Reference ET₀ Calculator & Crop Water Demand from Temperature
Estimates ET₀ for any latitude
Get the irrigation baseline from temperature alone — enter latitude, month and daily max/min temp for reference ET₀ (mm/day & mm/month) via the Hargreaves method, plus crop ETc from a Kc.
Reference ET₀ is about 5.39 mm/day (high demand) — the water a short green grass loses here, the baseline for irrigation. Over a month that's roughly 162 mm.
Next: multiply ET₀ by the crop coefficient (Kc) for its stage to get crop water use, subtract effective rainfall, then feed it into the Irrigation Scheduling tool to set how often and how much to water.
Hargreaves ET₀ uses temperature + FAO-56 solar radiation by latitude/month — a robust estimate where only temperature data is available.
Reference ET₀ — key facts
- Hargreaves
- 0.0023·Ra·(Tmean+17.8)·√ΔT
- ET₀ units
- mm per day
- Crop ETc
- ET₀ × Kc
- Kc peak
- ≈ 1.0–1.2 at full canopy
- Typical ET₀
- 2 (cool) – 8 (hot, dry)
- Needs
- only Tmax, Tmin, latitude, month
- Ra source
- FAO-56 radiation table
- Privacy
- Runs in your browser; nothing uploaded
The climatic baseline for irrigation
Before you can decide how much to irrigate, you need to know how fast the weather is pulling water out of the crop — that's reference evapotranspiration (ET₀), the daily water loss from a standard well-watered grass. The gold-standard FAO Penman-Monteith equation needs radiation, humidity and wind data most farmers don't have, so FAO recommends the Hargreaves method when only temperature is available: it combines the day's mean temperature, the day-night temperature range (a proxy for clear, dry skies) and the solar radiation reaching your latitude that month.
ET₀ is only the baseline. Multiply it by the crop's coefficient (Kc) for its growth stage to get the crop's real water use (ETc) — small at emergence, peaking at full canopy. Subtract effective rainfall, and that's the water to replace. Feed ET₀ or ETc into the Irrigation Scheduling tool to turn it into an interval and a depth per irrigation, and use the peak-month value when sizing pumps and systems.
From temperature only
Get ET₀ with just max/min temperature, latitude and month — no weather station needed.
Crop water use
Apply a Kc to turn ET₀ into your crop's actual daily water demand (ETc).
Schedule irrigation
Feed ETc into the Irrigation Scheduling tool for interval and depth.
Size for the peak
Use the hottest month's ET₀ to size pumps and system capacity.
Frequently Asked Questions
What is reference evapotranspiration (ET₀)?+
ET₀ is the rate of water loss, in millimetres per day, from a standardised short green grass surface that's well-watered. It's the climatic baseline for irrigation: multiply it by a crop coefficient (Kc) to get a real crop's water use (ETc). This tool estimates ET₀ from temperature using the Hargreaves method.
How does the Hargreaves method work?+
Hargreaves estimates ET₀ from temperature alone: ET₀ = 0.0023 × Ra × (Tmean + 17.8) × √(Tmax − Tmin), where Ra is extraterrestrial (top-of-atmosphere) solar radiation from a latitude/month table, and the temperature range stands in for sunshine and humidity. It's the recommended method when only temperature data is available.
Why does it need latitude and month?+
The amount of solar radiation reaching the top of the atmosphere (Ra) depends on the sun's angle, which varies with latitude and time of year. The tool looks Ra up from a FAO-56 table by your latitude and month — that's why both are needed even though you only enter temperatures.
What is a crop coefficient (Kc)?+
Kc scales ET₀ to a specific crop and growth stage: ETc = ET₀ × Kc. It's low at emergence (≈0.3–0.5), peaks at full canopy/flowering (≈1.0–1.2) and falls at maturity. Enter your crop's Kc for the stage and the tool gives the crop's actual daily water use.
How do I use ET₀ for irrigation?+
ET₀ × Kc gives the crop's daily water need (ETc) in mm. Subtract effective rainfall, then apply the balance — feed ETc into the Irrigation Scheduling tool to set how many days the soil lasts and how much depth to apply each time.
What's a typical ET₀ value?+
It ranges from under 2 mm/day in cool, humid conditions to 6–8 mm/day or more in hot, dry, windy weather. Around 4–5 mm/day is common in a warm growing season. The tool flags low/moderate/high/very-high demand bands.
Why does a bigger day-night temperature range raise ET₀?+
In Hargreaves, the gap between Tmax and Tmin is a proxy for clear skies and low humidity — clear, dry days have a wide range and high evaporation, while cloudy, humid days have a narrow range and lower evaporation. So a wider range increases the ET₀ estimate.
Is Hargreaves as accurate as Penman-Monteith?+
FAO Penman-Monteith is the standard and slightly more accurate where full weather data (radiation, humidity, wind) exists. Hargreaves needs only temperature and is recommended by FAO as the fallback; it's usually within about 10–15% and excellent for planning.
Does ET₀ change through the season?+
Yes — it rises into the hot, bright part of the year and falls in cool or cloudy months, because both temperature and solar radiation change. Re-run it for each month or use the peak month when sizing irrigation capacity.
Can I use it for the southern hemisphere?+
Yes — enter a negative latitude. The tool shifts the solar-radiation table by six months so the seasons line up correctly for the southern hemisphere.