A new multiple scattering scheme for the FLAIR forest radiative transfer model: Application to biochemical and biophysical parameter retrieval using hyperspectral data

This thesis investigated the development and assessment of a simple parameterization of the multiple scattering within canopies assuming the single scattering field is known and the background beneath the canopy is completely absorbing. The parameterization is based on the concept of spectral invariants related to recollision and escape probabilities from vegetation canopies. The simplified approach is evaluated against detailed 3-D ray tracing model, PARCINOPY, as well as reference datasets from the Radiation Modelling Intercomparison Experiment On-Line Checker. Comparison with homogenous canopies simulated with PARCINOPY showed that the model's performance is best in both the solar principal and perpendicular planes at low and mid LAI levels for all solar zenith angles. The comparison to the On-line Checker datasets shows also that the model is a suitable approach to describe the multiple scattering components of physically based models. This simple parameterization is then incorporated into the Four Scale Linear Model for Anisotropie Reflectance (FLAIR) canopy radiative transfer model to enhance the description of the spectrally dependant multiple scattered radiation field of a forest canopy. The contribution of the multiply scattered radiation between the canopy and the background is also added to the parameterization of the multiple scattering component. The validation of the new version of the FLAIR model was performed using the multi-angular data sets obtained by the airborne sensor POLarization and Directionality of the Earth's Reflectances (POLDER) during the BOReal Ecosystem-Atmosphere Study (BOREAS) campaign of 1994. The results indicate that this approach is well suited to the FLAIR model. It is also demonstrated that the multiple scattering problem can be parameterized by a limited number of architectural parameters and the leaf scattering coefficient. Finally, the combined canopy-leaf PROFLAIR (PROSPECT + FLAIR) model is used to investigate the potential of simulating broadleaf forest canopy spectral reflectance. The comparison between simulated data and Hyperion reflectance data showed the ability of the PROFLAIR model to realistically simulate canopy spectral reflectance. The model was then inverted with hyperspectral Hyperion data using a look-up-table (LUT) approach to retrieve canopy leaf area index (LAI), leaf chlorophyll content (Ca+b) and canopy integrated chlorophyll content (LAI x Ca+b). The LUT was populated by simulating the mode] in forward mode using a space of realization generated based on the specific distribution of the input parameters and based on a priori information from the field. The estimated variables were then compared to ground measurements collected in the field. The results showed a reasonable performance of the PROFLAIR model to the order of performances of other well-known models. When compared to ground measurements, the model showed its ability to retrieve canopy LAI from closed forest canopy with an RMSE of 0.47 and leaf chlorophyll content with an RMSE of 4.461mug/cm2