Methodology for determining fluxes of CO 2 and H 2 O vapor with the eddy-covariance method using data from instruments on a 447-m tower in the forest of northern Wisconsin is addressed. The primary goal of this study is the validation of the methods used to determine the net ecosystem exchange of CO 2 . Two-day least squares fits coupled with 30-day running averages limit calibration error of infrared gas analyzers for CO 2 and H 2 O signals to 2%-3%. Sonic anemometers are aligned with local streamlines by fitting a sine function to tilt and wind direction averages, and fitting a third-order polynomial to the residual. Lag times are determined by selecting the peak in lagged covariance with an error of 1.5%-2% for CO 2 and 1% for H 2 O vapor. Theory and a spectral fit method allow determination of the underestimation in CO 2 flux (5% daytime, 12% nighttime) and H 2 O vapor flux (21%), which is due to spectral degradation induced by long air-sampling tubes. Scale analysis finds 0.5-h flux averaging periods are sufficient to measure all flux scales at 30-m height, but 1 h is necessary at higher levels, and random errors in the flux measurements due to limited sampling of atmospheric turbulence are fairly large (15%-20% for CO 2 and 20%-40% for H 2 O vapor at lower levels for a 1-h period).
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Berger et al. (2001) studied this question.