Abstract
The Yangtze River is the third longest river in the world. Monitoring and protecting its water quality are important for economic and social development. Water clarity (Secchi disk depth, SDD) is an important reference index for evaluating water quality. In this study, Sentinel-2 multispectral instrument (MSI) remote sensing images were utilized together with the Forel-Ule index (FUI) and hue angle α to construct an SDD retrieval model, which was applied to the Yangtze River from 2017 to 2020, which was used to describe color in the International Commission on Illumination (CIE) color space to construct an SDD retrieval model that was applied to the Yangtze River for the period 2017–2020. Further, the spatial distribution, seasonal variation, inter-annual variation, and driving factors of the observed SDD variations were analyzed. The spatial distribution pattern of the Yangtze River was high in the west and low in the east. The main driving factors affecting the Yangtze River SDD was sediment runoff, water level, and precipitation. The upstream and downstream Yangtze River SDD were negatively correlated with the change in water level and sediment runoff, whereas the midstream Yangtze River SDD was positively correlated with the change in water level and sediment runoff. The upper and lower reaches of the Yangtze River and overall SDD showed a weak downward trend, and the middle reaches of the Yangtze River remained almost unchanged.
Highlights
Surface water is an important resource for daily human life and industrial production; its quality is closely related to human production and life
The water clarity of the three lakes in Northeast China was evaluated using Landsat 7 remote sensing images according to four regression equations, and the results showed that the water clarity of the lakes in this region had a strong geographical pattern [14]
Forel-Ule index (FUI) and hue angle α were calculated based on the water extraction results, and the results were retrieved based on the FUI and hue angle α of the Yangtze River
Summary
Surface water is an important resource for daily human life and industrial production; its quality is closely related to human production and life. The Yangtze River is the third largest river in the world, and it is an important water resource for coastal residents. It has many water conservancy projects, such as the Three Gorges Project and the Middle Route Project of South to North Water Diversion. Because the Yangtze River plays a very important role in social development in China, it is necessary to protect and monitor the water quality of the Yangtze River [1]. Water clarity (Secchi disk depth: SDD) is the most commonly used index for evaluating water quality. Extensive field survey work is required, which entails high
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