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

Abstract

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).

Land potential assessment and trend-analysis using 2000–2021 FAPAR monthly time-series at 250 m spatial resolution

“…This means that there is still a significant part of the variation (about 10%) that we could not map on this scale of work. Our validation of the aggregated monthly FAPAR against ground reference measurements shows agreement with the validation of GLASS FAPAR V6 by Ma et al (2022) against ground measurements, who reported an R 2 of 0.8, but for their 8-day product. The validation of our FAPAR model predictions show high agreement with ground reference measurements as well (R 2 = 0.75).…”

“…The GLASS FAPAR V6 data is a harmonized and consistent time series of FAPAR at 250 m resolution, which covers the time period from 26th February 2000 to 27th December 2021 in 8 day interval steps (Ma et al, 2022). It is an improved version of the GLASS FAPAR V5 in terms of time series trend stability.…”

“…Validation of FAPAR values in Ma et al (2022) using ground reference measurements (GBOV) showed an overestimation of FAPAR at low values. This is also in agreement with Brown et al (2020), who reported an overestimation of FAPAR derived from remote sensing of sparse vegetation.…”

“…FAPAR under no-human pressure in our study refers to the potential FAPAR we could observe when removing the impact of urbanization, intensive agriculture, fertilization, and irrigation. Ma et al (2022) have recently produced a consistent and harmonized time series of FAPAR at 250 m resolution covering 2000 to 2021 in 8-day interval steps by integrating several satellite products: the existing MODIS, PROBA-V, and GLASS V5 FAPAR products. This product showed improvements compared to GLASS V5 FAPAR in terms of higher spatio-temporal continuity and details in reflectance values.…”

Diffuse Fertilization Effect in Maize and Soybean Is Driven by Improved Light Use Efficiency Rather Than by Light Absorption

“…These data sets have been broadly used to investigate the influence of global environmental change on terrestrial ecosystem processes (Keenan et al., 2013, 2019; Q. Z. Liu et al., 2021; Z. Y. Zhang, Xiong, et al., 2023), and GPP and FAPAR from AmeriFlux are often used as reference benchmarks to validate remote sensing measurements (Ma et al., 2022; Ryu et al., 2018; Zheng et al., 2020). These three crop sites, US‐Ne1, US‐Ne2, and US‐Ne3, are equipped with an eddy covariance system, which includes a 3D sonic anemometer and an open‐ path or closed‐path CO 2 and H 2 O gas analyzer that is collocated with the sonic anemometer.…”