Exploitation of SENTINEL-2 time series for monitoring ecological quality parameters of french lakes and reservoirs (TELQUEL project)

International audienceThe ability to understand the close links between lacustrine environments and the human-altered landscapes in which they are embedded in depends strongly on the spatiotemporal availability and quality of key bio-optical parameters such as transparency, Chlorophyll A concentration (ChlA), Colored Dissolved Organic Matter (CDOM) absorption coefficient, Inorganic Suspended Matter (ISM) concentration. However, the current European monitoring based on the data collected from field observations do not characterize finely and reliably the spatiotemporal dynamics of these parameters. As an example, for the application of the Water Framework Directive in France, on the one hand only 380 lakes/reservoirs are monitored among the 480 involved water bodies, and on the other the collected measurements remains very punctual and discontinuous in time (four dates per year every 6 years). Remote sensing technique is often portrayed as a powerful tool to overcome this difficulty, as it provides synoptic and accurate spatio-temporal information on the desired bio-optical parameters. However, the different "ocean color" sensors that are able to monitor these parameters, such as SeaWiFS (1km, NASA), MODIS (250 m, NASA), MERIS (300 m, ESA), have a spatial resolution which is often too low to correctly study the majority of lake/reservoir ecosystems. Typically, only 2% of 480 French lakes/reservoirs are overlapped by at minimum 5 pixels of 250-m spatial resolution. The successful launch of Sentinel-2 satellite that carries the Multi-Spectral Instrument (MSI) sensor allows interesting research perspectives to monitor bio-optical properties of these lakes/reservoir due to (i) its high spatial resolution (typically between 10 m and 60 m); (ii) its suitable signal to noise ratios, and (iii) its good spectral performances (13 bands). The presentation aims at describing an ongoing project so-called TELQUEL. The main objectives of this project are to provide bio-optical algorithms for MSI/Sentinel-2 data to retrieve the water transparency, and ChlA, CDOM and ISM concentrations. The first step consists in developing a specific atmospheric correction algorithm over lake areas that takes into account the altitude effects and the adjacency effects induced by the land surface surrounding the lakes. The second step of the project consists of establishing bio-optical relationships between MSI/Sentinel-2 reflectances and the concentrations of biogeochemical parameters. A final step is dedicated to use the retrieved bio-optical properties and suspended matter concentrations for enhancing the lake water quality evaluation and simulation. We will focus our presentation on the methodological approach to correct for the atmospheric effects for lake/reservoir targets (dark target). Preliminary results on the bio-optical relationships obtained over the test-site of the project, namely the Naussac reservoir (10 km²) (close to Le Puy-en-Velay, France) will be presented