The terms on the right side of this equation can be computed from measured or estimated climatic and vegetation factors. The climatic factors include short wave and long wave radiant fluxes from and into the atmosphere (Rn), effects of horizontal air movement (wind speed) and air and surface temperatures on H, and soil heat fluxes (G). Vegetation factors include the resistance to diffusion of vapour from within plant leaves and stems and the resistance to diffusion of vapour from near the vegetation or soil surface upward into the atmosphere.

For general prediction purposes, the complex turbulent structures within and above vegetation canopies and the effects of partitioning of net radiation and energy within the canopies can be described in terms of simple exchange coefficients or their inverses, resistances. Generally this is accomplished using the linear "big leaf" model of Monteith [4, 5] and Rijtema [6] where two resistances, canopy and aerodynamic, operate in series between leaf interiors and some reference height above the vegetation (Fig. 5.1). Canopy,

or bulk surface resistance (rs) can be computed from the resistance of vapour flow through individual stomata openings (rj) and total leaf area of the vegetation.

The aerodynamic resistance (ra) describes the resistance to the random, turbulent transfer of vapour from the vegetation upward to the reference (weather measurement) height and the corresponding vertical transfer of sensible heat away from or toward the vegetation.

The energy balance equation can be arranged in terms of parameters Rn and G and parameters within the H and XETcomponents. If one assumes that eddy-diffusion transfer factors for XET and H are the same and that differences between transfer factors for momentum and those for heat can be quantified through a simple ratio, then the Penman-Monteith (PM) form of the combination equation [4] results:

where (es — ea) represents the vapor pressure deficit (VPD) of air at the reference (weather measurement) height (kPa), p represents mean air density (kg m—3), cp represents specific heat of air at a constant pressure (MJ kg-1 °C—*), A represents the slope of the saturation vapor pressure-temperature relationship at mean air temperature (kPa °C—*), y is the psychometric constant (kPa °C—*), rs is the bulk surface resistance (s m—*), and ra is the aerodynamic resistance (s m—*).

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