Uncertainties in eddy covariance flux measurements assessed from CH4 and N2O observations

The uncertainty in eddy covariance (EC) flux measurements is assessed for CH4 and N2O using data measured at a dairy farm site in the Netherlands in 2006 and 2007. An overview is given of the contributing uncertainties and their magnitude. The relative and absolute uncertainty of a 30 min EC flux are estimated for CH4 and N2O using N = 2185 EC fluxes. The average absolute uncertainty and its standard deviation are 500 ± 400 ng C m−2 s−1 for CH4 and 100 ± 100 ng N m−2 s−1 for N2O. The corresponding relative uncertainties have 95% confidence interval ranging from 20% to 300% for CH4 and from 30% to 1800% for N2O. The large relative uncertainties correspond to relatively small EC fluxes. The uncertainties are mainly caused by the uncertainty due to one-point sampling which contributes on average more than 90% to the total uncertainty. The other 10% includes the uncertainty in the correction algorithm for the systematic errors. The uncertainty in a daily and monthly averaged EC flux are estimated for several flux magnitude ranges. The daily and monthly average uncertainty are smaller than 25% and 10% for CH4 and smaller than 50% and 10% for N2O, respectively, based on fluxes larger than 100 ng C m−2 s−1 and 15 ng N m−2 s−1.