|Ruppert, J; Mauder, M; Thomas, C; Lüers, J: Innovative gap-filling strategy for annual sums of CO2 net ecosystem exchange, Agricultural and Forest Meteorology, 138, 5-18 (2006), doi:10.1016/j.agrformet.2006.03.003|
The determination of carbon dioxide net ecosystem exchange (NEE) with the eddy covariance (EC) method has become a fundamental tool for the investigation of the carbon balance of terrestrial ecosystems. This study presents a strategy for the processing, subsequent quality control and gap-filling of carbon dioxide eddy covariance flux measurements for the derivation of annual sums of NEE. A set of criteria used for quality assessment identified periods with instrumental or methodological failures. These criteria included tests for the methodological prerequisites of the EC method. The complete evaluation scheme was applied to data recorded above a spruce forest at the LUXNET-station Waldstein-Weidenbrunnen (DE-Wei) in 2003. Comparison of this new evaluation scheme to the use of a friction velocity (u*) threshold criterion of 0.3ms-1 indicates less systematic distribution of data gaps. The number of available high quality nighttime measurements increased. This effect was most pronounced during summer, when data is highly needed for a robust parameterization of respiratory fluxes. Non-linear regression analysis showed that air temperature and global radiation explain most of the variability of NEE and further seasonal segregation of the data based on an objective method did not significantly improve predictions at this evergreen forest site. The remaining pattern of systematic dependence of nighttime NEE on u* suggests that 'pressure pumping' has a significant influence on the release of respired carbon dioxide from the forest floor during nighttime.