Mass, heat and momentum exchange between stands of plants and their atmospheric environment

A model of mass and momentum transfer in the air layer occupied by a stand of plants is presented which takes account of the differing mechanisms of mass exchange and momentum dissipation near to the leaf surfaces. An analysis of momentum transfer leads to expressions for the vertical transport coefficient and windspeed in terms of the mean drag coefficient of the leaves, the drag coefficient of the stand as a whole and the area of leaf per unit volume of space. These expressions are used in the solution of the equations describing mass transfer with constant concentration of mass at the leaf surface. The effect of a chemical or physical resistance to transfer at the leaf surface is examined. The resistance to mass exchange between the stand and the atmosphere above is compared with that for momentum exchange. The transfer of water vapour in the stand is considered, the boundary conditions at the leaf surfaces being linked with those for sensible heat transfer. It is shown that, when leaf resistance to transpiration is negligible, the vapour and heat-fluxes are readily obtained from simple mass transfer theory and a knowledge of the distribution of radiation in the stand. The formal relations are extended to allow for finite leaf resistance. Computed profiles of the fluxes, temperature and humidity in a plant stand of assumed characteristics with defined boundary conditions are presented and compared with published observations. The interrelationship between leaf resistance, transpiration and leaf temperature is discussed. An expression for the total evaporation from a stand is derived and contrasted with a relation based on the analysis of Penman and Schofield (1951) and Monteith (1965).

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