Correlation-based flux partitioning of water vapor and carbon dioxide fluxes: Method simplification and estimation of canopy water use efficiency

Authors: SCANLON, TODD - University Of Virginia; SCHMIDT, DANIEL - University Of Virginia; Skaggs, Todd

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/23/2019
Publication Date: 8/30/2019
Citation: Scanlon, T.M., Schmidt, D.F., Skaggs, T.H. 2019. Correlation-based flux partitioning of water vapor and carbon dioxide fluxes: Method simplification and estimation of canopy water use efficiency. Agricultural and Forest Meteorology. 279. https://doi.org/10.1016/j.agrformet.2019.107732.
DOI: https://doi.org/10.1016/j.agrformet.2019.107732

Interpretive Summary: Over recent decades, knowledge of the Earth’s terrestrial carbon and water cycles has been significantly advanced by information generated by a worldwide network of eddy covariance systems, which measure the net ecosystem exchange of carbon dioxide and total evapotranspiration flux over a wide variety of ecosystems. As this information has been collected, the need has persisted for these measured fluxes to be partitioned into their constitutive components, as these components represent distinct biophysical process, driven by distinct environmental forcings. The current work describes a method to partition both carbon dioxide and water vapor fluxes based on the statistics of measured high-frequency time series. By leveraging recent theoretical advances and insights, we present a simplified approach that can facilitate the more widespread use of this method while providing additional information about the trade-offs between carbon uptake and water loss at the canopy scale. This research will be of interest to scientists and others concerned with improving the quantification of the terrestrial carbon and water budgets.

Technical Abstract: The partitioning of water vapor and carbon dioxide (CO2) exchange between vegetation and the atmosphere remains a current research priority. A technique that has been proposed to simultaneously partition these fluxes, based on the correlation between their high-frequency concentration time series, has been the subject of recent empirical evaluations and theoretical advances. The method assumes that flux-variance similarity can be applied separately to stomatal exchange (transpiration for water vapor and net photosynthesis for CO2) and non-stomatal exchange (direct evaporation for water vapor and soil and stem respiration for CO2). Here, we present a mathematical simplification of this approach, from which the partitioned fluxes can be derived from routine eddy covariance measurements. The simplification arises from the fact that the transpiration and net photosynthesis fluxes are linearly related in solution space with respect to variable canopy water use efficiency, W. Conditions that are amenable to successful partitioning can now be determined a priori for a given averaging period. The simplified framework also has the benefit of providing a means for estimating W based on optimization theory. This allow for the estimation of W without any preconceptions of how the intercellular CO2 concentration, ci, varies as a function of ambient conditions. The simplified partitioning framework is applied to eddy covariance measurements collected over a mixed deciduous forests for three growing season. Aside from being more computationally efficient, the partitioned results exhibit less scatter compared with prior implementations.