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Evapotranspiration Calculator

Estimate reference evapotranspiration (ETo) in millimetres per day with the Hargreaves equation from latitude, date, and air temperatures.

About Evapotranspiration Calculator

Reference evapotranspiration (ETo) is the water lost to evaporation and plant transpiration from a well-watered grass surface. This calculator uses the Hargreaves-Samani equation from FAO Irrigation and Drainage Paper 56: ETo = 0.0023 × (Tmean + 17.8) × √(Tmax − Tmin) × Ra, where Ra is extraterrestrial radiation in mm per day computed from latitude and day of year. It needs temperature only, so it is less accurate than Penman-Monteith in windy or humid climates.

How to use Evapotranspiration Calculator

  1. Fill in "Latitude", "Date", "Maximum temperature" and "Minimum temperature", and the other options.
  2. The result appears below and updates as you type.
  3. Copy the result with the copy button, or download it as a file where that is offered.

Questions about Evapotranspiration Calculator

How do I turn ETo into the water need of a crop?

Multiply ETo by the crop coefficient Kc for the crop and growth stage: ETc = Kc × ETo. FAO-56 tables list Kc values, typically 0.3 to 0.5 early in the season and 1.0 to 1.2 at full canopy.

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