Abstract
We investigate the benefits of assimilating in situ and satellite data of the fraction of photosynthetically active radiation (FAPAR) relative to eddy-covariance flux measurements for the optimization of parameters of the ORCHIDEE biosphere model. We focus on model parameters related to carbon fixation, respiration and phenology. The study relies on two sites – Fontainebleau (deciduous broadleaf forest) and Puechabon (Mediterranean broadleaf evergreen forest) – where measurements of net carbon exchange (NEE) and latent heat (LE) fluxes are available at the same time as FAPAR products derived from ground measurements or derived from spaceborne observations at high (SPOT) and medium (MERIS) spatial resolutions. We compare the different FAPAR products, analyze their consistency with the in situ fluxes, and then evaluate the potential benefits of jointly assimilating flux and FAPAR data. The assimilation of FAPAR data leads to a degradation of the model-data agreement with respect to NEE at the two sites. It is caused by the change in leaf area required to fit the magnitude of the various FAPAR products. Assimilating daily NEE and LE fluxes however has a marginal impact on the simulated FAPAR. The results suggest that the main advantage of including FAPAR data is the ability to constrain the timing of leaf onset and senescence for deciduous ecosystems, which is best achieved by normalizing FAPAR time series. The joint assimilation of flux and FAPAR data lead to a similar model-data improvement across all variables than when each data-stream is used independently, corresponding however to different and likely improved parameter values