Retrieval and Spatio-Temporal Variations Analysis of Yangtze River Water Clarity from 2017 to 2020 Based on Sentinel-2 Images

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