Momentum, mass and heat exchange of vegetation

Vegetation is treated as a complex surface roughness to which the transfer of mass or heat encounters greater aerodynamic resistance, γP, than the transfer of momentum, γD. The excess resistance (γP – γD) 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 Z0. 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 CO2 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 z0, 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 (es and Ts) is much more rigorous if extrapolation is made to u = −B−1u* rather than to u = 0; and (ii) that the surface resistance γS, proportional to (ew(Ts) − es) (Monteith 1965) exceeds the bulk physiological, or stomatal, resistance γST 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 = dew/dT.) In particular, for B−1 = 4 and β = 0: (i) γST = 1/3 to 1/2 of γS; and (ii) use of γS with γD in the Penman equation (instead of γS, with which γD is compatible) overestimates λE by about 15 per cent.