DEPENDENCE OF NDVI AND SAVI ON SUN SENSOR GEOMETRY AND ITS EFFECT ON FAPAR RELATIONSHIPS IN ALFALFA

This article describes ihs impacts of sensor view and solar zenith ang les on two vegetation indices - NDVI (normalized difference vegetation index) and SAVI (soil adjusted vegetation index). An evaluation of th ese geometric factors on the relationships between these VIs (vegetati on indices) and fAPAR (fraction of photosynthetically active radiation absorbed by the canopy) was performed. To accomplish this, an experim ent was conducted in Phoenix, Arizona, over plots of bare soil and low , medium, and high alfalfa Reflectances were measured from 0.4 mu m to 1.0 mu m in nine view angles (from - 40 degrees to + 40 degrees, in 1 0 degrees steps) over varying solar zenith angles. This was done simul taneously with fAPAR measurements. Changes in view angle caused variat ions in the indices to be as high as 50% in relation to nadir. However , there was an opposite view angle behavior between NDVI and SAVI, wit h the former Increasing from antisolar to forward scattering view dire ction. A derivative analysis of the indices showed the SAVI to exhibit a more linear relationship than NDVI with the individual bands, The r elationships between both Vis and fAPAR were, in general, linear. Howe ver view angle variations perturbed these relationships and caused an over- or underestimation of fAPAR, depending on view direction (antiso lar or forward), view angle, and vegetation index (NDVI or SAVI).