ET₀ (Reference Evapotranspiration) Calculator
Estimate reference evapotranspiration ET₀ from temperature alone using the FAO Hargreaves equation — the base for any crop water requirement.
Multiply ET₀ by a crop coefficient (Kc) to get the crop water requirement (ETc). Hargreaves needs only temperature, so it is ideal where humidity, wind and radiation data are missing — expect ±10–15% versus full Penman-Monteith.
Indicative planning figures based on published research averages. Local soil tests, varieties and weather change actual requirements — confirm with your agronomist or extension officer.
Agronomists and irrigation planners use the free ET₀ Calculator to estimate reference evapotranspiration from just max/min temperature when full weather data isn't available — the starting point for every crop water calculation.
About ET₀ (Reference Evapotranspiration) Calculator
Reference evapotranspiration (ET₀) is the rate at which a standard grass surface loses water in your climate — the foundation of all FAO-56 irrigation scheduling, since crop water use ETc = ET₀ × Kc. When humidity, wind and radiation data are missing, the Hargreaves equation estimates ET₀ from temperature alone: ET₀ = 0.0023 × Ra × (Tmean + 17.8) × √(Tmax − Tmin), where Ra is the extraterrestrial radiation for your latitude and season. This tool computes ET₀ in mm/day (and per month) so you can drive any crop-water or scheduling calculation.
How to use ET₀ (Reference Evapotranspiration) Calculator
- 1Enter the day's (or average) maximum and minimum temperature.
- 2Pick your latitude band and season to set the radiation term Ra.
- 3Read ET₀ in mm/day, then multiply by a crop's Kc to get its water requirement.
Why use ET₀ (Reference Evapotranspiration) Calculator?
- ✓Needs only max and min temperature — ideal for data-sparse locations
- ✓Uses the FAO-endorsed Hargreaves equation
- ✓Gives daily and monthly ET₀ to feed crop-water calculations
- ✓Free, instant and fully in-browser
Frequently asked questions
What is reference evapotranspiration (ET₀)?+
ET₀ is the evapotranspiration from a reference grass surface with unlimited water — a climate-only measure of evaporative demand. Multiplying ET₀ by a crop coefficient (Kc) gives the actual crop water requirement (ETc). It is the basis of FAO-56 irrigation scheduling.
When should I use the Hargreaves equation instead of Penman-Monteith?+
Use Hargreaves when you only have temperature data. Penman-Monteith is more accurate but needs humidity, wind and solar radiation. Hargreaves typically lands within ±10–15% of Penman-Monteith and is the recommended temperature-only fallback.
How do I turn ET₀ into a crop's water need?+
Multiply ET₀ by the crop's stage-specific Kc: ETc = ET₀ × Kc. For example, ET₀ of 6 mm/day with a mid-season Kc of 1.15 gives 6.9 mm/day of crop water use. Our crop water requirement calculators do this across the whole season.
Is this tool free and private?+
Yes — free, no sign-up, and all calculation runs in your browser, so it works offline and your data never leaves the device.
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