title: Estimating annual CO(2) flux for Lutjewad station using three different gap-filling techniques.
abstract: Long-term measurements of CO(2) flux can be obtained using the eddy covariance technique, but these datasets are affected by gaps which hinder the estimation of robust long-term means and annual ecosystem exchanges. We compare results obtained using three gap-fill techniques: multiple regression (MR), multiple imputation (MI), and artificial neural networks (ANNs), applied to a one-year dataset of hourly CO(2) flux measurements collected in Lutjewad, over a flat agriculture area near the Wadden Sea dike in the north of the Netherlands. The dataset was separated in two subsets: a learning and a validation set. The performances of gap-filling techniques were analysed by calculating statistical criteria: coefficient of determination (R(2)), root mean square error (RMSE), mean absolute error (MAE), maximum absolute error (MaxAE), and mean square bias (MSB). The gap-fill accuracy is seasonally dependent, with better results in cold seasons. The highest accuracy is obtained using ANN technique which is also less sensitive to environmental/seasonal conditions. We argue that filling gaps directly on measured CO(2) fluxes is more advantageous than the common method of filling gaps on calculated net ecosystem change, because ANN is an empirical method and smaller scatter is expected when gap filling is applied directly to measurements.
authors.affiliation: European Centre of Excellence for the Environment, Faculty of Sciences, Dunarea de Jos University of Galati, Street Domneasca No. 111, 800201 Galati, Romania.
authors.forename: Carmelia M
authors.initials: CM
authors.lastname: Dragomir
authors.forename: Wim
authors.initials: W
authors.lastname: Klaassen
authors.forename: Mirela
authors.initials: M
authors.lastname: Voiculescu
authors.forename: Lucian P
authors.initials: LP
authors.lastname: Georgescu
authors.forename: Sander
authors.initials: S
authors.lastname: van der Laan
mesh_terms: D000531:Altitude; D001272:Atmosphere / Q000032:analysis*; D002245:Carbon Dioxide / Q000032:analysis*; D019295:Computational Biology / Q000379:methods*; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods*; D009426:Netherlands; D016571:Neural Networks, Computer; D012044:Regression Analysis; D012621:Seasons; D012680:Sensitivity and Specificity
chemical_list: D002245:Carbon Dioxide