Fraction of Absorbed Photosynthetically Active Radiation (fAPAR)
Photosynthetically active radiation (PAR) is the spectral range from 400-700nm that is used by plants in photosynthesis. The fraction of PAR (fPAR) is a parameter used in remote sensing and in ecosystem modeling that signifies the portion of PAR used by plants. fPAR 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. fPAR can be measured on the ground with handheld instruments or inferred from satellite imagery over large spatial scales.
The major approaches to generating fPAR estimates from remotely sensed images are:
These methods require correction for atmospheric variation and sometimes require bidirectional reflectance normalization. The images are composited over multiple days (i.e., the value for any given pixel in the final image is taken from the highest-quality readings for that pixel across multiple images) to minimize the impact of atmosphere and screening by clouds or snow. The relationship between satellite measures of reflectance and estimates of fPAR will vary depending on the type of vegetation being considered, and thus major land cover type is an important input to calculating fPAR. Satellite measurements of reflected radiation are often used to estimate the fPAR values that are used as an intermediate variable in models of NPP.
The output from remotely sensed fPAR methods is a map with each pixel assigned a fPAR value between 0 and 1. Numbers close to 0 suggest that little of the PAR is absorbed by plants, and values close to 1 indicate that most of the PAR is absorbed by plants. fPAR and LAI data are commonly packaged together (e.g., in the MODIS products).
fPAR is used as an input in many ecosystem models to characterize:
Rangeland applications include:
Remotely sensed fPAR estimates are only approximations of true fPAR values. The mathematical models used to calculate fPAR vary widely, and each model contains assumptions and requires specific inputs. It is important to understand the model assumptions and assess the suitability of the model based on the available data, how well the model characterizes the vegetation compared to field measurements, and the desired output. Most models work optimally at a particular scale and in a particular ecosystem type, and the application of an existing model to a new location may require changes to the model. fPAR is often derived from spectral vegetation indices, such as NDVI, but there is no single equation with a set of coefficients that can be applied to images of different surface types. Estimation of fPAR by satellite imaging requires corrections for atmospheric effects, topography and diurnal variations, and values change rapidly throughout the season with changing phenology. fPAR estimates from visible/near-infrared images require a cloudless, clear image, and thus fPAR values are typically chosen from the best quality images over a multiple day period (often an 8 or 10-day window). For areas that are continually cloudy, the use of radar or lidar may be necessary to assess vegetation characteristics.
Remote sensing of fPAR requires an image with red and near-infrared bands.
Calculating fPAR requires image processing and statistical/mathematical modeling software.
Global fPAR from Modis imagery (source: Running, S.W. and NTSG. 2002 powerpoint presentation).
fPAR values for the Sahel derived from MODIS satellite images (source: Fensholt et al. 2006).
Images of the Ouachita Mountains in the southwestern US, showing how different indices can provide different types of information. The top three panels are images from April and the bottom three panels are from May of 2004. The leftmost panels (top and bottom) show a near natural color image, the middle panels show LAI (leaf area index) calculated from the images, and the right panels show fPAR . (image source: Short, N. 2009. The Remote Sensing Tutorial, Section 3. Online tutorial)
Strong correlation between NDVI and field measured fPAR across multiple sites (source: Fensholt et al 2004).
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