Abstract
Chlorophyll-a (Chl-a), total nitrogen (TN), and total phosphorus (TP) are important indicators to evaluate water environmental quality. Monitoring water quality and its variability can help control water pollution. However, traditional monitoring techniques of water quality are time-consuming and laborious, and can mostly conduct with sample point-to-point at the edge of lakes and rivers. In this study, an empirical (regression-based) model is proposed to retrieve Chl-a, TN, and TP concentrations in the Yangtze River by Landsat-8 images from 2014 to 2020. The spatial-temporal distribution and variability of water quality in the whole Yangtze River are analyzed in detail. Furthermore, the driving forces of water quality variations are explored. The results show that the mean absolute percentage error (MAPE) of the water quality parameters are 25.88%, 4.3%, and 8.37% for Chl-a, TN, and TP concentrations, respectively, and the root mean square errors (RMSE) are 0.475 μg/L, 0.110 mg/L, and 0.01 mg/L, respectively. The concentrations of Chl-a, TN, and TP in the upstream of the Yangtze River are lower than those in the midstream and downstream. These water quality parameters have a seasonal cycle with a maximum in summer and minimum in winter. The hydrological and meteorological factors such as water level, flow, temperature, and precipitation are positively correlated with Chl-a, TN, and TP concentrations. The larger the impervious surface and cropland area, the greater the cargo handling capacity (CHC), higher ratio of TP to TN will further pollute the water. The methods and results provide essential information to evaluate and control water pollution in the Yangtze River.
Highlights
The Yangtze River is the mother river of China, which originates from the QinghaiTibet Plateau
The hydrological and meteorological factors such as water level, flow, temperature, and precipitation are positively correlated with Chl-a, total nitrogen (TN), and total phosphorus (TP) concentrations
The results reveal that empirical models have accurate retrieval results for Chl-a, TN, and TP concentrations, which demonstrates that the Landsat-8 has adequate potential to retrieve Chl-a, TN, and TP concentrations in the river at a big scale
Summary
The Yangtze River is the mother river of China, which originates from the QinghaiTibet Plateau. The basin covers a total area of 1.8 million square kilometers and contains abundant resources. In the past two decades, due to the continuous increase in the total amount of sewage discharge in the river basin, some ecological conditions of rivers have gradually deteriorated, and the ecological balance of the Yangtze River has been destroyed [1]. The ecological environment of the Yangtze River basin and the residents around the Yangtze River have been severely affected [2]. According to the Bulletin of the Water Resources of the Yangtze River Basin and Southwestern Rivers in 2018, the main pollution items are ammonia nitrogen, TP, etc. It is of great significance to monitor river water quality and explore the mechanism of eutrophication for the management, control, and treatment of water bodies
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