Global land surface 250 m 8 d fraction of absorbed photosynthetically active radiation (FAPAR) product from 2000 to 2021

Abstract. The fraction of absorbed photosynthetically active
radiation (FAPAR) is a critical land surface variable for carbon cycle
modeling and ecological monitoring. Several global FAPAR products have been
released and have become widely used; however, spatiotemporal inconsistency
remains a large issue for the current products, and their spatial
resolutions and accuracies can hardly meet the user requirements. An
effective solution to improve the spatiotemporal continuity and accuracy of
FAPAR products is to take better advantage of the temporal information in
the satellite data using deep learning approaches. In this study, the latest
version (V6) of the FAPAR product with a 250 m resolution was generated from
Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance
data and other information, as part of the Global LAnd Surface Satellite
(GLASS) product suite. In addition, it was aggregated to multiple coarser
resolutions (up to 0.25∘ and monthly). Three existing global FAPAR
products (MODIS Collection 6; GLASS V5; and PRoject for On-Board Autonomy–Vegetation, PROBA-V, V1) were used to
generate the time-series training samples, which were used to develop a
bidirectional long short-term memory (Bi-LSTM) model. Direct validation
using high-resolution FAPAR maps from the Validation of Land European Remote
sensing Instrument (VALERI) and ImagineS networks revealed that the GLASS V6
FAPAR product has a higher accuracy than PROBA-V, MODIS, and GLASS V5, with
an R2 value of 0.80 and root-mean-square errors (RMSEs) of 0.10–0.11
at the 250 m, 500 m, and 3 km scales, and a higher percentage (72 %) of
retrievals for meeting the accuracy requirement of 0.1. Global spatial
evaluation and temporal comparison at the AmeriFlux and National Ecological
Observatory Network (NEON) sites revealed that the GLASS V6 FAPAR has a
greater spatiotemporal continuity and reflects the variations in the
vegetation better than the GLASS V5 FAPAR. The higher quality of the GLASS
V6 FAPAR is attributed to the ability of the Bi-LSTM model, which involves
high-quality training samples and combines the strengths of the existing
FAPAR products, as well as the temporal and spectral information from the
MODIS surface reflectance data and other information. The 250 m 8 d GLASS V6 FAPAR product for 2020 is freely available at https://doi.org/10.5281/zenodo.6405564
and https://doi.org/10.5281/zenodo.6430925 (Ma, 2022a, b) as well as at the University of Maryland for 2000–2021
(http://glass.umd.edu/FAPAR/MODIS/250m, last access 1 November 2022).
