get_Rs_daily: Get solar radiation (Rs)
In gopalpenny/fao56:

Description Usage Arguments Details Examples

Get solar radiation, Rs (FAO 56, Eq. 35) in MJ/m^2/day

1	get_Rs_daily(Ra, n, N, as = 0.25, bs = 0.5)

`Ra`	extraterrestrial radiation, MJ/m^2/day
`n`	actual duration of sunshine, hour
`N`	maximum possible duration of sunshine or daylight hours, hour
`as`	regression constant, expressing the fraction of extraterrestrial radiation reaching the earth on overcast days (n = 0)
`bs`	increase in solar radiation on clear days relative to overcast days, ie, as + bs is the fraction of extraterrestrial radiation reaching the earth on clear days (n = N).

This function calculates solar or shortwave radiation, in MJ/m^2/day. It uses Eq. 35 from FAO 56 in Chapter 3:

Rs = (as + bs * n / N) * Ra

Note that n/N is the relative sunshine duration (unitless). FAO56 suggests using as = 0.25 and bs = 0.5 if observed values are not available. But better to use observations if possible.

lat <- -22.9 # Rio de Janeiro
date <- "2019-05-15"
n <- 220 / 31 # 220 hours in a month / 31 days
N <- get_daylight_hours(lat, date)
Ra <- get_Ra_daily(lat, date)
Rs <- get_Rs_daily(Ra, n, N)