Solar Radiation under Cloudless Skies

When solar radiation enters the earth's atmosphere, a portion of the energy is removed by scattering and adsorption. The amount of energy lost is a function of the transmittance of the atmosphere, the composition and concentration of the constituents of air at the location, the path length the radiation travels through the air column, and the radiation wavelength.

Due to the complexity of the process and the detail of the information required to accurately predict the amount of radiant energy lost while passing through the atmosphere, SWAT+ makes a broad assumption that roughly 20% of the extraterrestrial radiation is lost while passing through the atmosphere under cloudless skies. Using this assumption, the maximum possible solar radiation, $H_{MX}$, at a particular location on the earth's surface is calculated as:

$H_{MX} = 30.0E_0[{\omega*T_{SR} }\sin\delta \sin\phi+\cos\delta\cos\phi\sin(\omega*T_{SR})]$ 1:1.2.7

where the maximum possible solar radiation, $H_{MX}$, is the amount of radiation reaching the earth's surface under a clear sky ($MJ m^{-2} d^{-1}$).