# Assimilation of Earth Observation Data Over Cropland and Grassland Sites into a Simple GPP Model
## 6 Conclusions  
A simple crop growth model composed by few equations building on the framework of the GRAMI model was coupled with the canopy radiative transfer model PROSAIL5B and reparametrized against MODIS NDVI observations to estimate GPP of crop and grass ecosystems over Europe. The model requires four meteorological variables as input (incident radiation, air temperature, precipitation and reference potential evapotranspiration) and a preliminary identification of the growing period. In this study, we used meteorological variable estimates from a global circulation model (ECMWF ERA-INTERIM) and the results of a satellite-based land surface phenology retrieval for the initial definition of growing period. GPP estimates from eddy covariance flux sites were used to (1) set the maximum light use efficiency value (2 crop and 2 grassland sites), and (2), validate GPP estimates (28 crop and 25 grassland sites). The approach resulted in acceptable accuracy of GPP 8-day estimates (RMSE = 2.45 gC m\({}^{-2}\) d\({}^{-1}\), R\({}^{2}\) = 0.67, n = 11209). Performances were shown to be improved of about 20% compared to those of standard MODIS MOD17 GPP product, used as benchmark.  
[PERSON]. ([PERSON]) developed and coded the coupled Sim Model and wrote the majority of the manuscript. [PERSON] provided essential contributions in preliminary mathematical development of the algorithm. [PERSON]-[PERSON] and [PERSON]. ([PERSON]) contributed to the conceptual development of the GPP model and provided substantial inputs for the set-up of the validation part. All authors contributed to the writing of the manuscript.  
This research received no external funding.  
This work used eddy covariance data acquired and shared by the FLUXNET community, including these networks: AmeriFlux, AfriFlux, AsiaFlux, CarboAfrica, CarboEuropeIP, Carboltaly, CarboMont, ChinaFlux, Fluxnet-Canada, GreenGrass, ICOS, KofFlux, LBA, NECC, OzFlux-TERN, TCGS-Siberia, and USCCC. The ERA-Interim reanalysis data are provided by ECMWF and processed by LSCE. The FLUXNET eddy covariance data processing and harmonization was carried out by the European Fluxes Database Cluster, AmeriFlux Management Project, and Fluxdata project of FLUXNET, with the support of CDIAC and ICOS Ecosystem Thematic Center, and the OzFlux, ChinaFlux, and AsiaFlux offices. We thank the following projects for supporting EC measurements and their harmonization: European Fluxes Database Cluster, NitroEurope IP and ECLAIRE. The authors would like to thank all the PIs of eddy covariance sites, including those that shared restricted access data with us. We thank the following PIs for sharing their data and their comments on the present paper: [PERSON] and [PERSON] (NERC, Edinburgh), [PERSON] (Mechnikov University, Odessa), and [PERSON] (CESBIO, Toulouse). We thank [PERSON], [PERSON], and [PERSON] (European Commission, Joint Research Centre) for collecting and harmonizing EC data and for their valuable suggestions. Finally, this research was supported by the Exploratory Research initiative of the European Commission--Joint Research Centre.  
The authors declare no conflict of interest.