Spatiotemporal Dynamics of Remote-Sensed Forel–Ule Index for Inland Waters Across China During the COVID-19 Pandemic

The 2019 coronavirus disease pandemic, called COVID-19, poses a novel research point for water body. Based upon the extracted 199 large lakes and reservoirs across China, this study developed the water color parameters, namely Forel–Ule index (FUI) retrieved from the Moderate Resolution Imaging Spectroradiometer imagery from 2018 and 2022 to explore the spatiotemporal variance of water quality for the first time in the context of COVID-19. The studied results demonstrated marked spatial and temporal patterns. Temporally, 68.8% of the lakes experienced an increase tendency in recent years and there was a noticeable upward trend in water quality during the postepidemic period. Spatially, various changes in lake regions were observed. Water quality of east lakes responded swiftly and showed an obvious improvement after the lockdown, whereas western lakes responded belatedly in the first epidemic year due to fewer human activity and smaller number of confirmed cases and had a clear benefit in the second year owing to the long-term sustaining of lower human activity intensity. This finding emphasized the significant influence of human activities than natural processes. In addition, this work quantitatively demonstrated the association in FUI in terms of new confirmed cases, temperature, precipitation, wind speed, and lake area, consistent with the extensive long-term researches. The study provided sufficient information for water color observation, and revealed the short-term improvement of water quality under epidemic situation. Additional attention of water resource management and sustainable strategies should be paid to inland water in order to maintain the short-term improvement.