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Reference Evapotranspiration Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Hargreaves-Samani simplified ET0 estimate. Last verified 15 June 2026.

Reference evapotranspiration (ET0) is the evaporative demand of the atmosphere, quantifying how much water a well-watered short grass crop would use per day. It is the foundation of irrigation scheduling: multiply ET0 by a crop coefficient to get actual crop water demand, then subtract effective rainfall to determine irrigation need. The Hargreaves-Samani method provides a reasonable ET0 estimate using only temperature data, making it practical when full weather station data is unavailable. Enter daily high and low temperatures and the extraterrestrial radiation for your location and date.

Typical mid-summer: 25-40 MJ/m2/day depending on latitude
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Hargreaves-Samani ET0 formula

ET0 = 0.0023 * Ra * (T_mean + 17.8) * (Tmax - Tmin)^0.5 * 0.408 [mm/day]

Ra is extraterrestrial radiation in MJ/m2/day, T_mean = (Tmax + Tmin)/2 in Celsius, and (Tmax - Tmin) is the daily temperature range. The 0.408 factor converts from MJ/m2/day to mm/day. Source: Hargreaves, G.H. and Samani, Z.A. (1985): Reference crop evapotranspiration from temperature. Applied Engineering in Agriculture, 1(2), 96-99.

ET0 reference values and irrigation implications

  • ET0 below 3 mm/day: low demand (cool, cloudy conditions); minimal irrigation needed.
  • ET0 3-5 mm/day: moderate demand; typical spring or fall in temperate climates.
  • ET0 5-8 mm/day: high demand; summer conditions in humid climates.
  • ET0 above 8 mm/day: very high demand; hot, sunny, dry or windy conditions; arid western US summer.
  • Weekly irrigation need: ET0 (mm/day) * 7 * crop coefficient (Kc) - effective rainfall.

Reference evapotranspiration: frequently asked questions

What is reference evapotranspiration (ET0)?

Reference evapotranspiration (ET0) is the evapotranspiration rate from a hypothetical short grass reference crop with an assumed height of 0.12 m, fixed surface resistance, and full ground cover. It represents the atmospheric demand for water and is the standard baseline used in irrigation scheduling. Actual crop water use equals ET0 multiplied by a crop coefficient (Kc).

What is the Hargreaves-Samani method?

The Hargreaves-Samani (1985) method estimates ET0 from temperature data alone, using the range between maximum and minimum temperature as a proxy for solar radiation and aridity: ET0 = 0.0023 * Ra * (T + 17.8) * (Tmax - Tmin)^0.5 * 0.408, where Ra is extraterrestrial radiation in MJ/m2/day and temperatures are in Celsius. It is a widely used simplified alternative to the FAO-56 Penman-Monteith equation when only temperature data is available.

How is extraterrestrial radiation (Ra) estimated?

Ra depends on latitude and day of year. For a simplified estimate, you can use the mid-season Ra value for your latitude or input a measured value. The FAO-56 method provides Ra tables by latitude and month. Typical mid-summer Ra values range from about 25 MJ/m2/day at 50 N latitude to 40 MJ/m2/day near the equator.

How accurate is the Hargreaves-Samani method?

The Hargreaves-Samani method typically has an accuracy of 15-25% compared to the FAO-56 Penman-Monteith standard. It tends to overestimate in humid climates and underestimate in arid climates. For irrigation scheduling, it provides a reasonable first estimate when humidity, wind, and solar radiation data are unavailable. The FAO-56 Penman-Monteith equation is the recommended standard when full meteorological data is available.

What is the difference between ET0, ETc, and ETa?

ET0 is the reference evapotranspiration from the hypothetical short grass reference crop. ETc (crop evapotranspiration) equals ET0 multiplied by the crop coefficient Kc, which varies by crop type and growth stage. ETa (actual evapotranspiration) is the real water lost from a specific crop under actual soil water availability, which may be less than ETc if soil moisture is limited. Irrigation replaces the difference between ETc and effective rainfall.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.