Common to both approaches is the partitioning of net radiation between the plant canopy and the soil surface assuming the Beer’s law to be valid (Impens & Lemeur, 1969):
where Rn,tot is the net radiation above the plant canopy, Rns is the net radiation at the soil surface, krn is an extinction coefficient and Al is the leaf area index. The partitioning of net radiation between plant canopies and the soil is calculated slightly different if the multiple plant option is used, which is described in detail below in section Radiation processes.
The energy fluxes and resistances in the soil-plant-atmosphere system are illustrated below (see Figure 4.1). The net radiation above the plant canopy, Rn,tot, is partly intercepted by the canopy according to Beer’s law described above. The remaining radiation at the soil surface, Rns, is balanced against latent heat flux to the air, LvEs, sensible heat flux to the air, Hs, and the heat flux to the soil, qh. The soil evaporation, Es, is thus estimated from the latent heat flux, LvEs, (i.e. the energy used for evaporating water from the surface). Several resistances act on the fluxes of energy e.g. soil surface resistance, rss, canopy resistance, rs, aerodynamic resistance above the canopy, ra and the aerodynamic resistance from the soil to the reference height above the canopy, ras.