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Focus Area on Biophysical Product Validation


Hongliang Fang, Chinese Academy of Sciences, China
Sylvain Leblanc, Natural Resources Canada
Marie Weiss, INRA, France

 

LAI and fAPAR Definitions

The biophysical focus area includes two variables at this time: leaf area index (LAI) and the fraction of absorbed photosynthetically active radiation (fAPAR).

Leaf area index is defined as one half the total green (i.e., photosynthetically active) leaf area per unit horizontal ground surface area.This definition, based on Chen and Black (1992), was accepted by GCOS, GTOS and CEOS in 2009. However, relatively to the way it is measured on the ground or derived from the satellite data, different definitions of LAI can be considered when derived from remote sensing instruments: GAI (Green Area Index, same definition as LAI but includes all the green elements, e.g., stems, ears,…), PAI (Plant Area Index, includes all elements, green and non-green).

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 vegetation. This definition was accepted by GCOS, GTOS and CEOS in 2009.

fAPAR actually corresponds to 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/diffuse 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 parts) for photosynthesis as well as other non-green elements such as trunks, senescent material, flowers, …

Note that fAPAR can be approximated by the fIPAR (fraction of Intercepted PAR radiation), by considering that the leaves are black in the PAR domain.

Units: LAI is a non-dimensional quantity, although units of m2/m2 are often quoted.
            fAPAR is expressed as a unitless fraction of the incoming radiation received at the land surface.


Highest Validation Stage Currently Reached for Satellite-Derived LAI and fAPAR Products

Validation stage 2 (LPV validation stage hierarchy) - The highest LPV validation stage reached for satellite-derived LAI and fAPAR products. Limitations to reach higher validation stage include limited number of validation sites, spatial and temporal gaps of in situ reference data coverage. LPV is collaborating with NEON, TERN, and ICOS to improve coverage of in situ reference data.
Note that less in situ measurements are available for fAPAR, which are mostly correspond derived from DHP measurements (and therefore actually correspond to fIPAR, see Definitions).
The BELMANIP2 sites ensure the fAPAR product inter-comparison over globally representative locations and time periods.


Validation Good Practices

The LPV Subgroup has just published a Good Practices Protocol document for validation of LAI Products. The document is available for download along with associated documents:

Fernandes, R., Plummer, S., Nightingale, J., Baret, F., Camacho, F., Fang, H., Garrigues, S., Gobron, N., Lang, M.,   Lacaze, R., LeBlanc, S., Meroni, M., Martinez, B., Nilson, T., Pinty, B., Pisek, J., Sonnentag, O., Verger, A., Welles, J.,   Weiss, M., & Widlowski, J.L. (2014). Global Leaf Area Index Product Validation Good Practices. Version 2.0.  In G.   Schaepman-Strub, M. Román, & J. Nickeson (Eds.), Best Practice for Satellite-Derived Land Product Validation (p.   76): Land Product Validation Subgroup (WGCV/CEOS), doi:10.5067/doc/ceoswgcv/lpv/lai.002
  (Download CEOS_LAI_PROTOCOL_Aug2014_v2.0.1.pdf, 5MB)


LAI and fAPAR Validation Reference Data Sets

Currently the best in situ reference data set for satellite-derived LAI and fAPAR product validation is available at the On Line Interactive Validation Exercise (OLIVE) validation platform (Weiss et al., 2014) . 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.

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

  • ImagineS : Implementation of Multi-scale Agricultural Indicators Exploiting Sentinels
  • NEON: National Ecological Observatory Network (US Only)
  • ICOS: Integrated Carbon Observation System (Europe)
  • EnviroNet

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.

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