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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 14, issue 12
Biogeosciences, 14, 3027-3050, 2017
https://doi.org/10.5194/bg-14-3027-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: OzFlux: a network for the study of ecosystem carbon and water...

Biogeosciences, 14, 3027-3050, 2017
https://doi.org/10.5194/bg-14-3027-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 22 Jun 2017

Research article | 22 Jun 2017

Interactions between nocturnal turbulent flux, storage and advection at an “ideal” eucalypt woodland site

Ian D. McHugh1, Jason Beringer2, Shaun C. Cunningham3,6,†, Patrick J. Baker4, Timothy R. Cavagnaro5, Ralph Mac Nally6, and Ross M. Thompson6 Ian D. McHugh et al.
  • 1School of Earth, Atmosphere and Environment, Monash University, Melbourne, 3800, Australia
  • 2School of Earth and Environment, University of Western Australia, Perth, 6907, Australia
  • 3School of Life and Environmental Sciences, Deakin University, Melbourne, 3125, Australia
  • 4Forest dynamics laboratory, University of Melbourne, Melbourne, 3052, Australia
  • 5Waite Research Institute and School of Agriculture, Food and Wine, University of Adelaide, Adelaide, 5005, Australia
  • 6Institute for Applied Ecology, University of Canberra, Canberra, 2617, Australia
  • deceased

Abstract. While the eddy covariance technique has become an important technique for estimating long-term ecosystem carbon balance, under certain conditions the measured turbulent flux of CO2 at a given height above an ecosystem does not represent the true surface flux. Profile systems have been deployed to measure periodic storage of CO2 below the measurement height, but have not been widely adopted. This is most likely due to the additional expense and complexity and possibly also the perception, given that net storage over intervals exceeding 24h is generally negligible, that these measurements are not particularly important. In this study, we used a 3-year record of net ecosystem exchange of CO2 and simultaneous measurements of CO2 storage to ascertain the relative contributions of turbulent CO2 flux, storage, and advection (calculated as a residual quantity) to the nocturnal CO2 balance and to quantify the effect of neglecting storage. The conditions at the site are in relative terms highly favourable for eddy covariance measurements, yet we found a substantial contribution (∼ 40%) of advection to nocturnal turbulent flux underestimation. The most likely mechanism for advection is cooling-induced drainage flows, the effects of which were observed in the storage measurements. The remaining ∼ 60% of flux underestimation was due to storage of CO2. We also showed that substantial underestimation of carbon uptake (approximately 80gCm−2a−1, or 25% of annual carbon uptake) arose when standard methods (u filtering) of nocturnal flux correction were implemented in the absence of storage estimates. These biases were reduced to approximately 40–45gCm−2a−1 when the filter was applied over the entire diel period, but they were nonetheless large relative to quantifiable uncertainties in the data. Neglect of storage also distorted the relationships between the CO2 exchange processes (respiration and photosynthesis) and their key controls (light and temperature respectively). We conclude that the addition of storage measurements to eddy covariance sites with all but the lowest measurement heights should be a high priority for the flux measurement community.

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We analysed a 3-year record of CO2 exchange at a eucalypt woodland and found that substantial nocturnal advective CO2 losses occurred, thus requiring correction. We demonstrated that the most common of these correction methods incurred substantial bias in long-term estimates of carbon balance if storage of CO2 below the measurement height was excluded. This is important because the majority of sites both in Australia and internationally lack such measurements.
We analysed a 3-year record of CO2 exchange at a eucalypt woodland and found that substantial...
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