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Focus Area on fAPAR Product Validation


Marie Weiss, INRA, France

 

fAPAR Definition

The fraction of absorbed photosynthetically active radiation (fAPAR) is defined as the fraction of photosynthetically active radiation (PAR; solar radiation reaching the surface in the 0.4-0.7µm spectral region) that is absorbed by a vegetation.

fAPAR is the sum of two terms, weighted by the diffuse fraction in the PAR domain: the 'black sky' fAPAR that corresponds to the direct component (collimated beam irradiance in the sun direction only) and the 'white sky' or the diffuse component. The closest approximation to white sky fAPAR occurs under a deep cloud cover that may generate an almost isotropic diffuse downward radiation. The adjectives of black and white are not related here to the color of the sky, but to the angular distribution of light intensity (see Martonchik et al., 2000 for more details).

fAPAR can be considered at a given time (e.g instantaneous fAPAR at the actual sun position of measurement) or daily integrated. All the vegetation elements contribute to PAR absorption, including the green elements (leaves, stems and other organs) for photosynthesis as well as other non-green elements such as trunks, senescent material, flowers, …

Units: fAPAR is expressed as a unitless fraction of the incoming radiation received at the land surface.


Highest Validation Stage Currently Reached for Satellite-Derived fAPAR Products

Validation stage 1 (LPV validation stage hierarchy) - The highest LPV validation stage reached for satellite-derived fAPAR products. For reaching validation stage 3 and higher, it is required to increase the number of global validation sites in order to cover a larger range of biomes representative of the Earth Surface, as well to extend the temporal coverage and provide continuous measurements over entire vegetation cycles. The BELMANIP2 sites ensure the fAPAR product inter-comparison over globally representative locations and time periods.

Validation Good Practices Document

The fAPAR focus area of CEOS LPV is currently reviewing existing validation methods and will consequently develop a good practice protocol for the validation of satellite-derived fAPAR products.


fAPAR Validation Reference Data Sets

Currently, there are few available in situ reference data sets for satellite-derived fAPAR product validation. There are some available on the On Line Interactive Validation Exercise (OLIVE) validation platform. The objective of this platform is to provide the user community consistent and reliable information on the accuracy and associated uncertainty of EO products. Two datasets are provided: the BELMANIP-2 dataset, used for product intercomparison, and DIRECT sites, where in situ datasets at medium resolution are available. References to the native data used in the DIRECT dataset are provided here. Recently, the FP7 ImagineS project has gathered a database of ground LAI and fAPAR measurements, upscaled using high spatial resolution imagery in agreement with CEOS LPV recommendations.

Additional ground-based measurements are available through the following links:

LPV Focus Areas

 

Meetings

Recent Developments in LAI and FAPAR Estimation and Validation, July 28 - August 2, 2019 Yokohama, Japan.

EGU 2017, 23-28 April 2017, Vienna, Austria. Abstract submission due 11 Jan 2017, details here.

ESA Annouces first gathering of the Sentinel-2 Validation Team (S2VT), 8-29 November 2016, at ESRIN, Frascati, Italy.

First Workshop of the CEOS LPV FAPAR focus area, 23-24 Jan 2014, JRC, Ispra, Italy

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