YielixReference Evapotranspiration (ET₀) Calculator — FAO-56 Penman-Monteith
Calculate reference evapotranspiration using the FAO-56 Penman-Monteith equation, the international standard for estimating atmospheric water demand. Enter your local temperature, humidity, wind speed, and solar radiation to determine how much water a standardized grass surface loses per day. ET₀ is the foundation of all irrigation scheduling — multiply it by a crop coefficient to estimate actual crop water use.
Inputs Explained
- Temperature (Min / Max)
- Daily minimum and maximum air temperature. Drives saturation vapor pressure — warmer air holds more moisture, increasing evaporative demand.
- Relative Humidity (Min / Max)
- Daily minimum and maximum relative humidity. Determines the vapor pressure deficit: drier air pulls more water from the soil and crop canopy.
- Wind Speed
- Average daily wind speed at 2 m height. Wind replaces moist air near the crop with drier air, accelerating transpiration.
- Solar Radiation
- Incoming shortwave radiation in MJ/m²/day. Provides the energy that powers evaporation from the soil and plant surfaces.
- Elevation & Latitude
- Site elevation adjusts atmospheric pressure; latitude adjusts extraterrestrial radiation for your location on the globe.
How This Calculator Works
Worked Example
Mid-summer irrigation planning in a temperate inland valley
- 1. Enter temperatures
Min 18 °C, Max 34 °C — gives a mean of 26 °C and high saturation vapor pressure.
- 2. Enter humidity
Min RH 30%, Max RH 75% — yields a significant vapor pressure deficit.
- 3. Enter wind speed
2.5 m/s at 2 m height — moderate wind enhances turbulent transfer.
- 4. Enter solar radiation
24 MJ/m²/day — typical clear-sky mid-summer value at 38° latitude.
- 5. Set location
Elevation 150 m, latitude 38° N — adjusts pressure and extraterrestrial radiation.
ET₀ ≈ 6.3 mm/day — multiply by crop Kc (e.g., 1.15 for mid-season corn) to get crop ET of about 7.2 mm/day.
How to Interpret Your Results
| Condition | What It Means |
|---|---|
| ET₀ < 2 mm/day | Low atmospheric demand — cool, humid, or overcast conditions. Irrigation frequency can be reduced. |
| ET₀ 2–4 mm/day | Moderate demand — typical of spring/autumn or coastal climates. Standard irrigation scheduling applies. |
| ET₀ 4–6 mm/day | High demand — warm, sunny conditions. Most irrigated crops need daily or every-other-day watering. |
| ET₀ > 6 mm/day | Very high demand — hot, dry, windy conditions. Irrigation systems must run at near-full capacity. |
Common Mistakes to Avoid
Using air temperature from a non-standard location (e.g., direct sun)
Always use shaded, ventilated temperature readings at standard height (1.5–2 m) per WMO guidelines.
Entering wind speed measured at 10 m height without converting
Convert 10 m wind to 2 m using the logarithmic profile: u₂ = u₁₀ × 4.87 / ln(67.8 × 10 − 5.42).
Confusing ET₀ with actual crop ET
ET₀ is the reference rate for grass. Multiply by the crop coefficient (Kc) to get actual crop water use.
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