ATMOSPHERIC EFFECTS AND SPECTRAL VEGETATION INDEXES

A vegetation / atmosphere radiative transfer method is employed to stu dy atmospheric effects in spectral vegetation indices. A one-dimension al turbid medium model of a vegetation canopy that includes specular r eflection and the hot spot effect is used to calculate canopy bidirect ional reflectance factors. These are then used to specify the lower bo undary condition of the atmospheric radiative transfer problem. A hori zontally 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 c onstituents were considered in this study. Spectral and angular distri bution of surface radiances above the canopy and atmosphere were evalu ated for different solar zenith angles and leaf area indices. The most frequently used spectral vegetation index, NDVI, and variants introdu ced recently to correct for atmospheric and soil brightness effects (A RVI, SAVI, and SARVI) were calculated to investigate the extent of atm ospheric distortion. The nature of the relationship between top-of-the -atmosphere and, top-of-the-canopy spectral vegetation indices is stud ied, and its sensitivity to various problem parameters assessed.