Rs Kr Tmax T05 Ra544

where Rs is the daily shortwave incoming solar radiation (MJ m-2 day-1); Ra is the daily extraterrestrial radiation (MJ m-2 day-1), Tmex and Tmin are the maximum and the minimum air temperatures (°C), and Kr is an adjustment coefficient (°C-1/2).

The adjustment coefficient Kr is empirical and takes different values for interior or coastal regions:

• For interior locations, where land mass dominates (i.e., where air masses are not strongly influenced by a large water body), Kr = 0.16.

• For coastal locations, situated on or adjacent to the coast of a large land mass that is greater than about 20 km wide in the inland direction (i.e., where air masses are influenced by a nearby water body but depend on the wind direction), Kr = 0.19.

This temperature difference method is recommended for locations where it is not appropriate to import Rs/Ra information from a nearby station [Eq. (5.43)], because no homogeneous climate conditions occur or because data are lacking in the region.

Caution is recommended when daily computations are required. Daily estimates should be used when summed or averaged to a several-day period (7 or 10 days, as in irrigation scheduling computations).

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