What is reference evapotranspiration (ET0)?

ET0 is the rate of evapotranspiration from a hypothetical reference surface of green, well-watered grass with a height of 0.12 m, surface resistance of 70 s/m, and albedo of 0.23. It quantifies the atmospheric demand for water and is used as a baseline to estimate actual crop water use by multiplying by a crop coefficient (Kc).

What is the Hargreaves-Samani method?

The Hargreaves-Samani (1985) equation estimates ET0 from maximum and minimum daily temperature and extraterrestrial radiation (Ra). It is recommended by FAO when only temperature data is available (FAO Irrigation and Drainage Paper No. 56, Chapter 3). With Ra entered in MJ/m²/day, this calculator multiplies Ra by 0.408 to convert it to equivalent evaporation before applying the equation.

What is extraterrestrial radiation (Ra)?

Extraterrestrial radiation (Ra) is the solar radiation received at the top of the atmosphere on a horizontal surface, expressed in MJ/m²/day. It depends on latitude and day of year and can be calculated from standard astronomical formulas. This calculator auto-computes Ra from latitude and month, or you can enter a measured value manually.

How do I get actual crop evapotranspiration (ETc)?

Multiply ET0 by the crop coefficient (Kc) for your specific crop and growth stage: ETc = ET0 × Kc. Kc values are published by FAO for hundreds of crops. For example, Kc for mid-season corn is about 1.20, so ETc ≈ 1.20 × ET0.

What units does the calculator use?

Temperature inputs accept both Celsius and Fahrenheit. Extraterrestrial radiation is in MJ/m²/day (standard for FAO equations). ET0 output is given in both mm/day and inches/day. When using the period mode, totals are shown in mm and inches.

How accurate is the Hargreaves method compared to Penman-Monteith?

The Hargreaves method typically agrees with the FAO-56 Penman-Monteith method within ±10–15% across most climates. It tends to slightly overestimate ET0 in humid coastal regions and underestimate in very arid regions. For critical infrastructure, use Penman-Monteith when humidity, wind, and radiation data are available.

Evapotranspiration Calculator

Enter daily temperature and solar radiation data to calculate reference evapotranspiration (ET0) using the Hargreaves-Samani method for irrigation planning.

ET0 Parameters

Temperature Unit

Max Daily Temperature (°C)

Min Daily Temperature (°C)

Extraterrestrial Radiation (Ra) Source

Latitude (°)

Negative = Southern Hemisphere

Month

Period (days) (for total ET0)

Enter 7 for weekly or 30 for monthly total ET0.

Enter temperature and radiation data, then click Calculate.

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Summary

This evapotranspiration calculator estimates reference evapotranspiration (ET0) using the Hargreaves-Samani method, a widely accepted FAO-endorsed approach requiring only maximum and minimum air temperature plus extraterrestrial solar radiation. ET0 represents the water demand of a standard short-grass reference surface and is the foundation for scheduling irrigation, estimating crop water needs (ETc = ET0 × Kc), and managing water resources in agriculture.

How it works

Enter the latitude of your field to auto-calculate extraterrestrial radiation (Ra) for a given month, or enter Ra manually.
Input the mean daily maximum and minimum air temperatures for the period.
The calculator computes the temperature range (TD = Tmax − Tmin) and the mean temperature.
ET0 is calculated using the Hargreaves-Samani equation after converting Ra from MJ/m²/day to equivalent evaporation: ET0 = 0.0023 × 0.408 × Ra × (Tmean + 17.8) × TD^0.5.
Results are shown in mm/day and inches/day; multiply by your crop coefficient (Kc) to get actual crop water use.
Use the multi-day mode to sum ET0 over a week or month for irrigation scheduling.

Use cases

Schedule drip or sprinkler irrigation based on daily crop water demand.
Estimate seasonal crop water requirements for water rights applications.
Plan irrigation in regions where only temperature data is available.
Calculate deficit irrigation thresholds to reduce water use in drought conditions.
Compare ET0 across months to identify peak irrigation periods.
Combine with crop coefficients (Kc) to calculate ETc for specific crops.
Validate irrigation schedules against weather-station evapotranspiration data.
Educate students on the Hargreaves-Samani method and ET0 fundamentals.

Frequently Asked Questions

Last updated: 2026-05-29 · Reviewed by Nham Vu