Evapotranspiration

Free Evapotranspiration calculator with clear step-by-step results.

About Evapotranspiration

Implements the FAO-56 Penman-Monteith reference evapotranspiration for a short grass surface. The result splits into two parts worth reading separately: the radiation term, which dominates in still humid conditions, and the aerodynamic term, which takes over when it is dry and windy.

Formulas

Saturation vapour pressure

es = 0.6108 x exp(17.27 T / (T + 237.3))

Reference ET

ET0 = (0.408 x slope x net radiation + gamma x (900 / (T + 273)) x wind x VPD) / (slope + gamma x (1 + 0.34 x wind))

Worked example

With the default values below, Evapotranspiration returns Reference evapotranspiration of 5.29. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean air temperatureaccepts -10–50 C: 22 C
Solar radiationaccepts 0.1–40 MJ/m2/day: 20 MJ/m2/day
Mean relative humidityaccepts 5–100 %: 60 %
Wind speed at 2 maccepts 0–20 m/s: 2 m/s

Results

Reference evapotranspiration: 5.29 mm/day
Vapour pressure deficit: 1.058 kPa
Radiation-driven component: 3.72 mm/day
Wind and humidity component: 1.57 mm/day

Step by step

1Values used
2Saturation vapour pressure
3Reference ET
4Reference evapotranspiration= 5.29 mm/day
5Vapour pressure deficit= 1.058 kPa
6Radiation-driven component= 3.72 mm/day
7Wind and humidity component= 1.57 mm/day

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
Figures are rounded for display to 3 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Is this the same as crop water use?

No. ET0 is for a reference grass surface. Multiply by the crop coefficient for your crop and growth stage to get actual crop evapotranspiration.

Why does wind increase evaporation so much?

Wind strips away the humid boundary layer above the surface, keeping the vapour pressure deficit at the leaf high. In dry conditions the aerodynamic term can exceed the radiation term.