terrestrial processes over the Pearl River basin and their impacts on the Pearl River estuary in South China

Abstract

Rapid population growth and socioeconomic development in past several decades have resulted in more people living in coastal regions, and estuaries have become critical regions for securing our sustainability. Therefore, it is necessary to have an integrated understanding of relationships between river basins and related estuaries. To this end, this study explores the terrestrial processes over the Pearl River Basin (PRB) and their impacts on the variations of sea water features in the Pearl River Estuary (PRE). In this study, the Soil and Water Assessment Tool (SWAT) model is applied to the PRB for the period from 2008 to 2018, and the model is calibrated, and validated with streamflow and sediment and nutrient observations. The results show that the West River Basin, one tributary of the PRB, contributes around 70% of the total sediment discharge. The nutrient modelling consists of point source (PS) and non-point source (NPS) pollution and the results indicate that April is a critical time with peak concentration of nutrients. It is found that agricultural land contributes to the largest sediment and NPS pollutant yields. A hydrodynamic model, Environmental and Fluid Dynamics Code (EFDC), is applied to the PRE. This study proposes a method of adjusting streamflow diversion ratio of each outlet, and the study proves that the method is reliable for rationally evaluation of freshwater discharges from the PRB to the PRE. It is observed that the temporal sediment variation basically follows that of freshwater discharge, reaching the peak in July, and also brings large amount of nutrients to the PRE. In addition, the surrounding conditions of sea current, salinity, sediment, and nutrient around two power stations in Hong Kong are explored using EFDC for detecting safety of the stations. Using the simulation results from SWAT and EFDC, this study explores the characteristics of salinity variation in the PRE. There are 8 outlets between the PRB and PRE, and four outlets are in the east and the others in the west. Notably, the main outlets in the east and west are Humen and Modaomen, respectively. The results indicate that salinity in the east is more sensitive to freshwater discharge with a further divide of freshwater and saltwater (25 km from Humen and 14 km from Modaomen). Also, there are a more stratified vertical profile and a larger affected area in the east than those in the west (the affected area of 1,265 km2 near the east and 227 km2 near the west). Moreover, the study reveals that water depth is a critical factor in determining the salinity variation near different outlets. Overall, this study contributes to improvement of understanding of terrestrial processes over the basin and the impacts on the estuary. The study confirms that adoption of streamflow diversion ratios of different outlets is necessary to improve the simulation accuracy of discharges from the river basin to the estuary. This study paves a new way in exploring the impacts of river basins on estuaries, which would be beneficial to other multi-outlet estuaries in the world.