Day 4

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Paper title Impact of sediment mineralogy and size distribution on forward modeling and inversion scheme for retrieval of optically active water constituents in moderate to highly turbid waters
  1. Tristan Harmel GET - CNRS - Magellium Speaker
  2. Guillaume Morin INRAE - PACA
  3. Pierre Gernez Nantes Université, Institut des Substances et Organismes de la Mer, ISOMER, UR 2160 Nantes, France
  4. Manuela Grippa Géosciences Environnement Toulouse, Toulouse
  5. Laurent Kergoat Geosciences Environnement Toulouse, GET, CNRS, University of Toulouse
  6. Elodie Robert CNRS/LETG
  7. Jean Michel Martinez CNRS
Form of presentation Poster
  • A7. Hydrology and Water Cycle
    • A7.06 EO for monitoring water quality and ecological status in inland waters
Abstract text Abstract: Superficial aquatic environments, including oceans, lakes and rivers, contain a great diversity of particulate and dissolved materials. The water-leaving radiance is directly driven by the optical properties of those in-water materials interacting with light, also known as optically active water constituents (OAWC). In turn, their inherent optical properties (IOPs), such as the absorption coefficient or the scattering matrix, are dependent on the nature of the particles in suspension (i.e., microalgae, sediments). More precisely, the IOPs of suspended sediments depend on their mineralogy including the spectral complex refractive index and size distribution. Nevertheless, the relationship between remotely measurable water reflectance and the IOPs is still to be better elucidated in turbid and very turbid waters. One of the goals of this study was to reassess the IOPs-reflectance forward model over a wide range of water turbidity, and accounting for the polarized nature of light. Moreover, a special focus was paid to evaluate the role of the viewing geometry (sun and viewing angles, and relative azimuth angle between Sun and sensor) and to provide the uncertainty attached to such widely used forward model.
A second part of this work was dedicated to hyperspectral and multispectral analysis of the performances of retrieval algorithms based on the developed forward model. A specific inversion scheme was applied to a series of in situ data sets of moderate to highly turbid waters. Results showed the need to consider the actual multimodal size distribution and spectrally dependent refractive index to accurately reproduce hyperspectral observations. However, the presence of very coarse particles (> 20 µm) produces ambiguities in the retrievals due to their minimal contribution to the water-leaving radiance. Conversely, those findings demonstrate the sensitivity of the measured reflectance to size distribution, thus providing a framework for size distribution retrieval from space. Based on those results, we argue that physically based analysis of the signal remains a fundamental step to gain more genericity and applicability of suspended sediment retrieval algorithms enabling to reconcile the exponentially increasing number of regional algorithms.