Atmospheric and viewing angle correction of vegetation indices and grassland fuel moisture content derived from NOAA/AVHRR

Abstract First, an algorithm for fast calculation of the visible and near-infrared bidirectional reflectances (R 1 and R 2 , respectively) of the ground from NOAA/AVHRR satellite observations is derived. It removes the effect of the atmosphere and is accurate to within a few percent for viewing angles less than 40°. Second, the use of vegetation indices as a measure of vegetation fuel-moisture content (FMC) is discussed by reference to Landsat observations of various grassland (i.e., cereal and pasture) sites in an area of intense FMC “ground truthing” in Victoria, Australia. It is concluded that the single measurement of ground-level (i.e., “atmosphere removed”) visible reflectance R 1 is potentially a more accurate measure of FMC than is a combined measure involving R 1 and R 2 such as a normalized difference vegetation index. Third, a numerical model of a grassland canopy is developed which provides a look-up table for normalization of AVHRR-derived R 1 at any solar zenith angle θ and viewing angle φ to the equivalent R 1 at a standard θ and φ such as θ = 0 and φ = 0. The table also refers R 1 to the reflectance r 1 of an assumed average Lambertian reflecting, randomly oriented, leaf within the canopy. Finally, comparison of the AVHRR-derived r 1 with the FMC measured at 21 sites about Victoria in an extensive ground sampling exercise over the spring and summer of 1988–1989 yields an r 1 versus FMC calibration which can be used operationally.