Calculator Inputs
Formula Used
This calculator uses the FAO‑56 Penman–Monteith method for daily reference evapotranspiration (ET0):
- Δ: slope of saturation vapor pressure curve (kPa/°C).
- Rn: net radiation at the surface (MJ/m²/day); G is ~0 daily.
- γ: psychrometric constant (kPa/°C) from pressure and altitude.
- u2: wind speed at 2 m (m/s), adjusted from measurement height.
- es and ea: saturation and actual vapor pressure (kPa).
Radiation and vapor pressure details
How to Use This Calculator
- Enter latitude, altitude, and a date or day-of-year.
- Provide daily temperatures and choose a humidity input type.
- Enter wind speed and the measurement height.
- Choose radiation input: measured Rs or sunshine hours.
- Press Calculate to view ET0 above the form.
- Download CSV or PDF for reports and records.
Example Data Table
| Latitude (°) | Altitude (m) | J | Tmax (°C) | Tmin (°C) | RHmean (%) | Wind (m/s @2m) | Rs (MJ/m²/day) | ET0 (mm/day) |
|---|---|---|---|---|---|---|---|---|
| 30.0 | 0 | 200 | 34 | 22 | 55 | 2.0 | 20.0 | ~6.2 |
| 25.3 | 450 | 120 | 28 | 16 | 60 | 1.5 | 17.5 | ~4.7 |
| -12.1 | 50 | 15 | 31 | 24 | 75 | 3.2 | 23.0 | ~7.0 |
Purpose of Reference ET0 in field planning
Reference ET0 estimates atmospheric demand for water from a short, well-watered grass surface. On construction sites, it standardizes daily water-loss assumptions for dust suppression, temporary irrigation, and landscape establishment. Using one index reduces disputes because inputs are visible and repeatable. ET0 is reported in millimeters per day and converts directly to liters per square meter for quantity takeoffs.
Weather inputs that control daily ET0
Temperature drives saturation vapor pressure, humidity sets vapor pressure deficit, wind increases turbulent transport, and radiation supplies energy. The calculator uses Tmax and Tmin to compute mean saturation vapor pressure and can derive mean temperature. Wind speed is converted to an equivalent 2 m height. Solar radiation can be entered directly or estimated from sunshine hours with the Angstrom relation.
Site factors and radiation corrections
Altitude affects atmospheric pressure, changing the psychrometric constant and the aerodynamic term. It also affects clear-sky radiation used to scale longwave losses. Latitude and day-of-year determine extraterrestrial radiation and daylight hours, capturing seasonal effects without local radiation records. Albedo defaults to 0.23 for the reference surface and can be adjusted for sensitivity checks.
Interpreting results for quantities and schedules
Multiply ET0 by the managed area to estimate daily depth loss. One millimeter over one square meter equals one liter, enabling quick volume conversions for water trucks or storage. For vegetation, apply a crop coefficient to convert ET0 to crop evapotranspiration. For curing and earthwork, ET0 supports drying-risk comparisons and watering-window planning. When coordinating crews, pair ET0 with forecast trends to stage hoses, pumps, and nozzles on-site before midday peak demand and avoid unexpected costly downtime.
Quality checks and practical constraints
Use consistent daily averaging periods from a single weather source. If ET0 looks low on clear, windy days, verify radiation units are MJ/m²/day and wind units are m/s. Negative ET0 can occur from inconsistent inputs; the tool clamps results to zero. Keep exported outputs with date, coordinates, and instrument notes for audits and claims.
FAQs
1) What does ET0 represent in this calculator?
ET0 is the daily reference evapotranspiration for a standardized grass surface. It describes atmospheric water demand, not a specific crop or soil condition.
2) Which humidity option should I use?
Use RHmean when only one daily humidity value is available. Use RHmax/RHmin when your station reports both. Use dew point when that value is more reliable.
3) My wind speed was measured above 2 meters. Is that okay?
Yes. Enter the measured height and the tool converts wind speed to an equivalent 2 m value using the standard logarithmic adjustment.
4) Can I estimate radiation without a pyranometer?
Yes. Select sunshine hours and enter daily sunshine duration. The tool estimates solar radiation using the Angstrom relation and calculated daylight hours.
5) How do I convert ET0 to water volume?
One millimeter over one square meter equals one liter. Multiply ET0 (mm/day) by area (m²) to get liters per day, then scale for efficiency or losses.
6) Why might my ET0 look unusually low or high?
Check unit consistency first: Rs in MJ/m²/day and wind in m/s. Confirm realistic temperatures and humidity. Incorrect day-of-year or latitude can also distort radiation and ET0.