Sources and Transfer Processes in the Air Layers Occupied by Vegetation

Abstract

A one-dimensional model for transfer processes within the air layers occupied by vegetation is presented. This involves a height-dependent apparent diffusivity and vertical profiles of (i) mean concentration in the air and (ii) mean concentration at foliage surfaces. The model enables an examination of the partition and dissipation of the net radiant energy absorbed by vegetation. In the present study diffusive resistances to H2O transfer within the plant and the soil are neglected. However, numerical examples indicate that, even in this case, the downward transfer of sensible heat tends to make the distribution of the transpiration load within vegetation more even than that of absorbed net radiation. The source distribution for sensible heat is generally of quite different shape to that for latent heat, and the ?foliage Bowen ratio? systematically increases with height within the canopy. The observations of Denmead (1964) suggest that smoothing of the transpiration load is greater than the model calculations suggest. It seems likely that diffusive resistances within the vegetation play a significant role in the smoothing process. Source strength distributions for various entities within vegetation way differ markedly, so that attempts to infer information about diffusive resistances within vegetation from concentration profiles above the canopy alone are not soundly based.