Expressions for the upwelling and downwelling fluxes of optical and thermal
radiation between soil, vegetation and the sky are derived, under certain
simple assumptions. These are that interception of radiation by the vegetat
ion is a purely geometric effect, while scattering is isotropic, with a str
ength given by a single-scattering albedo in the optical part of the spectr
um, and by Kirchoff's Law in the thermal. The soil is assumed to be a lambe
rtian reflector, also scattering according to an albedo and Kirchoff's Law.
The model, called RM, conserves energy exactly. As part of a SVAT, it is d
riven by measured insolation instead of net radiation, with little increase
in computational cost and number of parameters.