THE CEOS DATABASE : Missions, Instruments and Measurements
GLOBAL CLIMATE OBSERVING SYSTEM
ESSENTIAL CLIMATE VARIABLE - Fraction of absorbed photosynthetically active radiation (FAPAR)
DomainTerrestrialDescriptionSolar radiation in the spectral range from 400 to 700nm, known as Photosynthetically Active Radiation (PAR), provides the energy required by terrestrial vegetation to produce organic materials from mineral components. The part of this PAR that is effectively absorbed by plants is called the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR). It is a non-dimensional quantity varying from zero (over deserts) to up to one (for large, deep, homogeneous canopy layers observed by medium- to low-resolution sensors), although the maximum value is never witnessed in practice because some of the incoming light is always reflected back by the canopy or the underlying ground. FAPAR is related to, but different from, Leaf Area Index (LAI; covered in the following section), which describes the amount of leaf material in the canopy.

FAPAR plays a critical role in assessing the primary productivity of canopies, the associated fixation of atmospheric carbon dioxide, and the energy balance of the surface. As is the case with land surface albedo (section 6.3.9), FAPAR depends on the illumination conditions, i.e. the angular position of the sun with respect to the vegetation layer and the relative contributions of the direct and diffuse irradiances. Both black-sky (assuming only direct radiation) and white-sky (assuming that all the incoming radiation is in the form of isotropic diffuse radiation) FAPAR values may be considered. Models describing the primary productivity of plants and the energy balance of the land surface require either a characterisation of the diurnal evolution of FAPAR or the daily integrated value of FAPAR, depending on the time-step used. Other applications may only require cumulative or aggregated values over longer periods. [GCOS-195 6.3.11]
Sub-domain-
Full NameFraction of absorbed photosynthetically active radiation (FAPAR)
Satellite Signficant ContributionYes
GCOS Actions
Action
Status*
DescriptionWhoTime FramePerformance IndicatorCost Implications
T3
Cat-D
Development of a subset of current LTER and FLUXNET sites into a global terrestrial reference network for monitoring sites with sustained funding perspective, and collocated measurements of meteorological ECVs; seek linkage with Actions T4 and T29 as appropriate.Parties’ national services and research agencies, FLUXNET organizations, NEON, and ICOS, in association with CEOS WGCV, CGMS-GSICS, and GTOS (TCO and TOPC).Implementation started by 2011, completed by 2014.Plan for the development and application of standardised protocols for the30-100M US$ (40% in non-Annex-I Parties).
T29
Cat-C
Establish a calibration/validation network of in situ reference sites for FAPAR and LAI and conduct systematic, comprehensive evaluation campaigns to understand and resolve differences between the products and increase their accuracy.Parties’ national and regional research centres, in cooperation with space agencies coordinated by CEOS WGCV, GCOS and GTOS.Network operational by 2012.Data available to analysis centres.1-10M US$ (40% in non-Annex-I Parties).
T31
Cat-A
Operationalize the generation of FAPAR and LAI products as gridded global products at spatial resolution of 2 km or better over time periods as long as possible.Space agencies, coordinated through CEOS WGCV, with advice from GCOS and GTOS.2012One or more countries or operational data providers accept the charge of generating, maintaining, and distributing global FAPAR products.10-30M US$ (10% in non-Annex-I Parties).
*GCOS-195 Status Categories: Category A: Action completed, perhaps exceeding reasonable expectations. Very good progress on ongoing tasks. Category B: Action largely completed according to expectation. Good progress on ongoing tasks. Category C: Moderate progress overall, although progress may be good for some part of the action. Category D: Limited progress overall, although progress may be moderate or good for some part of the action. Category E: Very little or no progress, or deterioration rather than progress.
GCOS Products
ProductNameVariable/
Parameter
Related Measurements/
Instruments
from CEOS DB
T.7Maps of the Fraction of Absorbed Photosynthetically Active Radiation (FAPAR)FAPAR
Fractionally absorbed PAR (FPAR)
CEOS Response[T29 (T.7)]

2015 Update: The Land Product Validation Subgroup of the CEOS WGCV (LPV) has taken on a coordination role to establish fiducial reference data in collaboration with long-term in-situ networks for fAPAR and LAI. The LPV focus area for fAPAR and LAI are in contact with in-situ networks (e.g., NEON, ICOS) to coordinate field sampling protocols. A fAPAR workshop was held in 2014 to discuss details of a fAPAR intercomparison and validation protocol and field instrument set-up and sampling. In 2015, a few sites were instrumented and with calibrated PAR sensors that will allow for the generation of high-quality fAPAR reference data. However, the number of validation sites remains limited (see http://lpvs.gsfc.nasa.gov/Fpar_home.html).

Work on the generation of LAI in-situ reference data has been ongoing. A compiled reference data set has been extended recently for crop- and grassland sites in the framework of the EU Framework Programme 7 project ImagineS (http://fp7-imagines.eu) . It is planned to make these data available through the PLIVE platform.


[T30 (T.7)]

2015 Update: Completion or significant progress of all planned key activities related to this action has been achieved. The Land Product Validation subgroup of the CEOS Working Group on Calibration and Validation has coordinated the implementation pof an on-line validation system, supported by ESA (Weiss et al., 2014). The subgroup is coordinating with several long-term in-situ networks to improve the quantity and quality of validation data, for example by reviewing field sampling protocols (e.g., ICOS, NEON). Most importantly, the LPV subgroup compiled and distributed a community-reviewed best practices document for LAI intercomparison and validation. This document is referenced with a DOI (doi:10.5067/doc/ceoswgcv/lpv/lai.002). For more information see http://lpvs.gsfc.nasa.gov/LAI_home.html.


[T31 (T.7, T.8)]

2015 Update: Operational product generation has commenced through NOAA and EUMETSAT. For example, NOAA’s Climate Data Records Program has transitioned the production of LAI and fAPAR to operations (http://www.ncdc.noaa.gov/cdr/operationalcdrs.html). The records are global on a 0.05 by 0.05 degree grid and are produced daily from 1981-present. These records are also routinely updated and full documentation is available. However, to date community agreed specifications of uncertainty, reconciliation of algorithms and ancillary data have not been achieved for LAI and all contributing archives have not been therefore reprocessed to date.

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