If the solar radiation, Rs, is not measured, it can be calculated with the Angstrom formula, which relates solar radiation to extraterrestrial radiation and relative sunshine duration. This is a shortwave radiation.
cal_solarRadiation(as = 0.25, bs = 0.5, n, N, Ra)A vector for solar or shortwave radiation [MJ m-2 day-1]
regression constant, expressing the fraction of extraterrestrial radiation reaching the earth on overcast days (n = 0).Default is 0.25.
as+bs is fraction of extraterrestrial radiation reaching the earth on clear days (n = N). Default is 0.50.
actual duration of sunshine [hour].
maximum possible duration of sunshine or daylight hours [hour].from cal_daylightHours()
extraterrestrial radiation [MJ m-2 day-1]. From cal_extraterrestrialRadiation_for_daily()
Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. FAO Irrigation and drainage paper No. 56. Rome: Food and Agriculture Organization of the United Nations, 1998.