Analysis of water clarity decrease in Xin'anjiang Reservoir, China, from 30- Year Landsat TM, ETM+, and OLI observations

摘要

Water clarity controls a wide range of physical, chemical, and biological processes and thus is an important environmental variable for assessing both the climatic and human activity factors that affect lake ecosystems. Here, we use 30-year Landsat series data to investigate the patterns and trends of water clarity in the Xin'anjiang Reservoir, the largest freshwater, man-made reservoir in the Yangtze River Delta region of China. Three empirical models based on Landsat 5, 7 and 8 data were developed and validated to remotely estimate the water clarity. The validation results show that the three models performed well in deriving water clarity, with the mean absolute percent differences (MAPDs)<30. The long-term trends and spatial distribution patterns of water clarity from 1986 to 2016 were constructed using the three developed models. A significant decreasing trend in water clarity was observed, with a reduction of approximately 0.2 m, indicating a gradual water quality decrease. Eleven explanatory variables, including air temperature, rainfall, gross domestic product (GDP), population,chemical fertilizer usage, industrial sewage, and land-use change, were used to explore the driving force of the long-term trend of water clarity with the multiple regression and variation partitioning model. Air temperature, rainfall, cropland, and industrial sewage were the four potential driving forces and explained 63 of the long-term variation in water clarity. Increasing built-up land, air temperature and rainfall, and decreasing forest and grassland jointly caused the decreasing water clarity. Therefore, several reservoir management measurements should be implemented to stop water clarity decreasing.