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https://doi.org/10.5194/bg-2020-154
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-154
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 May 2020

13 May 2020

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This preprint is currently under review for the journal BG.

Calculating Canopy Stomatal Conductance from Eddy Covariance Measurements, in Light of the Energy Budget Closure Problem

Richard Wehr and Scott R. Saleska Richard Wehr and Scott R. Saleska
  • Ecology and Evolutionary Biology, University of Arizona, Tucson, 85721, USA

Abstract. Canopy stomatal conductance (gsV) is commonly estimated from eddy covariance (EC) measurements of latent heat flux (LE) by inverting the Penman-Monteith (PM) equation. That method implicitly represents the sensible heat flux (H) as the residual of all other terms in the site energy budget – even though H is measured at least as accurately as LE at every EC site while the rest of the energy budget almost never is. We argue that gsV should instead be calculated from EC measurements of both H and LE, using the flux-gradient formulation that defines conductance and underlies the PM equation. The flux-gradient formulation dispenses with unnecessary assumptions, is conceptually simpler, and provides more accurate values of gsV for all plausible scenarios in which the measured energy budget fails to close, as is common at EC sites. The PM equation, on the other hand, contributes biases and erroneous spatial and temporal patterns to gsV, skewing its relationships with drivers such as light and vapor pressure deficit. To minimize the impact of the energy budget closure problem on the PM equation, it was previously proposed that the eddy fluxes should be corrected to close the long-term energy budget while preserving the Bowen ratio (B = H/LE). We show that such a flux correction does not fully remedy the PM equation but should produce accurate values of gsV when combined with the flux-gradient formulation.

Richard Wehr and Scott R. Saleska

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AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

Richard Wehr and Scott R. Saleska

Model code and software

Software code for simulations and analyses concerning the calculation of canopy stomatal conductance at eddy covariance sites R. Wehr and S. R. Saleska https://doi.org/10.5061/dryad.h44j0zpgp

Richard Wehr and Scott R. Saleska

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Short summary
Water and carbon exchange between plants and the atmosphere is governed by stomata: adjustable pores in the surfaces of leaves. The combined gas conductance of all the stomata in a canopy has long been estimated using an equation that is shown here to be systematically incorrect because it relies on measurements that are generally inadequate. An alternative approach is shown to be more accurate in all probable scenarios and to imply different responses of stomatal conductance to the environment.
Water and carbon exchange between plants and the atmosphere is governed by stomata: adjustable...
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