Retrieval and Validation of Water Turbidity at Mete-Scale Using Pleiades Satellite Data: A Case Study in the Gironde Estuary  
[PERSON]  
1 College of Electronic and Information Engineering, Guangdong Ocean University, Zhanjiang 524088, China 12 Laboratoire d'Occanographie de Villefanche, UMR 7093 - CNRS/SU, 06230 Villefranche-sur-mer, France; [EMAIL_ADDRESS]  
[PERSON]  
2 Laboratoire d'Occanographie de Villefanche, UMR 7093 - CNRS/SU, 06230 Villefranche-sur-mer, France; [EMAIL_ADDRESS]  
[PERSON]  
3 Royal Belgian Institute of Natural Sciences, Operational Directorate Natural Environments, 1000 Brussels, Belgium; [EMAIL_ADDRESS]  
Correspondence: [EMAIL_ADDRESS]; Tel. : +86-0759-2396032  
to retrieve key water quality parameters such as surface temperature, turbidity, concentrations of suspended particulate matter (SPM) and chlorophyll-a (Chl-a) [2; 3; 4; 5; 6; 7; 8; 9; 10]. Over the last two decades, medium- (\(\sim\)300 m) and high- (\(\sim\)30 m) spatial-resolution satellite data have been used to map SPM concentrations and/or turbidity in estuaries and coastal waters [11; 12; 13; 14; 15]. For example, in the highly turbid Gironde Estuary (GE, France), several studies have demonstrated the potential of satellite data such as Satellite Pour I'Observation de la Terre (SPOT)/SPOT4 (Take5) [16; 17; 18], Landsat-Enhanced Thematic Mapper Plus (ETM+) and Landsat-Operational Land Imager (OLI) [17; 19] but also MODerate resolution Imaging Spectroradiometer (MODIS) [20; 21] to map concentration and seasonal dynamics of SPM. Recently, new satellite sensors with improved spectral, signal-to-noise and/or spatial resolutions such as Gaofen-1 (GF-1) [22; 23], Sentinel2-MultiSpectral Instrument (MSI) [24; 25; 26] have offered new capabilities to study coastal and inland waters. Increasing the spatial resolution from \(\sim\)300 m (MODIS, MEdium Resolution Imaging Spectrometer (MERIS), Ocean and Land Color Instrument (OLCI)) to \(\sim\)30 m (OLI, SPOT) or \(\sim\)10 m (MSI) allows for remote sensing of SPM in small estuaries and nearshore zones, and allows for the monitoring of ports and dredging operations. Very high-spatial-resolution satellite sensors such as IKONOS or WorldView-2 and airborne portable remote imaging spectrometers have been already used to map water quality parameters [27; 28; 29; 30; 31]. However, there was no operational atmospheric correction processing for such data over coastal or inland waters. The Pleiades constellation is composed of two satellites (Pleiades 1A and 1B) launched in 2011 and 2012. The spatial resolution is 2.8 m in multi-spectral mode, 0.7 m in the panchromatic band, and the pixel size is resampled to 2 m and 0.5 m [32]. The performance of Pleiades sensors improved compared to IKONOS and WorldView-2 in terms of spatial and temporal resolutions, as well as dynamic range (12 bits compared to 11 bits). Pleiades data can be recorded on-demand daily over a specific study area and can be corrected for atmospheric effects using the publicly available algorithm developed by [PERSON] and [PERSON] (2018) [33]. These characteristics give Pleiades the potential to operationally monitor SPM and turbidity dynamics in estuaries. Its capabilities are tested here in the GE, a macro-tidal system located in the south-west of France characterized by moderately to extremely turbid waters. SPM in the GE has already been successfully monitored with SPOT (20 m), Landsat ETM+/OLI (30 m) and MODIS (250 m) satellite data [16; 17; 18; 19; 20; 21]. The present study based on Pleiades satellite data represents one step further with observations recorded at 2 m spatial resolution. Moreover, the daily revisit associated to Pleiades data is a major advantage in order to observe and monitor small time-scale events such as tidal cycles, dredging activities, and peak floods [1; 34]. The limited number of Pleiades spectral bands is not a disadvantage for the monitoring of turbidity and sediment transport as only green, red and NIR spectral bands are needed for the estimation of SPM in turbid waters [19].