cal_solarRadiation {simET}

Calculating Solar radiation

Description

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.

Usage

cal_solarRadiation(as = 0.25, bs = 0.5, n, N, Ra)

Arguments

as

regression constant, expressing the fraction of extraterrestrial radiation reaching the earth on overcast days (n = 0).Default is 0.25.

bs

as+bs is fraction of extraterrestrial radiation reaching the earth on clear days (n = N). Default is 0.50.

n

actual duration of sunshine [hour].

N

maximum possible duration of sunshine or daylight hours [hour].from cal_daylightHours()

Ra

extraterrestrial radiation [MJ m-2 day-1]. From cal_extraterrestrialRadiation_for_daily()

Value

A vector for solar or shortwave radiation [MJ m-2 day-1]

Note

Rs is expressed in the above equation in MJ m-2 day-1. The corresponding equivalent evaporation in mm day-1 is obtained by multiplying Rs by 0.408 (Equation 20). Depending on atmospheric conditions (humidity, dust) and solar declination (latitude and month), the Angstrom values as and bs will vary. Where no actual solar radiation data are available and no calibration has been carried out for improved as and bs parameters, the values as = 0.25 and bs = 0.50 are recommended.

References

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.