Monitoring the maximum turbidity zone and detecting fine-scale turbidity features in the Gironde estuary using high spatial resolution satellite sensor (SPOT HRV, Landsat ETM+) data
Résumé
This study concerns the quantification of suspended particulate matter in the highly turbid estuarine waters of the Gironde, France, from high spatial resolution remotely sensed data, SPOT (Satellite Pour l'Observation de la Terre) High Resolution Visible (HRV), Landsat Enhanced Thematic Mapper Plus (ETM+). The methodology is based on calibration relationships established between the remote sensing reflectance (R-rs) signal and suspended particulate matter concentration (SPM), from in situ optical measurements. These relationships are valid in the long term as R-rs ratios between near-infrared (NIR) and visible wavebands are relatively independent of the particle grain-size and mineralogy. Consequently, they can be applied to satellite images, even if no simultaneous in situ measurements are carried out concurrently with the satellite overpass. Selected satellite sensor data are corrected for atmospheric effects using radiative transfer code, then converted into surface water SPM concentrations according to the established calibration relationships. Resulting SPM maps are presented for different river flow and tidal conditions. These maps are used to locate the maximum turbidity zone and observe its tidal and seasonal movements. The high spatial resolution of SPOT HRV and Landsat ETM+ satellite sensor data also shows detailed turbidity features in the estuary, resulting from re-suspension phenomena over banks and turbulent currents.