Inversion of Reflectance Spectra of Nonchlorophyllous Turbid Coastal Waters Philippe Forget,* Sylvain Ouillon,* Florence Lahet,* and Pierre Broche* This article deals with the inversion of reflectance spec- signatures characteristic of the different constituents of marine water related to terrigenous inputs, biogenic ac- tra measured in the Rho ˆne River mouth area during a flood event in terms of parameters characterizing the tur- tivities, and anthropic impacts as pollutions. The optically active constituents of sea water can be classified into bid water components. The optically active constituents considered are the total suspended matter, mainly com- three main distinct categories: yellow substance (or gelb- stoff or gilvin or CDOM), phytoplankton and associated posed of sediment, of known concentration ranging from 3.7 mg L -1 to 234 mg L -1 , and yellow substance. A sim- covarying byproducts, and nonchlorophyllous particulate matter consisting, in coastal areas, of terrigenous parti- ple reflectance model is presented. Scattering is treated by Mie theory applied to a collection of particles of real cles and resuspended sediments (Sathyendranath et al., 1989). Reflectance R(k), that is, the ratio of upwelling to refractive index m p and obeying a Junge size distribution law. A sensitivity study performed on the model shows downwelling irradiances just below the sea surface, is the radiometric quantity the most used for characterizing the that both concentration and refractive index and both cutoff particle diameters of the Junge size distribution optical properties of surface water by remote sensing. The measurement of R(k) allows, at least potentially, de- law cannot be accurately determined from a given reflec- tance spectrum. Two model parameters, among a total of termination of the size and concentration of the sedi- ments constitutive of the particulate load of the surface six, are estimated from model inversion: m p and a param- eter characterizing the yellow substance absorption spec- layer (e.g., Ferrier, 1995). The condition for this is to dispose of a sufficient number of narrow-band channels trum, a y0 =a y (440 nm). The derived values are realistic as compared to the values that can be found in literature. sampling the optical spectrum (Schiller and Doerffer, 1993). Many field instruments operated at sea level (e.g., Other combinations of parameters could be determined Althuis and Shimwell, 1995) or onboard an aircraft (e.g., by inversion but measurement of m p and a y0 from reflec- Carder et al., 1993) or a helicopter have already been tance spectra already has interesting applications in a sat- experimented with this goal. ellite remote sensing perspective. Increasing the number of This study addresses the inversion of reflectance parameters of the model is feasable, but much work is spectra of coastal sediment-dominated waters. The study needed to envisage the inverse problem. Elsevier Sci- uses measurements made in the Rho ˆne River mouth dur- ence Inc., 1999 ing a flood event by a spectroradiometer operated from a ship. After a description of the experimental data, which correspond to a significant variety of measured suspended INTRODUCTION matter concentrations, we present a model of spectral re- Visible remote sensing is a powerful means for monitor- flectance based on Mie theory for quantifying backscatter- ing coastal zones which manifest themselves by optical ing of light by sediment particles and on a semiempirical representation of absorption by yellow substance. We study the sensitivity of the model to the variations of its * Laboratoire de Sondages Electromagne ´ tiques de l’Environne- ment Terrestre, CNRS, Universite ´ de Toulon et du Var, La Garde, driving parameters. Two parameters, namely, yellow sub- France stance absorption at 440 nm and sediment refractive in- Address correspondence to P. Forget, Lab. de Sondages Electro- dex, are then estimated by inversion of the model. The magne ´ tiques de l’Environnement Terrestre, CNRS, Univ. de Toulon et results are discussed, and we conclude on the possibility du Var, bp132, 83957 La Garde, France. E-mail: forget@lseet.univ-tln.fr Received 4 August 1997; revised 24 November 1998. to extend the inversion procedure to other parameters. REMOTE SENS. ENVIRON. 68:264–272 (1999) Elsevier Science Inc., 1999 0034-4257/99/$–see front matter 655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(98)00117-5