Earth observation of lake colour dynamics across local to global scales: opportunities and challenges

Satellite remote sensing can potentially provide objective, broad scope, high frequency, and continuous measurements of inland water quality by capturing water colour information. However, challenges brought about by the optical complexity of inland waters and overlying atmosphere, and interference due to adjacency effects have hindered the development of valid Earth observation (EO) approaches for water quality monitoring in inland waters compared with the ocean applications. Water colour has been recognized as one of the most important Essential Climate Variables of the lake ecosystem, as it is directly related to changes in water constituents and almost all of the lake's ecological changes could alter water colour. Given the high retrieval accuracy from existing Earth observation satellite data, water colour, in terms of Forel Ule Index (FUI) and hue angle, can be a realistic indicator to track the long-term changes in the lake ecosystem and further explore the lake response to environmental changes. Through developing a global algorithm of FUI and hue angle for diverse types of inland waters and multiple source satellite datasets, datasets of FUI and hue angle for local and global lakes were constructed using satellite datasets. Lake colour have therefore emerged as a means to addressing scientific issues, such as the spatial patterns and long-term change trends of lake ecosystem, how water colour associated with climate change and local anthropogenic activities, spanning from local to global scales. Opportunities of leveraging water colour information observed from multisource satellite datasets in limnological research will be concluded and discussed in this report. We will also discuss in depth the challenges and possible countermeasures in estimating and reducing observation uncertainties associated with optical water types and multi-source satellite datasets.