Gap filling strategies for defensible annual sums of net ecosystem exchange

Authors

• Eva Falge, SPM, University of California, Berkeley, Berkeley, CA
• Dennis D. Baldocchi, University of California, BerkeleyFollow
• Richard Olson, Environmental Science Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
• Peter Anthoni, Richardson Hall, Oregon State University, Corvallis, OR
• Marc Aubinet, Unité de Physique, Faculté des Sciences Agronomiques de Gembloux, B-50 30 Gembloux, Belgium
• Christian Bernhofer, Technische Universität Dresden, IHM Meteorologie, Pienner Str. 9, 01737 Tharandt, Germany
• George Burba, University of Nebraska-LincolnFollow
• Reinhart Ceulemans, University of Antwerpen, Universiteitsplein 1, B-2610, Wilrijk, Antwerp, Belgium
• Robert Clement, University of Edinburgh, Edinburgh EH9 3JU, UK
• Han Dolman, Alterra, Postbus 47, 6700 AA Wageningen, The Netherlands
• Andre Grainer, INRA, Unité d’Ecophysiologie Forestière, F-54280 Champenoux, France
• Thomas Grunwald, Technische Universität Dresden, IHM Meteorologie, Pienner Str. 9, 01737 Tharandt, Germany
• David Hollinger, USDA Forest Service, 271 Mast Rd, Durham, NH 03824, USA
• Niels-Otto Jensen, Plant Biology and Biogeochemistry Department, Risoe National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark
• Gabriel Katul, Duke University, Durham, NC 27708-0328, USA
• Petri Keronen, University of Helsinki, P.O. Box 9, FIN-00014 Helsinki, Finland
• Andrew Kowalski, University of Antwerpen, Universiteitsplein 1, B-2610, Wilrijk, Antwerp, Belgium
• Chun Ta Lai, Duke University, Durham, NC 27708-0328, USA
• Beverly E. Law, Oregon State UniversityFollow
• Tilden Meyers, NOAA/ATDD
• Jon Moncrieff, Institute of Ecology and Resource Management, University of Edinburgh, Edinburgh EH9 3JU, UK
• Eddy Moors, Alterra, Postbus 47, 6700 AA Wageningen, The Netherlands
• J. William Munger, Harvard University
• Kim Pilegaard, SPM, University of California, Berkeley
• Ullar Rannik, University of Helsinki, P.O. Box 9, FIN-00014 Helsinki, Finland
• Corinna Rebmann, Max-Planck-Institut für Biogeochemie, Tatzendpromenade 1a, 07701 Jena, Germany
• Andrew E. Suyker, University of Nebraska - LincolnFollow
• John Tenhunen, Pflanzenökologie, Universität Bayreuth, 95440 Bayreuth, Germany
• Kevin Tu, University of New Hampshire, Durham, NH 03824, USA
• Shashi Verma, University of Nebraska - LincolnFollow
• Timo Vesala, University of Helsinki, P.O. Box 9, FIN-00014 Helsinki, Finland
• Kell Wilson, NOAA/ATDD, 456 S. Illinois Avenue, Oak Ridge, TN 37831-2456, USA
• Steve Wofsy, Harvard University

Document Type

Article

Date of this Version

2-8-2001

Comments

Published in Agricultural and Forest Meteorology 107 (2001) 43–69.

Abstract

Heightened awareness of global change issues within both science and political communities has increased interest in using the global network of eddy covariance flux towers to more fully understand the impacts of natural and anthropogenic phenomena on the global carbon balance. Comparisons of net ecosystem exchange (F_NEE) responses are being made among biome types, phenology patterns, and stress conditions. The comparisons are usually performed on annual sums of F_NEE; however, the average data coverage during a year is only 65%. Therefore, robust and consistent gap filling methods are required.

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 F_NEE is investigated. The difference between annual F_NEE filled by MDV compared to F_NEE filled by Regr. ranged from −45 to +200gCm⁻² per year (MDV−Regr.). Comparing LookUp and Regr. methods resulted in a difference (LookUp−Regr.) ranging from −30 to +150gCm⁻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 +77gCm⁻² per year, but in certain cases as much as +185gCm⁻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.