Reducing the influence of solar illumination angle when using active optical sensor derived NDVIAOS to infer fAPAR for spring wheat (Triticum aestivum L.)

Abstract

In recent years, the normalized difference vegetation index (NDVI), derived from active optical sensors (AOS) is being increasingly used to estimate the fraction of absorbed photosynthetically active radiation (fAPAR), a key variable in plant productivity modelling. The fAPAR of a plant canopy is dependent on the solar illumination angle. On the other hand AOSs when used to measure NDVIAOS are responding only to their integrated light source. The consequence is the NDVIAOS-fAPAR relationship is sensitive to solar illumination angle. In this work the effect of aligning the AOS view angle with the solar illumination angle on the NDVIAOS – fAPAR relationship is examined for a range of solar elevation angles (θse) of 40°, 55° and 70°, for spring wheat (Triticum aestivum L., bread var. Crusader), at four different growth stages (Zadoks 26, 29, 39 and 47). At each growth stage the fAPAR - NDVIAOS relationship was linear (R2 ≥ 0.80). The regression lines exhibited similar slopes with varying solar illumination angle, but with some variation in intercept. By aligning the AOS sensor view angle to the solar illumination angle there was no significant difference in the NDVIAOS-fAPAR relationship between Zadoks 26 and 29 nor between Zadoks 39 and 47. The presence of senescent leaves at later stages of crop growth was noted to influence the fAPAR – NDVIAOS relationship, which should be considered when applying a unique algorithm for all stages of crop growth.