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Reference Evapotranspiration (FAO-56 Penman-Monteith)

Source: R/drought.R
ck_pet_pm.Rd

Compute reference evapotranspiration ETo using the FAO-56 Penman-Monteith equation (Allen et al. 1998), the international standard for ETo estimation. Required inputs are daily Tmin and Tmax; optional inputs (humidity, wind speed, incoming solar radiation, elevation) increase accuracy. Where humidity, wind, or solar radiation are missing, FAO-56 fallback estimators are used.

Usage

ck_pet_pm(
  tmin,
  tmax,
  lat,
  dates,
  elev = 0,
  wind = 2,
  rh_min = NULL,
  rh_max = NULL,
  rs = NULL,
  albedo = 0.23,
  krs = 0.16
)

Arguments

tmin

Numeric vector of daily minimum temperatures (degrees C).

tmax

Numeric vector of daily maximum temperatures (degrees C), same length as tmin.

lat

Numeric. Latitude in decimal degrees.

dates

Date vector of the same length as tmin.

elev

Numeric. Elevation above sea level in metres (default 0).

wind

Numeric vector or single value. 2-m wind speed (m/s). Default 2 (FAO-56 fallback for unmeasured wind).

rh_min, rh_max

Optional numeric vectors of daily minimum and maximum relative humidity in percent. Both must be supplied together; otherwise vapour pressure falls back to e0(tmin).

rs

Optional numeric vector of daily incoming solar radiation (MJ m^-2 day^-1). If NULL, Hargreaves-Samani estimate is used.

albedo

Numeric. Surface albedo (default 0.23).

krs

Numeric. Hargreaves-Samani coefficient for the Rs fallback (default 0.16 for inland sites; 0.19 for coastal).

Value

A data frame with columns date, value, index ("pet_pm"), and unit.

Details

Inputs and units:

  • tmin, tmax: daily minimum and maximum temperature (degrees C).

  • lat: latitude in decimal degrees, used for the extraterrestrial-radiation calculation.

  • elev: elevation above sea level in metres (default 0).

  • wind: 2-metre wind speed (m/s). Default 2 m/s (the FAO-56 fallback value when wind data are unavailable).

  • rh_min, rh_max: minimum and maximum daily relative humidity (\ pressure is estimated as e0(tmin) (FAO-56 Eq. 48).

  • rs: daily incoming solar radiation (MJ m^-2 day^-1). If NULL, estimated by Hargreaves-Samani: Rs = krs * Ra * sqrt(Tmax - Tmin) with krs = 0.16 for inland sites.

  • albedo: surface albedo (default 0.23, the FAO grass reference).

References

Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration: guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56.

Examples

dates <- as.Date("2024-07-01") + 0:9
tmin <- c(15, 16, 14, 17, 15, 13, 16, 14, 15, 16)
tmax <- c(30, 32, 28, 33, 31, 27, 34, 29, 30, 32)
ck_pet_pm(tmin, tmax, lat = 45, dates = dates)
#>          date    value  index unit
#> 1  2024-07-01 5.582353 pet_pm   mm
#> 2  2024-07-02 6.006386 pet_pm   mm
#> 3  2024-07-03 5.150850 pet_pm   mm
#> 4  2024-07-04 6.136760 pet_pm   mm
#> 5  2024-07-05 5.839817 pet_pm   mm
#> 6  2024-07-06 4.996958 pet_pm   mm
#> 7  2024-07-07 6.545339 pet_pm   mm
#> 8  2024-07-08 5.391620 pet_pm   mm
#> 9  2024-07-09 5.520137 pet_pm   mm
#> 10 2024-07-10 5.936037 pet_pm   mm