UNESCO promotes the advancement of science and innovation for peace and sustainable development. With this mandate, the International Initiative on Water Quality (IIWQ) of UNESCO IHP promotes the innovative approach to freshwater quality monitoring through satellite Earth Observations for with the aim to enhance information on world’s freshwater resources and support science-based policy-making for sustainable water management. The approach, which builds on rapid developments in remote sensing, has great potential to improve the data availability on the quality of freshwater resources at the global, regional and basin scales.
The scientific approach used in the UNESCO World Water Quality Portal is based on monitoring water quality by means of spectral information of the reflectance of water surface captured by satellite Earth Observation images, which indicates the presence of optically-active components in a surface water body such as lakes and rivers. Suspended solids, photosynthetic pigments and organic matter in a water body alter water colour and thereby affect the reflectance of the water surface. Accordingly, these components present distinct spectral signatures on the reflectance of the water surface in the visible, near infrared and thermal infrared spectral regions. As a result, a quantitative assessment of some water quality parameters can be realized from satellite EO images.
Satellite EO-derived water quality data on the UNESCO World Water Quality Portal are generated by open source algorithms, which model the spectral reflectance of the water surface captured by satellite images into values of different water quality parameters. These satellite EO-derived data were calibrated and validated by in-situ water quality measurements and using algorithms previously published in scientific peer-reviewed journals.
The Portal provides satellite EO-derived data for key water quality parameters, such as: turbidity, suspended particulate matter, chlorophyll-a, cyanobacteria and Harmful Algal Blooms, dissolved organic matters, and water surface temperature. The Portal for water quality monitoring of Lake Chad also provides data on the seasonal water surface variability and water level of Lake Chad. In-situ monitoring data on water quality, along with their georeferenced coordinates, are also available on the Portal.
Water quality processing chain for satellite data
The Portal uses open-source satellite Earth Observation images from European Space Agency’s (ESA) Sentinel-2 and USA NASA/USGS Landsat-8 satellites for monitoring water quality.
Scientific validation and data calibration
Scientific validation and data calibration constitute an important and integral component of the use of EO data. In order to underpin the scientific approach applied in the Portal, satellite EO-derived data on water quality are calibrated and scientifically validated by in-situ monitoring data. Data calibration and validation is also aimed at assuring the quality and reliability of data for operational applications and decision-making by the basin organization, national agencies and local communities.
For the scientific validation and data calibration of the Portal on Lake Chad water quality monitoring, three field campaigns for in-situ water quality monitoring in Lake Chad and Chari-Logone rivers were carried out by experts of the Lake Chad Basin Commission (LCBC) between May 2020 and April 2021. These in-situ monitoring data on water quality of Lake Chad and Chari-Logone rivers are available on the Portal.
The Portal’s data production is based on open-source algorithms (one for each water quality parameter), developed by the different French research institutes with the support of the French National Centre for Space Studies (CNES).