Grain yield observations constrain cropland CO2 fluxes over 1 Europe

Carbon exchange over croplands plays an important role in the European carbon cycle over daily to seasonaltime scales. Not only do crops occupy one eighth of the global land area, but their photosynthesisand respiration are large and aect CO2 mole fractions at nearly every atmospheric CO2 monitoringsite. A better description of this crop carbon exchange in terrestrial biosphere models { most of whichcurrently treat crops as unmanaged grasslands { could strongly improve their calculated uxes. Availablelongterm observations of crop yield, harvest, and cultivated area allow such improvements, whencombined with the new crop-specic modeling framework we present here. In this framework we modelgross carbon uxes of the major European crops on a 25 x 25 km grid and daily time-step, while aiming tomake calculated seasonal grain yield agree with observations. For each crop species and region of Europe,this follows a two-step procedure. In the rst step, we calculate crop growth over the full growing seasonwith the process-based WOrld FOod STudies (WOFOST) agricultural crop growth model, which resultsin a simulated crop yield. Simulated yields are optimized by minimizing their dierence to regional cropyield observations from the Statistical Oce of the European Union (EUROSTAT) by estimating oneyearly regional scaling parameter for each crop species, the so-called \yield gap factor". In a second step,we run our WOFOST model for the full European 25km2 gridded domain using the optimized yield-gapfactors for each crop species and region, to make wall-to-wall carbon exchange uxes that also take intoaccount the local variations of weather, soil properties, and sowing calendar. We combine the resultingGPP and Raut uxes with a simple soil respiration expression to obtain the gridded total ecosystemrespiration (TER) and net ecosystem exchange (NEE). We assess our model's ability to represent theseasonal GPP, TER and NEE uxes using 40 site-years of observations at 7 European FluxNet croplandsites and compare it with cropland carbon uxes produced by a typical terrestrial biosphere model usedwidely for European carbon cycle studies. We conclude that our new model framework provides a moredetailed, realistic, and strongly observation-driven estimate of carbon exchange over European croplands.Its products will be made available to the scientic community through the ICOS Carbon Portal, andserve as a new cropland component in the CarbonTracker Europe inverse model ux estimates.