Water Quality from Space

The Colour of Water is determined by substances in the water, and measuring the water colour allows a retrieval of the water quality. Eutrophic waters appear green, while sediment loaded waters get a brown colour. Highly sensitive multispectral cameras, deployed on satellites, allow a quantification of such water constituents from space, at global scale and with a spatial resolution of down to 10s of meters.

The retrieval of water constituents, or its optical properties, is achieved by inversion of the water leaving reflectance spectrum, measured at top of atmosphere and thus requiring a correction for atmospheric effects. This multi-variate problem is extremely challenging in so-called optically complex, or Case-2, waters. The Case–2 Regional processor, originally developed by DOERFFER and SCHILLER, uses a large database of radiative transfer simulations inverted by neural networks as basic technology.

Through the ESA CoastColour project major improvements were introduced and subsequently further improved. The actual version of the processor is called C2RCC (Case-2 Regional CoastColour) and is capable of processing data from Sentinels–3 and 2, MERIS, VIIRS, MODIS, and Landsat-8.

The C2RCC processor is composed of a set of additional neural networks performing specific tasks and special neural networks have been trained to cover extreme ranges of scattering and absorption. C2RCC has been validated in various studies and is available through ESA’s Sentinel toolbox SNAP. It is also used in the Sentinel–3 OLCI ground segment processor of ESA for the generation of the Case–2 water products, as well as in the processor for the MERIS 4th reprocessing. More information can be found here.