Dependence of NDVI and SAVI on sun/sensor geometry and its effect on fAPAR relationships in Alfalfa

This article describes the impacts of sensor view and solar zenith angles on two vegetation indices -NDVI (normalized difference vegetation index) and SAVI (soil adjusted vegetation index). An evaluation of these geometric factors on the relationships between these VIs (vegetation indices) and fAPAR (fraction of photosynthetically active radiation absorbed by the canopy) was performed. To accomplish this, an experiment was conducted in Phoenix, Arizona, over plots of bare soil and low, medium, and high alfalfa. Reflectances were measured from 0.4 μm to 1.0 μm in nine view angles (from -40° to + 40°, in 10° steps) over varying solar zenith angles. This was done simultaneously with fAPAR measurements. Changes in view angle caused variations 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, with the former increasing from antisolar to forward scattering view direction. A derivative analysis of the indices showed the SAVI to exhibit a more linear relationship than NDVI with the individual bands. The relationships between both VIs and fAPAR were, in general, linear. However, view angle variations perturbed these relationships and caused an over- or underestimation of fAPAR, depending on view direction (antisolar or forward), view angle, and vegetation index (NDVI or SAVI).