Tham khảo tài liệu 'evapotranspiration remote sensing and modeling part 14', khoa học tự nhiên, công nghệ sinh học phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Modelling Evapotranspiration and the Surface Energy Budget in Alpine Catchments 379 Fig. 1. Scheme of the solar radiation components. where w is the solid angle seen from the point considered. The presence of shadows due to surrounding mountains can be expressed through a factor sw a function of topography and sun position defined as 1 if the point is in the sun sw X 0 if the point is in the shadow 5 All direct radiation terms have to be multiplied by this factor. In the next paragraphs we analyze in detail the parametrization of the single terms composing the radiation flux. Direct radiation R ịsw p and R ịiw p The direct long-wave radiation R ị w p is emitted directly by the sun and therefore it is negligible at the soil level differently from the long-wave diffuse radiation . Usually the short-wave radiation R .ịsw p is assumed as an input variable measured or calculated by an atmospheric model. The direct radiation can be written as the product of the extraterrestrial radiation Rpxir by an attenuation factor varying in time and space. R ịsw p FattRExtr 6 The extraterrestrial radiation can be easily calculated on the basis of geometric formulas Iqbal 1983 . The atmospheric attenuation is due to Rayleigh diffusion to the absorption on behalf of ozone and water vapor to the extinction both diffusion and absorption due to atmospheric dust and shielding caused by the possible cloud cover. Moreover the absorption entity depends on the ray path length through the atmosphere a function of the incidence angle and of the measurement point elevation. The effect of the latter can be very important in a mountain environment where it is necessary to consider the shading effects. Part of the dispersed radiation is then returned as short-wave diffuse radiation R ịsw D and part of the energy absorbed by atmosphere is then re-emitted as long-wave diffuse radiation R ịiw D . From a practical point of view according to the application type and depending on the measured data
