Uncertainties in eddy co-variance flux measurements assessed from CH4 and N2O

This study analyses the EC flux measurements of CH4 and N2O and its corrections for
changing calibration, density effects and for high and low frequency response losses. In addition, it
assesses the magnitude of the uncertainties associated with the precision of the measurement
instruments, measurement set-up and the methodology, which is partly based on literature. The
examples are given using EC flux data measured at a dairy farm site at Oukoop in the Netherlands in 2006 and 2007. The total uncertainty of a single 30 min CH4 EC flux measurement is estimated at 142% and 47% for U=1 m s-1 and 10 m s-1, respectively, and the total uncertainty of a single 30 min N2O EC flux is estimated at 269% and 87% for U=1 m s-1 and 10 m s-1, respectively. This uncertainly is dominated by the random uncertainty due to one-point sampling. For months with 100% coverage without gap filling, the monthly average EC flux is estimated within an uncertainty range of about 9 - 10% and 8 - 12% for CH4 and N2O, respectively, depending on weather conditions. This uncertainty is mainly due to the systematic uncertainty in the calibration factor. This means that EC flux measurements can contribute to more accurate estimations from net ecosystem exchange of both gasses than estimations based only on chamber measurements.