Evapotranspiration (ETo) is a calculated value that indicates the daily water use of a reference crop.
It is expressed as inch/day, and based on solar radiation, wind speed, relative humidity, and temperature.
ETo is estimated using the FAO Penman-Monteith approach:
ET0 reference evapotranspiration [mm]
Rn net radiation at the crop surface
G soil heat flux density [MJ m2 day-1]
T air temperature at 2m height [˚C]
U2 wind speed at 2 m height [m s-1]
es saturation vapor pressure [kPa]
ea actual vapor pressure [kPa]
es – ea saturation vapor deficit [kPa]
Δ slope vapor pressure curve [kPa ˚C-1]
ϒ psychometric constant [kPa ˚C-1]
Cn numerator constant [-]
Cd denominator constant [-]
Cn and Cd are reference constants for the weather station environment.
CIMIS (California Irrigation Management Information System) weather stations require a well kept lawn around the station for this reason.