Computational representations of canopy flow

The first attempts to model flow and transport in plant canopies that accommodated (i) the distinct microclimates of different stands of vegetation (ii) the separation of soil surface and layers of canopy as distinct sources and sinks of heat and mass and (iii) the influence of atmospheric stability or advection effects, applied gradient transfer to diffusion within the canopy space ([493]). In this procedure, a flux density is expressed as the product of a diffusion coefficient (turbuient or eddy diffusivity) and the gradient of the time average of the quantity of interest, as in the following examples  

The shortcoming of such descriptions of exchange is that diffusion is not a local phenomenon, as implied by these formulations, and eddy sizes most important to exchange within the canopy can be many times larger than scales associated with the distribution of sources and sinks of heat, water vapour, etc. As an obvious example, gradient diffusion would not permit a secondary maximum in the wind profile in an extensive canopy, because such a profile would require a counter-gradient flux of momentum. 

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