Vegetation variables are important for evaluating the status and temporal processes of vegetated land surfaces. Using the AVHRR reflectance data it is possible to derive such vegetation variables consistently over large regions (regional and continental) and for long time periods.
The Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) is the photosynthetically active radiation which is absorbed by living green vegetation (Myneni et al. 1997). It depends on canopy structure, vegetation element optical properties, and atmospheric and illumination conditions. It plays an essential role in the estimation of the global energy and carbon balance and is an Essential Climate Variable (ECV) as recognized by the Global Climate Observing System (WMO 2011). FAPAR assessments can be retrieved from remote sensing data by numerically inverting physically-based models (GTOS 2009) or by establishing land cover specific statistical relationships between the NDVI and FAPAR.
Example of the FAPAR product from August 1990.
Myneni RB, Nemani RR, Running SW (1997) Estiamtion of global lead index and absorbed par using radiative transfer models. IEEE Trans Geosci Remote Sens, 35, 1380–1393
GLOBAL TERRESTRIAL OBSERVING SYSTEM (GTOS) (2009) FAPAR. Assessment of the status of the development of the standards for the terrestrial essential climate variables. GTOS 65. Available online at: http://www.fao.org/gtos/doc/ECVs/T10/T10.pdf
WMO (2011) GCOS Systematic Observation Requirements For Satellite-Based Data Products for Climate - 2011 Update