ATMOSPHERIC CORRECTION OF AVHRR DATA FOR BIOPHYSICAL REMOTE-SENSING OF THE SAHEL

The importance of atmospheric correction of reflectances measured with the Advanced Very High Resolution Radiometer (AVHRR) for biophysical studies using the normalized difference vegetation index (NDVI) is examined for a study area in the Sahel for which measurements of aerosol and water vapor were available. During the rainy season atmospheric aerosols were relatively more variable than water vapor. Atmospheric corrections were applied to Channel 1 (red) and Channel 2 (near-infrared) for the effects of molecular absorption and Rayleigh scatter, aerosol scatter and absorption, and water vapor absorption. The results were expressed as the difference between corrected and uncorrected reflectances (Delta rho). In Channel 1 the magnitude and variability of Delta rho was mostly caused by aerosols. In Channel 2 the magnitude of Delta rho was caused by water vapor, but most of the variability was caused by aerosols. Most of the degradation in the NDVI signal (Delta nu iota) was caused by water vapor but the variability in Delta nu iota was caused by both water vapor and aerosol. Atmospheric corrections using seasonal averages of atmospheric water vapor and aerosol optical depths resulted in corrections that were similar to the full corrections using daily values. In the Sahel it may therefore be acceptable to use average values for the atmospheric variables to correct satellite data when sunphotometer data are not available, although the effects of interannual variability in mean atmospheric conditions are not known.