NRT Product available since Jan 2013
Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) defines the fraction of PAR (400-700 nm) absorbed by the green parts of the canopy, and thus expresses the canopy's energy absorption capacity. FAPAR depends both on canopy structure, leaf and soil optical properties and irradiance conditions. FAPAR has been recognized as one of the fundamental terrestrial state variables in the context of the global change sciences (Steering Committee for GCOS, 2003; Gobron et al., 2006). It is a key variable in models assessing vegetation primary productivity and, more generally, in carbon cycle models implementing up-to-date land surfaces process schemes. Besides, FAPAR it is an indicator of the health of vegetation. FAPAR is generally well correlated with the LAI, the more for healthy fully developed vegetation canopies.
This operational product is documented in the following documents:
Please see Product Peer-Review publications in References.
The use of LSA SAF products in publications is kindly requested to be duly acknowledged:
FAPAR and MTFAPAR were provided by the EUMETSAT Satellite Application Facility on Land Surface Analysis (LSA SAF; Trigo et al., 2011)
Trigo, I. F., C. C. DaCamara, P. Viterbo, J.-L. Roujean, F. Olesen, C. Barroso, F. Camacho-de Coca, D. Carrer, S. C. Freitas, J. García-Haro, B. Geiger, F. Gellens-Meulenberghs, N. Ghilain, J. Meliá, L. Pessanha, N. Siljamo, and A. Arboleda, 2011: The Satellite Application Facility on Land Surface Analysis. Int. J. Remote Sens., 32, 2725-2744, doi: 10.1080/01431161003743199
The FAPAR product is currently generated daily at the full spatial resolution of the MSG/SEVIRI instrument, and will be later provided on a 10-days basis. The product is based on the red and NIR channels (VIS 0.6 µm, NIR 0.8 µm) using as input the three parameters of a parametric BRDF (Bi-directional Reflectance Distribution Function) model (Roujean et al. 1992). The MSG BRDF provides cloud-free observations over the SEVIRI disk based on an iterative scheme with a characteristic time scale of five days. The FAPAR product is expressed in the range from 0% to 100%. It is corrected from uncertainty derived of the view/sun angles variations and the anisotropy effects of surface's reflectance because it is derived using the same geometry for the whole SEVIRI disk. The FAPAR product includes routine quality check and error estimates on a pixel basis. The product will be validated in order to define the product uncertainties over a range of global conditions.
For the retrieval of daily FAPAR from space data without any prior knowledge on the land cover, a statistical relationship (Roujean and Bréon, 1995) general enough for global applications is used. The principle of the algorithm is based on simulations of visible and near infrared spectral reflectance values in optimal angular geometries identified based on numerical experiments (simulations of the SAIL radiative transfer code). A vegetation index, called RDVI (Renormlized Difference Vegetation Index), is introduced, which shows to be less sensitive to background reflectance variability. A pre-established relationship is then applied between RDVI computed in an optimal angular geometry in the solar principal plane (s=45°, v=60°, j=0°) and daily FAPAR. Theoretical FAPAR uncertainties are estimated propagating input error through the model.
The FAPAR product is computed within the area covered by the MSG disk, over 4 specific geographical regions (Europe, Africa - N_Africa and S_Africa- and South America). For each day and geographical region, the FAPAR product, its error estimate and the processing flag are disseminated in HDF5 format. The relevant information concerning the data fields is included in the HDF5 attributes.
Automatic Quality Control (QC) is performed on FAPAR product and the quality information is provided on a pixel basis. QC contains general information about input data quality, and specific information related with the limits of application. The error estimate defines the confidence level of the different products on the basis of the theoretical model assumption and considering the statistical uncertainties of the observations and model parameters. The quality of FAPAR product depends on input quality (signal-to-noise ratio, accuracy of atmospheric corrections, number of cloud-free observations), accuracy of cloudy and snow pixels identification, and spectral variation in reflectance of different land-surface elements. Validation includes the assessment of the product in a systematic and statistically robust way representing global conditions. The specification for the overall accuracy of the FAPAR product is 20%. Results of detailed validation studies will be given in the Validation Report.
Gobron, N. et al. (2006). Evaluation of fraction of absorbed photosynthetically active radiation products for different canopy radiation transfer regimes: Methodology and results using Joint Research Center products derived from SeaWiFS against ground-based estimations. Journal of Geophysical Research, 111, D13110, doi:10.1029/2005JD006511.
Roujean, J.-L. and F.-M. Bréon, (1995). Estimating PAR absorbed by vegetation from Bidirectional Reflectance Measurements. Remote Sensing of Environment, 51: 375-384.