Uncertainty analysis of eddy covariance CO$_{2}$ flux measurements for different EC tower distances using an extended two-tower approach

Post, H.; Hendricks-Franssen, Harrie-Jan; Schmidt, M.; Vereecken, H.; Graf, Alexander

doi:10.5194/bgd-11-11943-2014

Journal Article

FZJ-2015-00513

Uncertainty analysis of eddy covariance CO$_{2}$ flux measurements for different EC tower distances using an extended two-tower approach

Post, H. (Corresponding Author)FZJ* ; Hendricks-Franssen, H.-J.FZJ* ; Graf, A.FZJ* ; Schmidt, M.FZJ* ; Vereecken, H.FZJ*

2014
Copernicus Katlenburg-Lindau [u.a.]

Biogeosciences discussions 11(8), 11943 - 11983 (2014) [10.5194/bgd-11-11943-2014]

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Please use a persistent id in citations: http://hdl.handle.net/2128/8273 doi:10.5194/bgd-11-11943-2014

Abstract: The use of eddy covariance CO2 flux measurements in data assimilation and other applications requires an estimate of the random uncertainty. In previous studies, the two-tower approach has yielded robust uncertainty estimates, but care must be taken to meet the often competing requirements of statistical independence (non-overlapping footprints) and ecosystem homogeneity when choosing an appropriate tower distance. The role of the tower distance was investigated with help of a roving station separated between 8 m and 34 km from a permanent EC grassland station. Random uncertainty was estimated for five separation distances with an extended two-tower approach which removed systematic differences of CO2 fluxes measured at two EC towers. This analysis was made for a dataset where (i) only similar weather conditions at the two sites were included and (ii) an unfiltered one. The extended approach, applied to weather-filtered data for separation distances of 95 m and 173 m gave uncertainty estimates in best correspondence with the independent reference method The introduced correction for systematic flux differences considerably reduced the overestimation of the two-tower based uncertainty of net CO2 flux measurements, e.g. caused by different environmental conditions at both EC towers. It is concluded that the extension of the two-tower approach can help to receive more reliable uncertainty estimates because systematic differences of measured CO2 fluxes which are not part of random error are filtered out.

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