Evapotranspiration (Hargreaves) Calculator
Estimate reference-crop evapotranspiration (ETo) from just the daily maximum and minimum temperature using the Hargreaves equation — a practical stand-in when you have no humidity, wind or solar-radiation data.
Enter Values
How to use this calculator
- Enter the day's maximum temperature in °C (negatives allowed).
- Enter the day's minimum temperature in °C — it must be below the maximum.
- Optionally set the extraterrestrial radiation Ra for your latitude and date; leave it blank to use 25 MJ/m²/day, then read ETo in mm/day.
How it works
The Hargreaves method needs only temperature. Extraterrestrial radiation Ra (top-of-atmosphere sunlight) is converted to an equivalent water depth by multiplying by 0.408, the mean temperature is taken as the average of max and min, and ETo = 0.0023 × Ra_mm × (Tmean + 17.8) × √(Tmax − Tmin). The temperature range acts as a proxy for cloudiness and actual sunshine.
Worked example
Worked example. With Tmax 30 °C, Tmin 15 °C and Ra 25 MJ/m²/day: Ra_mm = 25 × 0.408 = 10.2 mm/day, Tmean = 22.5 °C, range = 15 °C, so ETo = 0.0023 × 10.2 × (22.5 + 17.8) × √15 = 3.66 mm/day.
Common mistakes
- Leaving Ra at the 25 default — Ra changes a lot with latitude and season, so look it up in a standard table for your location and date.
- Treating ETo as the crop's actual water use; you must multiply it by the crop coefficient Kc for the crop and growth stage.
- Entering a minimum temperature equal to or above the maximum, which makes the √(Tmax − Tmin) term invalid.
Frequently asked questions
What is the difference between ETo and actual crop water use?
ETo is the demand of a standard short, well-watered grass reference. Actual crop use is ETo × Kc, where the crop coefficient Kc depends on the crop and its growth stage.
Where do I get the extraterrestrial radiation Ra?
Ra is top-of-atmosphere solar radiation and is not measured at the ground. It is read from a table (or computed) using your latitude and the day of the year — it does not depend on cloud or weather.
Related tools
- Rainfall Intensity Calculator
- Vapour Pressure Deficit (VPD) Calculator
- Apparent Temperature Calculator
- Degree Days Calculator
- Absolute Humidity Calculator
- Air Density Calculator
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Tip: Enter any known values to calculate the remaining results.
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