YielixET0 Calculator — Reference Evapotranspiration (FAO-56 Penman-Monteith)
This ET0 calculator (also written ET₀) estimates reference evapotranspiration using the FAO-56 Penman-Monteith equation, the international standard for estimating atmospheric water demand. As a reference evapotranspiration calculator, it takes your local temperature, humidity, wind speed, and solar radiation and determines how much water a standardized grass surface loses per day. ET0 is the foundation of all irrigation scheduling — multiply it by a crop coefficient (Kc) to convert reference ET0 into actual crop ET for your field.
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 ET for irrigation scheduling.
Mixing metric and imperial units between inputs
Keep temperature, wind speed, and solar radiation in one unit system (metric mm/°C/m/s or imperial in/°F/mph) — the calculator converts internally, but mismatched manual entries produce a wrong ET0.
Related Calculators
Irrigation Water Requirement Calculator
Calculate crop water needs from ET₀, crop coefficient, and irrigation efficiency
Soil Moisture Depletion Calculator
Calculate irrigation scheduling from soil properties, root depth, and daily crop water use
Effective Rainfall Calculator
Calculate effective rainfall using the USDA-SCS method to determine irrigation reduction
Irrigation Runtime Calculator
Calculate how long to run your irrigation system based on flow rate, area, and target application depth
Drip Emitter Spacing Calculator
Calculate emitter count, total flow, and lateral length for drip irrigation layout
Irrigation Pump Sizing Calculator
Calculate required pump power from flow rate and total dynamic head for irrigation systems
Pipe Friction Loss Calculator
Calculate friction head loss in irrigation pipes using the Hazen-Williams equation
Water Cost Calculator
Calculate seasonal irrigation water and energy costs per hectare or acre