Rs [as bs n N Ra532

where Rs is the incoming solar radiation (MJ m-2 day-1), as is the fraction of extraterrestrial radiation Ra on overcast days, bs is the proportionality factor, n is the bright sunshine hours per day (h), and N is the total length of the day (h), as = 0.25 and bs = 0.50 for average climate conditions.

The daylight hours N are calculated from the sunset hour angle as

Net Shortwave Radiation Rns. Representing the balance between incoming and reflected solar radiation, Rns (MJ m-2 day-1) is given by

where a is the albedo or canopy reflection coefficient (); a = 0.23 for the grass reference crop.

Net Long-wave Radiation (Rn). Defined as the balance between the downcoming longwave radiation from the atmosphere (Rld[) and the outgoing long-wave radiation emitted by the vegetation and the soil (Rlut), Rnl (MJ m-2 day-1) is computed as

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