Remotely Sensed Water Color as a Proxy for Monitoring Water Quality in Inland Lakes

Constituents in water control the amount of light reflected from and absorbed by natural water bodies. This interaction is used as a basis for water quality monitoring using remotely sensed data. Recent studies have shown that water color derived from satellite data can be used to investigate water quality changes due to human and climate change impacts. However, how the change in satellite-based water color corresponds with specific water quality variables needs to be better understood. We analyzed timeseries (2013-2022) satellite-derived water color. We compared it with in-situ measured water quality variables (Secchi depth, turbidity, chlorophyll a, and total suspended solids) for lakes in the Midwest and Northeast regions of the USA. One of the focuses of this study, Lake Erie, observed for size, movement, and toxicity of harmful algal blooms (HABs) at multiple stations. Four bands (ultra blue, blue, green, and red) were extracted from harmonized Landsat and Sentinel-2 data to obtain the tristimulus values. These values are mapped on a chromaticity diagram to get the dominant color wavelength and the Forel–Ule Index (FUI). Results showed a strong relationship between in-situ water quality variables (e.g., Secchi depth and turbidity) and satellite-based FUI. Spatially, the relationship between in-situ water quality variables and water color is not consistent, as there is high variability in the concentration of the observed variables between the sampling locations. For example, measurement stations characterized by yellow to brown colors exhibited a strong relationship with TSS. However, generally, peak chl a concentration corresponds with yellowish green color. Typically, stations with blue to green water color are characterized by lower Chl a concentrations. This in-situ validation is used to infer the water quality of water bodies with no available in-situ monitoring.