Atmospheric Effects and Spectral Vegetation Indices

A vegetation/atmosphere radiative transfer method is employed to study atmospheric effects in spectral vegetation indices. A one-dimensional turbid medium model of a vegetation canopy that includes specular reflection and the hot spot effect is used to calculate canopy bidirectional reflectance factors. These are then used to specify the lower boundary condition of the atmospheric radiative transfer problem. A horizontally homogeneous cloudless midlatitude continental atmosphere with both molecular and aerosol loading is assumed throughout. The canopy and atmospheric radiative transfer equations are numerically solved by the discrete ordinates method. A total of 13 discrete wavelengths in the solar spectrum outside the absorption bands of major atmospheric constituents were considered in this study. Spectral and angular distribution of surface radiances above the canopy and atmosphere were evaluated for different solar zenith angles and leaf area indices. The most frequently used spectral vegetation index, ND VI, and variants introduced recently to correct for atmospheric and soil brightness effects (ARVI, SA VI, and SAR VI) were calculated to investigate the extent of atmospheric distortion. The nature of the relationship between top-of-the-atmosphere and top-ofthe-canopy spectral vegetation indices is studied, and its sensitivity to various problem parameters assessed.