NEE and gross primary production (GPP) can be estimated by crop net primary production based on inventories of biomass at these sites, independent of species and regions. NEE can also be estimated by the product of photosynthetic capacity and the number of days with the average air temperature >5 °C. Yield measured at these sites or reported at the NUTS2 level dataset of EUROSTAT is a relatively poor predictor of NEE.
To investigate the difference in the variability in CO2 emissions of different crops at the same location and to compare this variation with the variation of the same crop at different locations and with the inter-annual variation the measured dataset at the flux sites was extended with simulated data.
These simulations show that the variability in carbon exchange is determined by: firstly the choice of crop and the location and to a lesser extent by the yearly differences in climate.