On the relationship between FAPAR and NDVI

The influence of pixel heterogeneity, background, atmospheric and bidirectional effects on the relationship between fraction of photosynthetically active radiation absorbed by the photosynthesizing tissue in a canopy (FAPAR) and normalized difference vegetation index (NDVI) is investigated using a three-dimensional model of radiation transfer. Top of the canopy (TOC) NDVI and FAPAR increase with ground cover and plant leaf area. Their functional response to leaf orientation, solar zenith angle and atmospheric optical depth is similar. For instance, planophile canopies (mostly horizontal leaves) have a higher FAPAR and TOC NDVI than erectophile canopies (mostly erect leaves). However, FAPAR and TOC NDVI respond differently to other parameters such as soil reflectance and leaf optical properties. For example, an increase in soil reflectance increases FAPAR but decreases TOC NDVI. Atmospheric and bidirectional effects confound the interpretation of top of the atmosphere (TOA) NDVI. The transmissivity of NDVI, defined as the ratio TOA/TOC NDVI, decreases with increasing atmospheric turbidity and solar zenith angle. Sensing about the nadir directions under clear sky conditions and moderate solar incidence angles can result in transmissivities as high as 0.8. There are sufficient causal grounds for relating FAPAR to NDVI. The relationship is independent of pixel heterogeneity, parameterized here with ground cover, plant leaf area, and variations in leaf orientation and optical properties. On the other hand, the relationship is sensitive to background, atmospheric, and bidirectional effects. A simple linear model relating FAPAR to TOC NDVI is proposed, and its validity is discussed.