varwg.meteo.meteox2y.penman_monteith

varwg.meteo.meteox2y.penman_monteith(at, u, Rn, rh)

Calculates the reference crop evaporation following the Penman-Monteith method and the FAO-56 determinations published in ASCE Standardized Reference Evapotranspiration Equation

Parameters:

atnp.array of floats

mean daily air temperature at 2m-height [deg C]

unp.array of floats

mean daily wind speed at 2m-height [m/s]

Rnnp.array of floats

measured net radiation at the crop surface [MJ m^-2 d^-1]

rhnp.array of floats

relative humidity [%]

Returns:

etonp.array of floats

FAO Penman-Monteith standardized reference crop evapotranspiration for

short (~=0.12m) surfaces [mm d^-1]

Notes

• Units for the 0.408 coefficient are m^2 mm MJ^-1

• The FAO-56 Penman-Monteith equation is a grass reference equation that was derived from the Penman-Monteith form of the combination equation (Monteith 1965, 1981) by fixing h = 0.12 m for clipped grass and by assuming measurement heights of z = 2 m (at, rh, u) and using a latent heat of vaporization of 2.45 MJ kg-1. The result is an equation that defines the reference evapotranspiration from a hypothetical grass surface having a fixed height of 0.12 m, bulk surface resistance of 70 s m-1, and albedo of 0.23.

• in relationship to the net radiation, the soil heat flux is very small and is fixed with G = 0.1*Rn