Effects of different eddy covariance correction schemes on a energy balance closure and comparisons with the modified Bowen ratio system

Eddy covariance (EC) and modified Bowen ratio (MBR) systems have been shown to yield subtly different estimates of sensible heat (H), latent heat (LE), and CO2 fluxes (Fc). Our study analyzed the discrepancies between these two systems by first considering the role of the data processing algorithm used to estimate fluxes using EC and later examining the effects of atmospheric stability on discrepancies between EC and MBR. We found that EC correction algorithms disproportionately increase the magnitude of LE and Fc, and consequently have a strong effect on the Bowen ratio measured by EC. Two corrections not universally employed were each found to account for up to 20% of LE and Fc: the correction for signal asynchrony by adjusting the lag between the IRGA and the sonic anemometer, and the frequency domain correction for path-length averaging in both instruments. Comparison of fluxes between two EC systems 10 m apart showed that LE and Fc are inherently more variable than H, highlighting the role of field heterogeneity in determining canopy gas exchange at very small spatial scales. When all relevant corrections were applied, there was no bias for H and LE between EC and MBR. An examination of discrepancies between EC and MBR showed that the discrepancies were highest during neutral periods when shear drove vertical mixing much more than buoyancy. During these neutral periods, the correlation between T, H2O, and CO2 was much lower on average, which violates the similarity assumption exploited in the MBR technique. The largest discrepancies in Fc in both systems were clearly visible when plotted against a light-response curve, which creates the possibility of using a screening technique to exclude data that depart markedly from other recently collected data. The EC system tended to have the largest departures from the expected value at low light and during the night, while the MBR system tended to show occasionally spikes of high photosynthetic uptake. (C) 2008 Elsevier B.V. All rights reserved.