Momentum, mass and heat exchange of vegetation

Abstract Vegetation is treated as a complex surface roughness to which the transfer of mass or heat encounters greater aerodynamic resistance, γ P, than the transfer of momentum, γ D. The excess resistance (γ P – γ D) is equated to B / u *, where B −1 is the non‐dimensional bulk parameter introduced by Owen and Thomson (1963) and used by Chamberlain (1966, 1968). A general expression is obtained for B −1 in terms of the exchange characteristics of the individual elements of a vegetative canopy: this expression does not contain the surface roughness parameter Z 0. Using exchange coefficients of individual bean leaves (Thom 1968) and the bulk momentum absorption properties of a particular bean crop (Thom 1971) the relation B −1 = (constant) u * 1/3 is derived. With u * in cm s −1, the constant is 1. 35 for heat exchange and transpiration, 2. 18 for CO 2 exchange, and 1. 13 for evaporation from the crop when wet. It is suggested, partly on the basis of the lack of dependence of B −1 on z 0, that the same set of equations may provide a first approximation to B −1 for many types of vegetation. Demonstrated are (i) that Monteith's (1963) method of extrapolating to zero wind speed to determine representative surface values of vapour pressure and of temperature (e s and T s) is much more rigorous if extrapolation is made to u = − B −1 u * rather than to u = 0; and (ii) that the surface resistance γ S, proportional to (e w (T s) − e s) (Monteith 1965) exceeds the bulk physiological, or stomatal, resistance γ ST of vegetation by an amount 1 − (Δ/γ). β. B −1 / u *, significant only when the Bowen ratio β is less than about 3/4 (γ/Δ). (γ = 0. 66 mb °C −1 ; Δ = de w /dT. ) In particular, for B −1 = 4 and β = 0: (i) γ ST = 1/3 to 1/2 of γ S ; and (ii) use of γ S with γ D in the Penman equation (instead of γ S, with which γ D is compatible) overestimates λ E by about 15 per cent.