Comparison of flux measurement by open-path and close-path eddy covariance systems

摘要

For flux measurement, the eddy covariance technique supplies a possibility to directly measure the exchange between vegetation and atmosphere; and there are two kinds of eddy covariance systems, open-path and close-path systems. For the system error, it may result in difference of flux measurements by two systems. Therefore, it is necessary to compare the measured results from them. ChinaFLUX covers of eight sites applied the micrometeorological method, in which Changbai Mountains (CBS) and Qianyanzhou (QYZ) carried out open-path eddy covariance (OPEC) and close-path eddy covariance (CPEC) measurements synchronously. In this paper the data sets of CBS and QYZ were employed. The delay time of close-path analyzer to the open-path analyzer was calculated; the spectra and cospectra of time-series data of OPEC and CPEC were analyzed; the open-path flux measurement was used as a standard comparison, the close-path flux measurement results were evaluated. The results show that, at two sites the delay time of CO_2 density for close-path analyzer was about 7.0 — 8.0 s, H_2O density about 8.0 — 9.0 s; the spectrum from the open-path, close-path and 3D sonic anemometer was consistent with the expected -2/3 slope (log-log plot), and the cospectra showed the expected slope of -4/3 in the internal subrange; the CO_2 flux measured by the close-path sensor was about 84% of that of open-path measurement at QYZ, about 80% at CBS, and the latent heat flux was balanced for two systems at QYZ, 86% at CBS. From the flux difference between open-path and close-path analyzers, it could be inferred that the attenuation of turbulent fluctuations in flow through tube of CPEC affected H_2O flux more significantly than CO_2 flux. The gap between two systems was bigger at CBS than at QYZ; the diurnal variation in CO_2 flux of two measurement systems was very consistent.