The impact of solar illumination angle when using active optical sensing of NDVI to infer fAPAR in a pasture canopy

The fraction of absorbed photosynthetically active radiation (fAPAR) for plant canopies is often inferred from top-of-canopy, spectral reflectance, vegetation indices like the normalized difference vegetation index (NDVI). Such measures are derived using passive optical sensors and solar illumination of the canopy. However both the passive sensor-derived NDVI and the accompanying fAPAR measurements are affected by the solar elevation angle ( θ s ). In many cases the effect of θ s on both NDVI and fAPAR measurements is similar and the effect of θ s is often cancelled out. The new class of active optical sensors (AOS) that contain their own radiant light sources to produce equivalent measurements of NDVI are not influenced by θ s even though the accompanying values of fAPAR, as derived using a passive sensor are. This means the fAPAR–NDVI AOS relationship will invariably be sensitive to θ s . By way of example, this paper investigates the correlations between the NDVI AOS and fAPAR under conditions of varying solar illumination angle for a tall fescue ( Festuca arundinacea) pasture. The NDVI AOS was observed to retain a strong linear correlation with fAPAR ( R 2 ≥ 0.85) but fAPAR was highly sensitive to θ s . Subsequently, simple models can be utilized to predict the fAPAR–NDVI AOS relationship for any solar elevation angle between 30 and 80°.