Towards a quantitative interpretation of vegetation indices Part 1: Biophysical canopy properties and classical indices

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Abstract
The information content of vegetation indices currently derived from satellite observations is discussed using a physically‐based approach. These indices can, in principle, be related to some of the biophysical properties of the canopy through a complex set of equations, provided atmospheric effects, the anisotropy of the coupled surface‐atmosphere system, and the morphology and optical heterogeneity of the canopy are taken into account. Atmospheric effects are discussed on the basis of an approximate analytical expression applicable to vegetation indices built from AVHRR channels 1 and 2. These indices strongly depend on the aerosol and water vapor content, a fact that limits their quantitative interpretation in terms of physical and biophysical properties of the canopy. The interaction between the electromagnetic solar radiation and the surface is discussed using physical models of albedos developed for vegetation canopies. Finally, the biophysical leaf model PROSPECT is used to parameterize relationships between leaf reflectance and some physiological properties of the canopy elements. Clearly, the biological information that may be contained in vegetation indices is partly hidden or distorded by a number of physical processes. Properly deconvoluting these effects requires either a fully physical approach based on model inversions, or at least a careful analysis of the factors affecting classical vegetation indices.