Evaluation of the first year of operational sentinel-2A data for retrieval of suspended solids in medium- to high-turbidity waters

In this study, we apply high-resolution Sentinel-2A imagery to assist in the monitoring of the southwestern Spanish coast during its first year of data. The aim is to test suitability of MultiSpectral Imager (MSI) at higher resolution (10 m) for mapping Total Suspended Solids (TSS). Several field campaigns are carried out to collect TSS at three different sites in the Guadalquivir estuary, Cadiz Bay and Conil port. A regional multi-conditional remote sensing algorithm with a switching method that automatically selects the most sensitive TSS vs. water reflectance relationship is developed to estimate TSS concentration while avoiding saturation effects. An existing semi-analytical algorithm is calibrated by means of a cross-validation procedure based on both red 664 nm (r = 0.8, NRMSE of 25.06%) and near-infrared (NIR) 865 nm (r = 0.98, NRMSE of 10.28%) parts of the spectrum, showing the MSI sensor’s great potential to estimate TSS even though it was not designed for aquatic remote sensing. The first year of data reveals improved monitoring along the coastal region at unprecedented resolution with accuracy to detect the Estuarine Turbidity Maximum (ETM). ACOLITE and POLYMER Atmospheric Correction strategies are applied over this coastal region (no in-situ data on water reflectance). The results confirm that the flexible POLYMER algorithm can address intense sun-glint effects. These findings encourage further research of water quality studies relying on both operational Sentinel-2A and Sentinel-2B, with great implications to improve the understanding of turbid coastal and inland water environments.We would like to thank the ESA and the Copernicus programme for distributing the Sentinel-2 data used in this study. Thanks to Quinten Vanhellemont for his assistance in using ACOLITE processor. We thank the Junta de Andalucía for the in-situ data archives: the Regional Water Management Agency (http://www.chguadalquivir.es/saih/) and the Regional Agroclimatic Station Network (http://www.juntadeandalucia.esagriculturaypesca/ifapa/ria/servlet/FrontController), as well as Puertos del Estado (http://www.puertos.es/). We acknowledge the CSIC Sampling Unit OPECAM (http://www.icman.csic.es/en/facilities/opecam-field-unit/) and Josefa Perez dredging ship for the campaign assistance. Isabel Caballero was supported by a postdoctoral grant from the Junta de Andalucía fellowship program. This work was financially supported by the Junta de Andalucía Projects P09-RNM-4853 and PR11-RNM-7722, PIE 201530I012 and the National Project CTM2014-58181-R. The authors acknowledge the anonymous reviewers, whose comments helped to greatly improve an earlier version of this manuscript.Peer reviewe