A global land surface 250-m 8-day Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) product (2022-part1)

The Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) is a critical land surface variable for carbon cycle modeling and ecological monitoring. We generated the latest version (V6) of the FAPAR product at a 250-m resolution, which is also aggregated to multiple coarser resolutions (up to 0.25° and monthly), from MODIS surface reflectance data and other information, as part of the Global Land Surface Satellite (GLASS) products suite. Three existing global FAPAR products (MODIS Collection 6, GLASS V5, and PROBA-V V1) were used to generate the time-series training samples for developing the Bidirectional Long Short-Term Memory (Bi-LSTM) model. Direct validation using high-resolution FAPAR maps from VALERI and ImagineS networks revealed that the GLASS V6 FAPAR product achieves higher accuracy than PROBA-V, MODIS, and GLASS V5, with R² of 0.80 and RMSE of 0.10-0.11 at the 250-m, 500-m, and 3-km scales, and a higher percentage (72%) of retrievals in meeting accuracy requirement of 0.1. Global spatial evaluation and temporal comparison at the Ameriflux and NEON sites indicated that the GLASS V6 FAPAR is more spatio-temporal continuous and better reflects the variations of vegetation than the GLASS V5 FAPAR. 

The temporal span of GLASS V6 FAPAR product is from 2000 to 2020 and the temporal frequency is 8-day. The 250-m and 500-m data are in the Sinusoidal projection, while the 0.05°, 0.1° and 0.25° data are in geographic latitude/longitude. Data files are provided in HDF−EOS format.