Energy Balance Closure at FLUXNET Sites

Authors

Kell Wilson, Atmospheric Turbulence and Diffusion Division, NOAA, P.O. Box 2456, Oak Ridge, TN
Allen Goldstein, ESPM, University of California at Berkeley, Berkeley, CA 94720, USA
Eva Falge, Pflanzenökologie, University Bayreuth, 95440 Bayreuth, Germany
Marc Aubinet, Unité de Physique, Faculté des Sciences Agronomiques de Gembloux, B-50 30 Gembloux, Belgium
Dennis D. Baldocchi, University of California - BerkeleyFollow
Paul Berbigier, Unité de Bioclimatologie, INRA Bourdeaux, Gazinet, France
Christian Bernhofer, Insitute für Hydrologie und Meteorologie, 01737 Tharandt, Germany
Reinhart Ceulemans, Free University, Amsterdam, The Netherlands
Han Dolman, INRA Ecophysiology, Pierroton, France
Chris Field, Department of Plant Biology, Carnegie Institution of Washington, Stanford, CA 94305, USA
Achim Grelle, Department of Ecology and Environmental Research, Swedish University of Agricultural Sciences, S-750 07 Uppsala, Sweden
Andreas Ibrom, Institut für Bioklimatologie, Georg-August University, Göttinen, Germany
B. E. Law, Oregon State UniversityFollow
Andy Kowalski, Free University, Amsterdam, The Netherlands
Tilden Meyers, Atmospheric Turbulence and Diffusion Division, NOAA, P.O. Box 2456, Oak Ridge, TN 37831, USA
John Moncrieff, Department of Ecology and Environmental Research, University of Edinburgh, Edinburgh, UK
Russ Monson, DEPOB, University of Colorado, Boulder, CO 80309, USA
Walter Oechel, Global Change Research Group, San Diego State University, San Diego, CA 92182, USA
John Tenhunen, Pflanzenökologie, University Bayreuth, 95440 Bayreuth, Germany
Shashi Verma, University of Nebraska - LincolnFollow
Riccardo Valentini, Department of Forest Science and Resources, University of Tuscia, 1-01100 Viterbo, Italy

Document Type

Article

Date of this Version

2002

Comments

Published in Agricultural and Forest Meteorology 113 (2002) 223–243. Copyright (c) 2002. Used by permission.

Abstract

A comprehensive evaluation of energy balance closure is performed across 22 sites and 50 site-years in FLUXNET, a network of eddy covariance sites measuring long-term carbon and energy fluxes in contrasting ecosystems and climates. Energy balance closure was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat (LE)) against available energy (net radiation, less the energy stored) and by solving for the energy balance ratio, the ratio of turbulent energy fluxes to available energy. These methods indicate a general lack of closure at most sites, with amean imbalance in the order of 20%. The imbalance was prevalent in all measured vegetation types and in climates ranging from Mediterranean to temperate and arctic. There were no clear differences between sites using open and closed path infrared gas analyzers. At a majority of sites closure improved with turbulent intensity (friction velocity), but lack of total closure was still prevalent under most conditions. The imbalance was greatest during nocturnal periods. The results suggest that estimates of the scalar turbulent fluxes of sensible and LE are underestimated and/or that available energy is overestimated. The implications on interpreting long-term CO₂ fluxes at FLUXNET sites depends on whether the imbalance results primarily from general errors associated with the eddy covariance technique or from errors in calculating the available energy terms. Although it was not entirely possible to critically evaluate all the possible sources of the imbalance, circumstantial evidence suggested a link between the imbalance and CO₂ fluxes. For a given value of photosynthetically active radiation, the magnitude of CO₂ uptake was less when the energy imbalance was greater. Similarly, respiration (estimated by nocturnal CO₂ release to the atmosphere) was significantly less when the energy imbalance was greater.