VEGETATION ATMOSPHERE INTERACTION AND SURFACE CONDUCTANCE AT LEAF, CANOPY AND REGIONAL SCALES

The problem of areally averaging descriptions of land-atmosphere energy and mass exchange is common to both the leaf-canopy and canopy-region scale translations. This paper attempts a unified treatment. The starting point is a review of both the basic and equilibrium-departure forms of the Penman-Monteith or combination equation (CE) for the latent and sensible heat fluxes at an evaporating surface; the equilibrium-departure form expresses the evaporation as a linear combination of two other scalar fluxes, those of available energy and saturation deficit. Next, tools for scale translation are established, including (1) the basic conservation requirement that net scalar fluxes average linearly over the land surface and (2) the matching of model form between scales. These tools are applied to the CE for latent and sensible heat fluxes F-E and F-H leading to a 'flux matching' averaging scheme based on term-by-term matching of the linearly additive scalar flux terms in the CE for F-E Several variations of this scheme are discussed. For F-E, all variations satisfy the scalar conservation requirement that fluxes average linearly, but for F-H, none satisfies this requirement exactly, except when longwave radiative coupling is ignored. By considering a range of canopy-region scale translations, errors in the averaging of F-H are shown to be small in practice. Finally, three averaging schemes (the flux-matching scheme and two schemes based on parallel sums of elemental aerodynamic and surface conductances, respectively) are compared for leaf-canopy scale translations, by constructing a sequence of analytic model canopies at increasing levels of detail. For every canopy model considered, all three averaging schemes give nearly identical definitions of the bulk canopy conductance for typical dry canopies. However, schemes other than the flux matching scheme can become inconsistent over partially wet surfaces, yielding undefined or negative bulk conductances.