Long-term time series of global vegetation products: challenges and lessons learnt from AVHRR to Sentinel-3

Abstract

Long term global terrestrial vegetation monitoring from satellite Earth Observation system is a critical issue within global climate and earth science modelling applications. A set of Essential Climate Variables was identified as being both accessible from remote sensing observations and intervening within key processes. Among those related to land surfaces, the leaf area index (LAI) and the fraction of absorbed photosynthetic active radiation (FAPAR) are derived from observations in the reflective solar domain. These vegetation biophysical variables play a key role in several processes, including photosynthesis, respiration and transpiration. LAI is defined as half the total developed area of leaf elements per unit horizontal ground area. It controls the exchanges of energy, water and greenhouse gases between the land surface and the atmosphere. FAPAR is defined as the fraction of radiation absorbed by the canopy in the 400 - 700 nm spectral domain under specified illumination conditions. It is one of the main inputs in light use efficiency models. The cover fraction (FCOVER) defined as the fraction of background covered by green vegetation as seen from nadir appears also as a very pertinent variable that can be used in surface energy balance models to separate the contribution of the soil from that of the canopy.This paper describes the GEOVx products consisting in LAI, FAPAR and FCOVER derived every 10 days at the global scale at kilometric and hectometric resolution within THEIA and Copernicus Global Land Service (CGLS) initiatives. GEOV2/AVHRR is derived from Long Term Data Record (LTDR) AVHRR data. It provides a global coverage at 0.05° ground sampling distance (~4km) every 10 days from 1981 to 2021. The GEOV2/CGLS Collection 1km of LAI, FAPAR and FCOVER products starts in 1999 with SPOT/VEGETATION data, and continues from 2014 to June 2020 with PROBA-V. The GEOV3/CGLS Collection 300m of LAI, FAPAR and FCOVER products is available from 2014 with PROBA-V and from July 2020 to present with Sentinel-3. The products are delivered with associated uncertainties and quality indicators. The products are accessible free of charge respectively through the THEIA (https://www.theia-land.fr/product/serie-de-variables-vegetales-avhrr-fr/) and GCLS (http://land.copernicus.eu/global/) websites, along with documentation describing the physical methodologies, the technical properties of products, and the quality of variables based on the results of validation exercises.This talk will focus on the retrieval algorithms used to generate the GEOVx LAI, FAPAR and FCOVER products. The GEOVx products will be assessed based on the comparison with other existing satellite products and ground data. The consistency of the time series will be evaluated with due attention to the switchover from different sensors (from AVHRR to SPOT/VEGETATION, from SPOT/VEGETATION to PROBA-V and from PROBA-V to Sentinel-3). Finally, some applications of the GEOVx biophysical products will be presented.