Assessment of five global satellite products of fraction of absorbed photosynthetically active radiation: Intercomparison and direct validation against ground-based data

The fraction of absorbed photosynthetically active radiation (FAPAR) is a critical input parameter in many climate and ecological models. The accuracy of satellite FAPAR products directly influences estimates of ecosystem productivity and carbon stocks. The targeted accuracy of FAPAR products is 10%, or 0.05, for many applications. It is important to evaluate satellite FAPAR products and understand differences between the products to effectively use them in carbon cycling models. In this study, five global FAPAR products, namely MODIS, MISR, MERIS, SeaWiFS, and GEOV1 are intercompared over different land covers and directly validated with ground-based measurements at VAlidation of Land European Remote sensing Instruments (VALERI) and AmeriFlux sites. Intercomparison results show that MODIS, MISR, and GEOV1 agree well with each other and so do MERIS and SeaWiFS, but the difference between these two groups can be as large as 0.1. The temporal trends of these products agree better with each other in the Northern Hemisphere than in the Southern Hemisphere. The trends in the Northern Hemisphere are similar to those globally. However, the conclusions from the northern hemispheric scale could not be extended to the global scale for land covers such as savannahs and broadleaf evergreen forests. The differences between the products are consistent throughout the year over most of the land cover types, except over the forests, because of the different assumptions in the retrieval algorithms and the differences between green and total FAPAR products over forests. Direct validation results show that MERIS, MODIS, MISR, and GEOV1 FAPAR products have an uncertainty of 0.14 when validating with total FAPAR measurements, and 0.09 when validating with green FAPAR measurements. Overall, current FAPAR products are close to, but have not fulfilled, the accuracy requirement, and further improvements are still needed.