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.