Investigation of a model inversion technique to estimate canopy biophysical variables from spectral and directional reflectance data

The objective of this study was to assess the ability to estimate canopy biophysical variables from remote sensing data observed at the top of the canopy in several directions and wavebands within the visible-near infrared domain. The variables considered were the leaf area index, leaf chlorophyll content, the fraction of photosynthetically active radiation absorbed by the canopy and the cover fraction. The SAIL radiative transfer model was inverted using a simple technique based on look-up-tables. The size of the look-up-table, and the number of its elements selected to get a distribution of the solution were first determined. The nadir reflectance in the red and near-infrared bands was considered to evaluate the retrieval performances in terms of the distributions and co-distributions of the solutions. The optimal spectral and directional sampling to estimate the variables considered was investigated Finally, the impact of spatial heterogeneity on the retrieval performances, the effect of the model assumptions used to generate the look-up table and the effect of radiometric noise were evaluated. These results were discussed in view of the definition of future satellites and the selection of the best measurement configuration for accurate estimation of canopy characteristics.