Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/9/2008
Publication Date: 6/30/2008
Citation: Wolf, A., Saliendra, N., Akshalov, K., Johnson, D.A., Laca, E.A. 2008. Effects of Different Eddy Covariance Correction Schemes on Energy Balance Closure and Comparisons with the Modified Bowen Ratio System. Agricultural and Forest Meteorology 148:942-952. Interpretive Summary: Accurate measurements of carbon dioxide (CO2) and water vapor fluxes are important to better understand the processes involved in carbon and water cycling and their possible role in global change. We used two eddy covariance (EC) sytems and a modified Bowen ratio system (MBR) to measure CO2 and water vapor fluxes above a shortgrass steppe in northeastern Kazakhstan. We processed these data and examined differences between these three measurement systems. We found that EC corrections have a strong effect on the Bowen ratio measured by EC. Two important corrections need to be used in processing EC data: 1) correction for signal asynchrony that accounts for the lag time between the infrared gas analyzer and sonic anemometer, and 2) the frequency domain correction for path-length averaging. Our comparisons showed the importance of field heterogeneity in determining field canopy gas exchange. Discrepancies between EC and MBR systems were highest during neutral periods when correlations between water vapor and CO2 were low, which violates the similarity assumption for the MBR technique. Our analysis of these data suggested that a screening technique could be used to exclude outlier data. The EC systems exhibited their largest discrepancies at night, while the MBR system exhibited occasional spikes of high photosynthetic uptake.
Technical Abstract: Eddy covariance (EC) and modified Bowen ratio (MBR) systems typically yield subtly different estimates of H, LE, and Fc. Our study analyzed the discrepancies between EC and MBR 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 increased the magnitude of LE and Fc, and consequently had 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: 1) the correction for signal asynchrony by adjusting the lag between the infrared gas analyzer and the sonic anemometer, and the frequency domain correction for path-length averaging in both instruments. Comparison of fluxes between two EC systems located 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, no bias for H and LE was observed 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 critical for the MBR technique. The largest discrepancies in Fc in both systems were clearly visible when plotted against a light-response curve, which suggested that a screening technique could be used to exclude data that depart markedly from other recently collected data. The EC system tended to have the largest departures from the light-response curve at night, while the MBR system tended to show occasional spikes of high photosynthetic uptake.