Open- vs. closed-path eddy covariance measurements of the net ecosystem carbon dioxide and water vapour exchange: a long-term perspective.

The differential design, deployment and data post-processing of open- (OP) and closed-path (CP) eddy covariance systems is a potential source of bias for ongoing global flux synthesis activities. Here we use a unique six year data set of concurrent CP and OP carbon dioxide (CO2) and water vapour (H2O) eddy covariance flux measurements above a temperate mountain grassland in Austria to explore the consequences of these differences on a long-term basis. The theoretically based transfer function approach was able to account and correct for the differences in low-pass filtering between the two systems. Corrected CO2 and H2O fluxes exhibited excellent 1:1 correspondence, but the CP system tended to underestimate OP H2O fluxes during conditions of high air temperature, wind speed and global radiation, large sun angles and low relative humidity. Corrections for self-heating of the OP infra-red gas analyser had a very small effect on these relationships. Energy balance closure was slightly more favourable for the OP system. No significant differences were found for the random flux uncertainty of both systems. A larger fraction of OP data had to be excluded because of obstructions of the infra-red path by water and snow. This, however, did not translate into a correspondingly larger fraction of accepted CP flux values, because of a larger percentage of CP flux data failing on the stationarity test. Integrated over the annual cycle, the CP system yielded on average a more positive net ecosystem CO2 exchange (25 vs. 0 gC m-2 y-1) and a lower evapotranspiration (465 vs. 549 mm y-1) as compared to the OP system.