Monthly reference evapotranspiration by Hargreaves (FAO-56)

FAO-56 equation 52, ET0 = 0.0023 (Tmean + 17.8) (Tmax - Tmin)^0.5 Ra, with Ra the extraterrestrial radiation (equation 21, converted to mm/day by 0.408) on the month's representative day J = floor(30.4 M - 15), times the days in the month from wet_month_days(). Allen et al. (1998), Crop Evapotranspiration, FAO Irrigation and Drainage Paper 56, chapter 3.

Usage

wet_pet_hargreaves(tmax, tmin, lat, month)

Arguments

tmax, tmin: Monthly mean daily maximum and minimum temperature (C): numeric vectors or SpatRasters.
lat: Latitude (degrees, south negative): numeric, or a SpatRaster such as terra::init(tmax, "y") on a geographic grid.
month: Month, 1 to 12 (one value).

Value

Reference evapotranspiration over the month (mm), numeric or a SpatRaster like the inputs.

Details

Hargreaves needs only temperature, so on climr's monthly Tmax/Tmin normals it gives a demand on the same climate as the precipitation. A negative diurnal range (independently downscaled Tmin above Tmax) and Tmean below -17.8 C both give 0, never NaN or a negative demand.

Examples

# a July at 50 N with a 20 C diurnal range: about 170 mm
wet_pet_hargreaves(tmax = 25, tmin = 5, lat = 50, month = 7)
#> [1] 171.0623
# the annual cycle at one place
round(vapply(1:12, function(m) wet_pet_hargreaves(c(-3, 0, 5, 11, 16, 20, 24, 24, 18, 10, 2, -3)[m],
  c(-10, -9, -5, -1, 3, 7, 9, 9, 5, 1, -4, -9)[m], 50.7, m), 0))
#>  [1]   7  15  37  70 111 133 155 131  75  33  11   6