Global Radiation Calculation as applied in WOFOST/CGMS1
When the sunshine duration is known, the global radiation is calculated using the Ångström-Prescott formula. Constants depend on the geographic location.
where Rg : Incoming daily global solar radiation [MJ m-2 d-1]

Ra : Daily extra-terrestrial radiation  [MJ m-2 d-1]

Aa : Empirical constant  [-]

Ba : Empirical constant  [-]

n : Bright sunshine hours per day [hr]

<Ld : Astronomical day length  [hr]
Constants:
Spatial variation:

<When sunshine duration is not available but minimum and maximum temperature and cloud cover are known, the Extended Hargreaves (a.k.a. Supit ) formula is applied. Constants depend on geographic location:

where <Rg Incoming daily global solar radiation [MJ m-2 d-1]

Ra
Daily extra-terrestrial radiation  [MJ m-2 d-1]

CC
Mean total cloud cover during daytime [octa]

Tmax
Maximum temperature [°C]

Tmin : Minimum temperature [°C]

As : Empirical constants [°C-0.5]

Bs : Empirical constants [-]

Cs : Empirical constants [MJ m-2 d-1]
Constants:
Spatial variation:

When only minimum and maximum temperatures are known, the Hargreaves equation is applied. Again, the constants depend on geographic location:

where Rg Incoming daily global solar radiation [MJ m-2 d-1]

Ra
Daily extra-terrestrial radiation [MJ m-2 d-1]

Tmax
Maximum temperature [°C]

Tmin : Minimum temperature [°C]

Ah : Empirical constants [°C-0.5]

Bh : Empirical constants [MJ m-2 d-1]
Constants:
Spatial variation:

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1CGMS is the Crop Growth Monitoring System as applied by the European Commission

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