We review several methods of gap filling and apply them to data sets available from the EUROFLUX and AmeriFlux databases. The methods are based on mean diurnal variation (MDV), look-up tables LookUp, and nonlinear regressions (Regr.), and the impact of different gap filling methods on the annual sum of FNEE is investigated. The difference between annual F-NEE filled by MDV compared to F-NEE filled by Regr. ranged from -45 to +200 g C m(-2) per year (MDV-Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp-Regr.) ranging from -30 to +150g Cm-2 per year.
We also investigated the impact of replacing measurements at night, when turbulent mixing is insufficient. The nighttime correction for low friction velocities (u(*)) shifted annual F-NEE on average by +77 g C m(-2) per year, but in certain cases as much as +185 g C m-2 per year.
Our results emphasize the need to standardize gap filling-methods for improving the comparability of flux data products from regional and global flux networks. (C) 2001 Elsevier Science B.V. All rights reserved.