Photo-synthetically active radiation (PAR) is the spectral range from 400-700nm that is used by plants in photosynthesis. The absorbed fraction of PAR (fAPAR) is a parameter used in remote sensing and in ecosystem modeling that signifies the portion of PAR absorbed by plants. fAPAR is commonly used in ecosystem models because it has an important influence on exchanges of energy, water vapor and carbon dioxide between the surface of the earth and the atmosphere. Precipitation and temperature are two of the major factors that determine the proportion of PAR absorbed by plants. It is an important parameter in measuring biomass production because vegetation development is related to the rate at which radiant energy is absorbed by vegetation. fAPAR can be measured on the ground with handheld instruments or inferred from satellite imagery over large spatial scales.
The major approaches to generating fAPAR estimates from remotely sensed images are:

  • linear modeling - linear modeling approaches attempt to relate reflectance data recorded by a sensor to field measurements of fAPAR using linear regression techniques. Such approaches may correlate field-measured fractional cover with sensor reflectance bands, or to vegetation indices like NDVI
  • physical modeling - Physical models use principles of how light energy is absorbed or reflected from different surfaces to estimate physical characteristics of vegetation. Biophysical models incorporate parameters related to how light interacts with processes like photosynthesis, evapo-transpiration, stress, and decay of plant material

The relationship between satellite measures of reflectance and estimates of fAPAR will vary depending on the type of vegetation being considered, and thus major land cover type is an important input to calculating fAPAR. The fAPAR values are here calculated using a physical model and used as an intermediate variable in NPP modeling.