Focus Area on Biophysical Product Validation
Luke Brown, University of Salford, UK
Richard Fernandes, Natural Resources Canada
Hao Tang, National University of Singapore
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 of a plant canopy or ecosystem is defined as one half of the total green leaf area per unit horizontal ground surface area and measures the area of leaf material present in the specified environment (projection to the underlying ground along the normal to the slope). Leaf Area Index should be abbreviated as LAI and symbolized as L.
The fraction of absorbed photosynthetically active radiation (fAPAR) is defined as the fraction of photosynthetically active radiation (PAR, i.e. the solar radiation reaching the surface in the 0.4-0.7 µm spectral region) that is absorbed by vegetation canopy with a stated associated illumination condition. Commonly used illumination conditions are direct illumination only ('direct fAPAR', 'black-sky fAPAR' ), diffuse isotropic illumination only ('diffuse fAPAR', sic 'white-sky fAPAR'), or ambient illumination (sic 'blue-sky'). The fraction of photosynthetically active radiation is abbreviated as fAPAR and symbolized as FAPAR.
For clarity, green leaf fAPAR (fAPARg) corresponds to the fraction of photosynthetically active radiation (PAR, i.e. the solar radiation reaching the surface in the 0.4-0.7 µm spectral region) that is absorbed by green leaves in the vegetation canopy. Woody matter and non-vascular vegetation such as bryophytes, lichen and algae are not leaves for the purpose of fAPARg and LAI.
Units: LAI is a non-dimensional quantity, although units of m2/m2 (half foliage area/horizontal ground area) are often quoted.
fAPAR is expressed as a unitless fraction of the incoming radiation received at the land surface.
Proposal for additional Vegetation Biophysical Parameters
The biophysical focus area is opening a call for proposals to include additional vegetation biophysical parameters within its scope. These parameters must already be associated with systematically generated continental or global products and have user requirements specified by an international organization recognized by CEOS. Please contact a focus area leads for more information.
Highest Validation Stage Currently Reached for Satellite-Derived LAI and fAPAR Products
Validation stage 3 (LPV validation stage hierarchy) - The highest LPV validation stage reached for satellite-derived LAI and Fapar products. LPV is collaborating with NEON, TERN, and ICOS to improve coverage of in situ reference data.
Note that fewer in situ measurements are available for fAPAR, which are mostly derived from DHP measurements (and therefore actually correspond to fIPAR, see Definitions).
The BELMANIP2 sites ensure that product inter-comparison occurs 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.), Good Practices 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)
Validation Reference Data Sets
The DIRECT V2.1 database compiles Leaf Area Index (LAI), fraction of absorbed photosynthetically active radiation (FAPAR) and fraction of vegetation cover (FCover) averaged values over a 3 km x 3 km area. The ground data were upscaled using high-resolution imagery following the LPV LAI good practices protocol to properly account for the spatial heterogeneity of the sites.
Ground measurements included in the original DIRECT dataset were a result of several international activities, including VALERI, BigFoot, SAFARI-2000, CCRS, Boston University and ESA campaigns compiled by S. Garrigues (Garrigues et al., 2008). These were later ingested into the LPV OLIVE tool (Weiss et al., 2014) for accuracy assessment. The DIRECT sites were reviewed by F. Camacho to remove sites lacking understory measurements (Camacho et al., 2013). In an effort several years later, the sites were expanded and incorporated into a database with ImagineS sites (Camacho et al., 2021). DIRECT V2.1 is the latest update, that includes 44 new sites from China (Fang et al., 2019; Song et al., 2021) and 2 new sites from ESA FRM4Veg (Brown et al., 2021).
The CEOS WGCV LPV DIRECT V2.1 database constitutes a major effort of the international community to provide ground reference for the validation of LAI and FAPAR ECVs, with a total of 176 sites around the world (within 7 main biome types) and 280 LAI, 128 FAPAR and 122 FCover values spanning the period from 2000 to 2021.
Additional details and access to the new LPV DIRECT 2.1 dataset can be found on the WGCV Cal/Val portal page dedicated to DIRECT V2.1.
Additional ground-based measurements are available through the following links:
LPV Focus Areas
Meetings
Land Product Validation and Evolution, June 12 -14, 2023 ESA/ESRIN, Italy.
IGARRS 2023, 16-21 July 2023, Pasadena, CA. Abstract submission due 18 Nov 2022.
EGU 2023, 23-28 April 2023, Vienna, Austria. Abstract submission due 10 Jan 2023.
