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The net radiation flux at the surface (R n ) represents the actual radiant energy available atthe surface. It is computed by subtracting all outgoing radiant fluxes from all incomingradiant fluxes (Figure 2). This is given in the surface radiation balance equation:R n = R S↓ - α R S↓ + R L↓ - R L↑ - (1-ε o )R L↓ (2)where; R S↓ is the incoming shortwave radiation (W/m 2 ), α is the surface albedo(dimensionless), R L↓ is the incoming longwave radiation (W/m 2 ), R L↑ is the outgoinglongwave radiation (W/m 2 ), and ε o is the surface thermal emissivity (dimensionless).shortwave radiationR S↓(incidentshortwave)α R S↓(reflectedshortwave)R L↓(incidentlongwave)longwave radiation(1-ε o )R L↓(reflectedlongwave)R L↑(emittedlongwave)vegetation surfaceNet surface radiation = gains – lossesR n = (1 - α) R S↓ + R L↓ - R L↑ - (1-ε o )R L↓Figure 2. Surface Radiation BalanceIn Equation (2), the amount of shortwave radiation (R S↓ ) that remains available at thesurface is a function of the surface albedo (α). Surface albedo is a reflection coefficientdefined as the ratio of the reflected radiant flux to the incident radiant flux over the solarspectrum. It is calculated using satellite image information on spectral radiance for eachsatellite band. The incoming shortwave radiation (R S↓ ) is computed using the solarconstant, the solar incidence angle, a relative earth-sun distance, and a computedatmospheric transmissivity. The incoming longwave radiation (R L↓ ) is computed using amodified Stefan-Boltzmann equation with atmospheric transmissivity and a selectedsurface reference temperature. Outgoing longwave radiation (R L↑ ) is computed using theStefan-Boltzmann equation with a calculated surface emissivity and surface temperature.Surface temperatures are computed from satellite image information on thermal radiance.10