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The following summarizes the discussion by Dubayah and Rich on the topographic effects that modelers should examine. The three types of irradiance being received at the Earth's surface are direct irradiance, diffuse sky irradiance and reflected irradiance due to direct and diffuse irradiance.
Direct irradiance is function of several factors which may vary by date, atmospheric conditions which vary with time, space and elevation. The solar zenith angle - 90 degrees minus the solar elevation angle and exoatmospheric flux - the solar flux at the top of the atmosphere - vary in a predictable manner by date. The optical depth varies according to the presence and concentration of atmospheric absorbers and scatterers. These gases and particulates vary greatly in both time and space. Elevation also influences optical depth because the concentrations ob absorbers and scatterers decrease with decreased pressure of higher elevations. Shading is results from the directness of the sunlight on the slope. Shadowing occurs when topography obstructs the sunlight. That is, the solar zenith angle is not greater than the horizon angle in the direction of the Sun.
Diffuse irradiance is a function of the factors mentioned above as well as the amount of the sky that is visible at a given point and the anisotropy in the diffuse irradiance field. Generally diffuse irradiance is anisotropic not isotropic, is not the same in all directions. Anisotropy can be explicitly modeled but it is dificult. The portion of the sky that is blocked by topography will reduce the diffuse irradiance due to self shading on slopes and by shadowing from adjacent terrain.
Reflected irradiance from surrounding terrain is calculated by using a average reflected radiation term and then by adjusting this by a terrain configuation factor. This factor takes into account both the anisotrophy of the diffuse radiation field and the geometric affects between locations.
See the map of US Daily Direct Solar Radiation 1961-1990 and the map of US Daily Total Solar Radiation 1961-1990, These maps are from measured and not modeled data.
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