An Assessment of Water Color for Inland Water in China Using a Landsat 8-Derived Forel–Ule Index and the Google Earth Engine Platform

Abstract

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 km2. 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 km2 (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 (R2 = 0.90, P < 0.001). Also, it had a high degree of consistency with the in situ Secchi depth (R2 = 0.90, P < 0.001) and trophic level index (R2 = 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 km2) found to be yellow was much larger than for large lakes (area ≥ 1 km2) (50% against 28%). Our results will provide important information that can be used for preserving and restoring inland water resources.

Keywords

• China
• Forel–Ule index (FUI)
• Google earth engine
• inland water color
• lake
• Landsat 8