FAO56 (version 1.0)

ETo_FPM: FAO Penman-Monteith Reference Evapotranspiration (ETo) Equation

Description

ETo_FPM returns the value of evapotranspiration rate from the reference surface.

Usage

ETo_FPM(
  Delta = SlpSVPC(T_mean),
  T_mean = (T_min + T_max)/2,
  R_n = NULL,
  G = 0,
  gamma = PsyCon(AtmPres(elev)),
  u_2 = NULL,
  u_z = NULL,
  z = NULL,
  e_s = MSVP(T_max, T_min),
  T_dew = NULL,
  e_a = NULL,
  T_min = NULL,
  T_max = NULL,
  phi_deg = NULL,
  elev = NULL,
  date = NULL,
  n = NULL,
  N = NULL,
  a_s = 0.25,
  b_s = 0.5
)

Value

The function returns the value of evapotranspiration rate from the reference surface as a numeric scalar.

Arguments

Delta

Optional. A numeric scalar that denotes the slope vapour pressure curve [kPa/C].

T_mean

Optional. A numeric scalar that denotes the average temperature [C].

R_n

Optional. A numeric scalar that denotes the net radiation at the crop surface [\(MJ/(m^2\times day)\)].

G

Optional. A numeric scalar that denotes the soil heat flux density [\(MJ/(m^2\times day)\)]. The default is G=0.

gamma

Optional. A numeric scalar that denotes the psychrometric constant [kPa/C].

u_2

A numeric scalar that denotes the wind speed at the height 2m above the ground surface [m/s].

u_z

A numeric scalar that denotes the wind speed at the height z above the ground surface [m/s].

z

A numeric scalar that denotes the height above the ground surface where the wind speed has been measured [m].

e_s

Optional. A numeric scalar that denotes the saturation vapour pressure [kPa].

T_dew

Optional. A numeric scalar that denotes the dew point temperature [C].

e_a

Optional. A numeric scalar that denotes the actual vapour pressure [kPa].

T_min

Optional. A numeric scalar that denotes the daily minimum temperature [C].

T_max

Optional. A numeric scalar that denotes the daily maximum temperature [C].

phi_deg

Optional. A numeric scalar that denotes the latitude in terms of degree [degree].

elev

Optional. A numeric scalar that denotes the elevation above the sea level [m].

date

Optional. A character string that denotes the date in the format "Year-Month-Day" or "Year/Month/Day".

n

Optional. A numeric scalar that denotes actual duration of sunshine [hour]

N

Optional. A numeric scalar that denotes maximum possible duration of sunshine or daylight hours [hour]

a_s

Optional. A numeric scalar that denotes regression constant, expressing the fraction ofextraterrestrial radiation reaching the earth on overcast days (n = 0). The default is a_s = 0.25.

b_s

Optional. A numeric scalar that denotes fraction of extraterrestrial radiation reaching the earth on clear days (\(n = N\)). The default is b_s = 0.5

Reference

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. Fao, Rome, 300(9), D05109.

Details

This is a function to calculate the evapotranspiration rate from the reference surface (ETo) by using FAO Penman-Monteith equation which is one of the most-widely used equations for this purpose. If Delta is missing, the function uses the argumnet T_mean to compute its value. If T_mean is missing, the function needs T_min and T_max to compute T_mean. If R_n is missing, the arguments phi_deg, date, n, N, elev, T_min, T_max, and e_a must be present. If gamma is missing, the function needs elev to compute gamma. If e_s is missing, the arguments T_min and T_max must be present for computation of e_s. If e_a is missing, one of the arguments T_dew or T_min must be present in order to compute e_a. If T_dew is missing and T_min is present, then T_dew is computed based on the T_min value. If u_2 is missing, the function needs the values of the arguments u_z and z to compute u_2.

See Also

ETo_Hrg for Hargreaves Equation.

Examples

ETo_FPM(u_2 = 2, e_a = 2.85, T_min = 25.6, T_max = 34.8, phi_deg = 13.73,
             elev = 2, date = '2002-04-15', n = 8.5, N = 12.31)