ATMOSPHERIC AND VIEWING ANGLE CORRECTION OF VEGETATION INDEXES AND GRASSLAND FUEL MOISTURE-CONTENT DERIVED FROM NOAA AVHRR

First, an algorithm for fast calculation of the visible and near-infrared bidirectional reflectances (R1 and R2, 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 R1 is potentially a more accurate measure of FMC than is a combined measure involving R1 and R2 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 R1 at any solar zenith angle θ and viewing angle φ to the equivalent R1 at a standard θ and φ such as θ = 0 and φ = 0. The table also refers R1 to the reflectance r1 of an assumed average Lambertian reflecting, randomly oriented, leaf within the canopy. Finally, comparison of the AVHRR-derived r1 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 r1 versus FMC calibration which can be used operationally. © 1990.