Daily ET0 Calculator (FAO-56 Penman-Monteith)
Estimate reference evapotranspiration (ET0) in mm/day using standard meteorological inputs. Use metric units as listed in each field.
What is ET0?
ET0 (reference evapotranspiration) is the amount of water that would evaporate and transpire from a well-watered reference grass surface under current weather conditions. It is expressed in millimeters per day (mm/day).
In practical terms, ET0 is a climate demand signal. When ET0 is high, the atmosphere is “pulling” more water from soil and plants. When ET0 is low, water demand is lower. Farmers, landscape managers, greenhouse teams, and irrigation designers use ET0 as a core input to estimate irrigation needs.
Method used in this calculator
This tool uses the FAO-56 Penman-Monteith daily equation, widely accepted as the standard approach for reference evapotranspiration:
ET0 = [0.408Δ(Rn − G) + γ(900/(T + 273))u2(es − ea)] / [Δ + γ(1 + 0.34u2)]
- Δ: slope of saturation vapor pressure curve
- Rn: net radiation at crop surface
- G: soil heat flux (assumed 0 for daily timestep)
- γ: psychrometric constant
- T: mean air temperature (°C)
- u2: wind speed at 2 m height
- es − ea: vapor pressure deficit
How to use the ET0 calculator
1) Enter weather inputs
Fill in Tmax, Tmin, RHmax, RHmin, wind speed, and solar radiation for the day you want to evaluate. You can also select a date to auto-fill the day-of-year field.
2) Enter location
Latitude and altitude are used to compute extraterrestrial radiation and clear-sky radiation terms, both needed for radiation balance.
3) Click “Calculate ET0”
The result panel shows ET0 (mm/day) and key intermediate values such as net radiation and vapor pressure deficit.
From ET0 to crop water use (ETc)
ET0 is a reference value. To estimate water use of a specific crop, multiply by a crop coefficient:
ETc = Kc × ET0
Example: if ET0 is 5.2 mm/day and your crop coefficient is 0.85, then ETc is about 4.4 mm/day. This is often the starting point for daily irrigation scheduling.
Input guidance and typical ranges
- Tmax/Tmin: measured daily highs/lows in °C.
- RHmax/RHmin: daily max/min relative humidity in %.
- u2: wind speed at 2 m height in m/s.
- Rs: measured or estimated incoming solar radiation (MJ/m²/day).
- Latitude: decimal degrees, north positive / south negative.
- Altitude: station elevation above sea level in meters.
- Day of year: integer from 1 to 366.
Common mistakes to avoid
- Mixing units (for example, wind in km/h instead of m/s).
- Using ET0 directly as final irrigation depth without Kc and system efficiency adjustments.
- Ignoring effective rainfall and soil water storage.
- Applying one day’s ET0 as a fixed weekly value during rapidly changing weather.
Practical interpretation
A rough daily interpretation for many climates:
- < 2 mm/day: low atmospheric demand
- 2 to 5 mm/day: moderate demand
- > 5 mm/day: high demand, closer irrigation monitoring needed
These ranges are only directional. Always calibrate with local agronomic data, soil type, rooting depth, and irrigation system performance.
Final note
This ET0 calculator is ideal for fast planning and educational use. For operational irrigation management, pair ET0 with crop stage coefficients, rainfall accounting, and field observations (plant stress, soil moisture, and application uniformity) to make better day-to-day decisions.