Grain Yield Observations Constrain Cropland CO2 Fluxes Over Europe

Carbon exchange over croplands plays an important role in the European carbon cycle over daily to seasonal time scales. A better description of this exchange in terrestrial biosphere models -- most of which currently treat crops as unmanaged grasslands -- is needed to improve atmospheric CO~2~ simulations. In the framework we present here, we model gross European cropland CO~2~ fluxes with a crop growth model constrained by grain yield observations. Our approach follows a two-step procedure. In the first step, we calculate day-to-day crop carbon fluxes and pools with the WOrld FOod STudies (WOFOST) model. A scaling factor of crop growth is optimised regionally by minimizing the final grain carbon pool difference to crop yield observations from the Statistical Office of the European Union. In a second step, we re-run our WOFOST model for the full European 25 x 25 km gridded domain using the optimized scaling factors. We combine our optimized crop CO~2~ fluxes with a simple soil respiration model to obtain the net cropland CO~2~ exchange. We assess our model’s ability to represent cropland CO~2~ exchange using 40 years of observations at 7 European FluxNet sites and compare it with carbon fluxes produced by a typical terrestrial biosphere model. We conclude that our new model framework provides a more realistic and strongly observation-driven estimate of carbon exchange over European croplands. Its products will be made available to the scientific community through the ICOS Carbon Portal, and serve as a new cropland component in the CarbonTracker Europe inverse model.