Evapotranspiration Calculator
Evapotranspiration is the water lost from a landscape through evaporation from the soil and transpiration from plants combined. Reference evapotranspiration (ET0) standardises this to a well-watered grass surface so it can be compared across sites and used to schedule irrigation. The Hargreaves equation estimates ET0 from temperature alone, needing only daily maximum, minimum, and mean temperature plus the extraterrestrial radiation for the latitude and day of year. This makes it a practical choice where humidity, wind, and solar-radiation data are missing. Enter the temperatures and the extraterrestrial radiation (in MJ per square metre per day) to get ET0 in millimetres per day.
Hargreaves evapotranspiration formula
Tmean = (Tmax + Tmin) / 2
ET0 = 0.0023 * (Tmean + 17.8) * sqrt(Tmax - Tmin) * Ra_mm
where Ra_mm = Ra (MJ/m2/day) * 0.408
The Hargreaves equation, recommended by the FAO as a temperature-based alternative when full data are unavailable, multiplies a temperature term by the square root of the daily temperature range and by extraterrestrial radiation. The factor 0.408 converts radiation from megajoules per square metre per day to equivalent millimetres of water. The daily temperature range stands in for cloudiness and humidity, since clear, dry days swing more widely.
Worked example
On a day with a maximum of 30 degrees Celsius, a minimum of 15, and extraterrestrial radiation of 30 MJ per square metre per day: Tmean = 22.5, the range is 15, and Ra in water depth is 30 * 0.408 = 12.24 mm. ET0 = 0.0023 * (22.5 + 17.8) * sqrt(15) * 12.24 = 0.0023 * 40.3 * 3.873 * 12.24 = 4.39 mm/day. A crop would need roughly this much water replaced that day, before crop-coefficient adjustment.
Frequently asked questions
What is reference evapotranspiration?
Reference evapotranspiration (ET0) is the rate of water loss from a standardised, well-watered short-grass surface under given weather. It removes the effect of the specific crop so weather-driven demand can be compared between places and dates. Actual crop water use is ET0 multiplied by a crop coefficient that depends on the plant and its growth stage.
Why use the Hargreaves equation?
The FAO Penman-Monteith equation is the global standard but needs humidity, wind, and solar-radiation data that many sites lack. The Hargreaves equation needs only temperature and extraterrestrial radiation (a function of latitude and date), so it is the FAO-recommended fallback where data are limited, giving reasonable ET0 estimates for irrigation planning.
Where do I get extraterrestrial radiation?
Extraterrestrial radiation is the solar radiation arriving at the top of the atmosphere for a given latitude and day of year. It is tabulated in FAO Irrigation and Drainage Paper 56 and computed from astronomical formulas, not measured locally. Look up the value for your latitude and date, or use a published table, and enter it here.
How accurate is a temperature-only estimate?
Hargreaves ET0 is generally within about 10 to 15 percent of the full Penman-Monteith result under typical conditions, and works best for periods of ten days or longer. It can be less accurate in very humid, very windy, or coastal settings. Where full weather data exist, the Penman-Monteith method is preferred.
Official sources
- UN FAO Irrigation and Drainage Paper 56: Crop evapotranspiration guidelines.
- USDA Natural Resources Conservation Service: irrigation water management.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.