Penman-Monteith evapotranspiration model

If daily air temperature solar radiation, maximum and minimum relative humidity (RHmax and RHmin) (or dew point temperature from which relative humidity can computed) and wind speed are available, then the Penman-Monteith equation may be used to calculate potential evapotranspiration.

RefET_pot =

Slope_vpf · (Rad_net - SoilHeatFlux) + (DayFract · VPD · C_va) / r_a

Slope_vpf + g · (1 + r_c/r_a)

where

SoilHeatFlux ((MJ/m²)/day) is the soil heat flux.
DayFract (0-1) is the fraction of the day that is in daylight.
C_va ((MJ/m3)/°C) is the volumetric heat capacity of air taken as 1.2x10^-3.
g (kPa/°C) is the psychometric constant.
Slope_vpf (kPa/°C) is the slope of saturation vapor pressure function of temperature.
VPD (kPa) is the Vapor pressure deficiet
Rad_net ((MJ/m²)/day) is the net radiation .
r_a, r_c (day/m) are the aerodynamic and canopy resistances to vapor transfer of the reference crop. For a short clipped grass (0.12m height), theses are

r_a = ln((hs - 0.744 · hp) / (0.026 · hp)) ·ln((hs - 0.640 · hp))/(0.013 · hp)
0.16 · ws

r_c = 0.000787034 (this may be adjusted in the case of CO₂ simulation).
where

ws (m/s) is the wind speed. (Note that wind speed is stored in daily weather files a m/day but will be converted to m/s).
hs (m) is the screening height (Screening height is the height of the wind measurement device at the weather station).
hp (m) is the plant height (the reference plant height).