Estimating the fraction of absorbed photosynthetically active radiation from the MODIS data based GLASS leaf area index product

The fraction of absorbed photosynthetically active radiation (FAPAR) is an essential biophysical variable and plays a critical role in the carbon cycle. Existing FAPAR products from satellite observations are spatially incomplete and temporally discontinuous, and also insufficiently accurate to meet the requirements of various applications. In this study, a new method is proposed to calculate high quality, accurate FAPAR from the Global LAnd Surface Satellite (GLASS) leaf area index (LAI) to ensure physical consistency between LAI and FAPAR retrievals. As a result, a global FAPAR product (denoted by GLASS) was generated from the GLASS LAI data from 2000. With no missing values, GLASS FAPAR product is spatially complete. Comparison of the GLASS FAPAR product with the MODerate Resolution Imaging Spectroradiometer (MODIS), Geoland2/BioPar version 1 (GEOV1), and the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) FAPAR products indicates that these FAPAR products exhibit similar spatial distribution pattern. However, there were relatively large discrepancies between these FAPAR products in equatorial forest regions and around 50-60°N, where the SeaWiFS FAPAR values were lower than the other products and GLASS FAPAR product showed the largest values. Temporal consistency analysis indicates that GLASS FAPAR product has continuous trajectories, while MODIS FAPAR product shows more unstable profiles, especially during the growing season. Direct comparison with ground-based estimates demonstrated that GLASS FAPAR values were more accurate (R2 = 0.9292 and RMSE = 0.0716) than GEOV1 (R2 = 0.8681 and RMSE = 0.1085), MODIS (R2 = 0.8048 and RMSE = 0.1276) and SeaWiFS FAPAR values (R2 = 0.7377 and RMSE = 0.1635).