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Aerodynamic Resistance and Penman–Monteith Evapotranspiration over a Seasonally Two-Layered Canopy in Semiarid Central Australia

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  • 1 Plant Functional Biology and Climate Change Cluster, School of the Environment, and Australian Supersite Network and OzFlux, Terrestrial Ecosystem Research Network, University of Technology, Sydney, Broadway, New South Wales, Australia
  • | 2 Plant Functional Biology and Climate Change Cluster, School of the Environment, University of Technology, Sydney, Broadway, New South Wales, Australia
  • | 3 Plant Functional Biology and Climate Change Cluster, School of the Environment, and Australian Supersite Network and OzFlux, Terrestrial Ecosystem Research Network, and National Centre for Groundwater Research and Training, University of Technology, Sydney, Broadway, New South Wales, Australia
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Abstract

Accurate prediction of evapotranspiration E depends upon representative characterization of meteorological conditions in the boundary layer. Drag and bulk transfer coefficient schemes for estimating aerodynamic resistance to vapor transfer were compared over a semiarid natural woodland ecosystem in central Australia. Aerodynamic resistance was overestimated from the drag coefficient, resulting in limited E at intermediate values of vapor pressure deficit. Large vertical humidity gradients were present during the summer, causing divergence between momentum and vapor transport within and above the canopy surface. Because of intermittency in growth of the summer-active, rain-dependent understory and physiological responses of the canopy, leaf resistance varied from less than 50 s m−1 to greater than 106 s m−1, in which the particularly large values were obtained from inversion of drag coefficient resistance. Soil moisture limitations further contributed to divergence between actual and reference E. Unsurprisingly, inclusion of site-specific meteorological (e.g., vertical humidity gradients) and hydrological (e.g., soil moisture content) information improved the accuracy of predicting E when applying Penman–Monteith analysis. These results apply regardless of canopy layering (i.e., even when the understory was not present) wherever atmospheric humidity gradients develop and are thus not restricted to two-layer canopies in semiarid regions.

Corresponding author address: James Cleverly, Plant Functional Biology and Climate Change Cluster, School of the Environment, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007, Australia. E-mail: james.cleverly@uts.edu.au

Abstract

Accurate prediction of evapotranspiration E depends upon representative characterization of meteorological conditions in the boundary layer. Drag and bulk transfer coefficient schemes for estimating aerodynamic resistance to vapor transfer were compared over a semiarid natural woodland ecosystem in central Australia. Aerodynamic resistance was overestimated from the drag coefficient, resulting in limited E at intermediate values of vapor pressure deficit. Large vertical humidity gradients were present during the summer, causing divergence between momentum and vapor transport within and above the canopy surface. Because of intermittency in growth of the summer-active, rain-dependent understory and physiological responses of the canopy, leaf resistance varied from less than 50 s m−1 to greater than 106 s m−1, in which the particularly large values were obtained from inversion of drag coefficient resistance. Soil moisture limitations further contributed to divergence between actual and reference E. Unsurprisingly, inclusion of site-specific meteorological (e.g., vertical humidity gradients) and hydrological (e.g., soil moisture content) information improved the accuracy of predicting E when applying Penman–Monteith analysis. These results apply regardless of canopy layering (i.e., even when the understory was not present) wherever atmospheric humidity gradients develop and are thus not restricted to two-layer canopies in semiarid regions.

Corresponding author address: James Cleverly, Plant Functional Biology and Climate Change Cluster, School of the Environment, University of Technology, Sydney, PO Box 123, Broadway, NSW 2007, Australia. E-mail: james.cleverly@uts.edu.au
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