Title: Intercomparison of Fractional Absorbed Photosynthetically Active Radiation Products derived from Satellite Data over Europe
Citation: REMOTE SENSING OF ENVIRONMENT vol. 142 p. 141-154
Publication Year: 2014
JRC N°: JRC82055
ISSN: 0034-4257
URI: http://dx.doi.org/10.1016/j.rse.2013.12.005
DOI: 10.1016/j.rse.2013.12.005
Type: Articles in periodicals and books
Abstract: The Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) is recognized as an essential climate variable (ECVs), playing a critical role in the estimation of the global energy and carbon balance. With multiple space-borne remote sensing FAPAR global products available from several sources the need for continual comparison and validation has become imperative. In this study, the performance of three global FAPAR algorithms (JRC-TIP, ESA/JRC MGVI and Boston University FAPAR) was evaluated over Europe for the year 2011. Uncertainty assessments were carried out using both, quantitative quality indicators as proposed by 25 the individual product teams as well as independent theoretical uncertainty estimates obtained with the triple collocation error model. Multitemporal analysis for the 2009-2011 period was further performed over four European flux tower sites. Results show an overall agreement among FAPAR products on sites having high and low FAPAR values, except for the north-eastern region of Europe where the Boston product overestimates values by other products by up to 0.5. Overall we observe a systematic negative bias for JRC-TIP and MGVI products as 32 compared to the Boston product across all land cover types and particularly over forest biomes. These findings suggest that differences between the assumptions and approximations implicitly made by using different radiative transfer models translate in differences in estimated fluxes. In particular assumptions on the optical properties of land surfaces, i.e., the absorbing/scattering efficiency of leaves and background, showed important in remote sensing derived FAPAR products. The uncertainty analysis using product-based information for JRC-TIP and Boston datasets revealed consistent spatial patterns but large differences in magnitudes (up to 0.1) with systematic lower uncertainties for the Boston product. Independent uncertainty estimates suggest instead similar uncertainty ranges among the three products. Investigations at the four flux tower sites highlighted good agreement of seasonal variations with higher FAPAR values in the photosynthetically active period and lower values during the dormant period. Findings suggest that differences in absolute values and inconsistency in uncertainty representation among FAPAR products are still considerable. Standardization frameworks quantifying the impact of different radiative transfer formulations on the estimation of biophysical variables, independent uncertainty estimation methods and well-defined ground measurement protocols, need to be put in place before FAPAR products can be reliably fed into existing biogeochemical process models.
JRC Directorate:Sustainable Resources

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