remote sensing Article Retrieval and Spatio-Temporal Variations Analysis of Yangtze River Water Clarity from 2017 to 2020 Based on Sentinel-2 Images Yelong Zhao 1,2, Shenglei Wang 2, Fangfang Zhang 2,* , Qian Shen 2 and Junsheng Li 2,3,4 1 School of Geomatics, Liaoning Technical University, Fuxin 123000, China;
[email protected] 2 Key Laboratory of Digital Earth Science, Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100049, China;
[email protected] (S.W.);
[email protected] (Q.S.);
[email protected] (J.L.) 3 Key Laboratory of Earth Observation Hainan Province, Sanya 572029, China 4 School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences, Beijing 100049, China * Correspondence:
[email protected]; Tel.: +86-137-1765-5892 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 Citation: Zhao, Y.; Wang, S.; Zhang, driving factors of the observed SDD variations were analyzed.