F acf bc536

Rso where Rso is the shortwave solar radiation for a clear-sky day (MJ m-2 day-1), ac ^ 1.35, and bc ^ -0.35 for average climate conditions, with ac + bc ^ 1.0.

Rso for daily periods can be estimated as

where0.75 = as+bs Eq. (5.36), z is the station elevation (m), and Ra is the extraterrestrial radiation (MJ m-2 day-1). This equation is valid for station elevations less than 6,000 m that have low air turbidity. Alternative equations for locations with high air turbidity and variable humidity are given by Allen [78].

The net emissivity of the surface, e', represents the difference between the emissivity by the vegetation and the soil and the effective emissivity of the atmosphere and is computed as e'= 0.34 - 0.14Vea, (5.38)

where ea is the actual vapor pressure (kPa). The coefficients ai = 0.34 and bi = -0.14 are recommended for average atmospheric conditions.

Net Radiation Rn. This is computed as the algebraic sum of the net shortwave radiation Rns and the net long-wave radiation Rnl:

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