Aerodynamic resistance and Penman-Monteith evapotranspiration over a seasonally two-layered canopy in semi-arid central Australia

American Meteorological Society
Publication Type:
Journal Article
Journal of Hydrometeorology, 2013, 14 (1), pp. 1562 - 1570
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Accurate prediction of evapotranspiration E depends upon representative characterization of meteoro- logical conditions in the boundary layer. Drag and bulk transfer coefficient schemes for estimating aero- dynamic 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 re- sponses of the canopy, leaf resistance varied from less than 50 sm21 to greater than 106 sm21, 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 PenmanMonteith analysis. These results apply re- gardless 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.
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