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The use of eddy covariance (EC) CO2 flux measurements in data assimilationand other applications requires an estimate of the random uncertainty. Inprevious studies, the (classical) two-tower approach has yielded robustuncertainty estimates, but care must be taken to meet the often competingrequirements of statistical independence (non-overlapping footprints) andecosystem homogeneity when choosing an appropriate tower distance. The roleof the tower distance was investigated with help of a roving stationseparated between 8 m and 34 km from a permanent EC grassland station.Random uncertainty was estimated for five separation distances with theclassical two-tower approach and an extended approach which removedsystematic differences of CO2 fluxes measured at two EC towers. Thisanalysis was made for a data set where (i) only similar weather conditions atthe two sites were included, and (ii) an unfiltered one. The extendedapproach, applied to weather-filtered data for separation distances of 95and 173 m gave uncertainty estimates in best correspondence with anindependent reference method. The introduced correction for systematic fluxdifferences considerably reduced the overestimation of the two-tower baseduncertainty of net CO2 flux measurements and decreased the sensitivityof results to tower distance. We therefore conclude that corrections forsystematic flux differences (e.g., caused by different environmentalconditions at both EC towers) can help to apply the two-tower approach tomore site pairs with less ideal conditions

biogeosciences

Uncertainty analysis of eddy covariance CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; flux measurements for different EC tower distances using an extended two-tower approach