FAO-56 Reference ET Calculator

Daily grass reference evapotranspiration from the FAO Penman-Monteith equation. Not a toy Hargreaves-only widget unless solar data are missing.

Latitudedecimal degrees, positive north Elevationmetres Day of year1–365. 15 April = 105. Tmax°C Tmin°C Wind at 2 m u2m/s daily average Actual vapour pressure eakPa. Preferred if you measured it. Leave blank to use RH. RHmax / RHmin% . Used only if ea is blank. Enter one mean RH in RHmax if you have only one value. Solar RsMJ m⁻² day⁻¹. Leave blank if you will use sunshine hours. Sunshine hours nhours. Used with FAO as=0.25, bs=0.50 when Rs is blank. Soil heat flux GMJ m⁻² day⁻¹. Daily = 0. Monthly example uses 0.14.

Formula

ETo = (0.408 Δ (Rn − G) + γ 900/(T+273) u2 (es − ea))
      / (Δ + γ (1 + 0.34 u2))

e°(T) = 0.6108 exp(17.27 T / (T + 237.3))
es = (e°(Tmax) + e°(Tmin)) / 2
Δ = 4098 e°(Tmean) / (Tmean + 237.3)²
P = 101.3 ((293 − 0.0065 z)/293)^5.26
γ = 0.665×10⁻³ P

Rn from FAO-56 shortwave and net longwave (Eq. 38–39).
If Rs is blank, Rs = (0.25 + 0.50 n/N) Ra.
If Rs and n are both blank, the page falls back to Hargreaves
(FAO-56 Eq. 52) and says so.

Worked check: Bangkok April (FAO-56 Example 17) with G = 0.14
gives ETo = 5.72 mm/day.

References

Allen RG, Pereira LS, Raes D, Smith M. Crop evapotranspiration — Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56. Rome: FAO; 1998. Equation 6 and Chapter 4 Example 17.

Field planning aid only. Confirm rates with your agronomist, label, and local extension. Not a pesticide or fertilizer recommendation.