Water color is an important parameter in water quality assessment. However, the existing water color investigations have mostly focused on the lakes with areas greater 1 km 2 . In order to improve the understanding of the color of water bodies in China, a cloud-free composite image of China for the summer of 2015 was generated using time-series of Landsat-8 imagery and the best-available-pixel (BAP) compositing algorithm.Then, the first Forel-Ule index (FUI) water color product with a resolution of 30 m was produced for China using the generated BAP composite and the Google Earth Engine computing platform.Finally, the first national-scale assessment of the FUI of natural lakes with an area > 0.01 km 2 (N = 60026) was conducted based on the generated FUI product. The generated FUI product was shown to have a high degree of consistency with in-situ water surface reflectance-derived FUI (R 2 = 0.90, P < 0.001). Also, it had a high degree of consistency with the in-situ Secchi depth (SD) (R 2 = 0.90, P < 0.001) and Trophic Level Index (TLI) (R 2 = 0.62, P < 0.001) datasets. In addition, we found that the most prevalent lake colors in China were yellow (about 49%) and green (about 41%). Besides, the proportion of small lakes (areas < 1 km 2 ) found to be yellow was much larger than for large lakes (area ≥ 1 km 2 ) (50% against 28%). Our results will provide important information that can be used for preserving and restoring inland water resources.
Index Terms-Inland water color; Forel-Ule index (FUI); Landsat 8; Google Earth Engine; China; lake I. INTRODUCTION nland water bodies have a direct interface with human society and are important to the food supply, commerce, transportation, and human health [1]. However, with the growth of the population and the rapid development of the economy, China has been facing increasingly severe water scarcity and security issues in recent years [2-5]. Because of the close relationship between water bodies and humans, water bodies in China are under great pressure due to human activities Manuscript received January 14, 2021.