A Comprehensive Evaluation of Global Fraction of Absorbed Photosynthetically Active Radiation (FAPAR) Products of MODIS, GLASS, and GEO Over China

The fraction of absorbed photosynthetically active radiation (FAPAR) is defined as the fraction of incident solar radiation absorbed by vegetation for photosynthesis. Despite the proliferation of global FAPAR products, comparative analyses of their performance across varying terrain morphological conditions remain limited. The aim of this study is to compare three FAPAR products GEOLAND (GEO), global land surface satellite (GLASS), and moderate resolution imaging spectroradiometer (MODIS) with spatial resolutions of 500 m and 1 km, respectively, and to conduct a comprehensive evaluation of their accuracy, temporal continuity, and spatial consistency. The results showed that the data quality of FAPAR products tended to decline as elevation increases in mountainous areas compared to areas with flat terrain. The GLASS FAPAR product had the best temporal continuity, while the GEO FAPAR product had a higher proportion of high-quality data than MODIS but with a missing data rate of up to 12$\%$. GLASS and GEO FAPAR products exhibited good data consistency with the highest correlation coefficient of 0.89. In the southwestern mountainous regions, the most significant differences are observed between MODIS and both the GEO and GLASS products. Compared to in-situ FAPAR measurements, the 500 m resolution GEO FAPAR product demonstrated the best performance, with the R$^{2}$ of 0.90, the RMSE of 0.09, and the MAD of 0.08. The results of this research will help to understand the effect of terrain on FAPAR products in mountainous areas and provide guidance for selecting the most suitable FAPAR products over different terrains and land cover types.