Evapotranspiration Calculator
Calculate daily grass-reference evapotranspiration (ETo) using the FAO-56 Penman-Monteith method.
Required Data
North positive; south negative.
Used only if solar radiation must be estimated from temperature range.
Humidity Data (optional)
Priority: dew point → RHmax + RHmin → RHmean. If all are blank, actual vapour pressure is estimated from Tmin as recommended by FAO-56 for missing humidity data.
Wind Data (optional)
Wind measured above 2 m is adjusted to the FAO standard 2 m height. If wind speed is blank, 2.0 m/s is used as the FAO default for general conditions.
Solar Data (optional)
Priority: measured Rs → sunshine duration using Angstrom-Prescott (as=0.25, bs=0.50) → FAO temperature-range estimate.
—
FAO-56 grass reference evapotranspiration
Calculated Parameters
How to Use
Enter latitude, elevation, date, and daily minimum and maximum air temperature. For the most accurate result, also provide measured humidity, wind speed and solar radiation.
If some weather data are unavailable, the calculator applies FAO-56 procedures for missing data: humidity can be estimated from Tmin, wind defaults to 2 m/s, and solar radiation can be estimated from sunshine duration or temperature range.
Use the Interior / inland or Coastal setting only when solar radiation is estimated from Tmax−Tmin. For small islands, provide radiation or sunshine data.
Method: Daily FAO-56 Penman-Monteith reference evapotranspiration for the standard grass reference surface. Daily soil heat flux G is taken as 0.
Radiation: Measured Rs is preferred. If unavailable, sunshine duration uses the default Angstrom-Prescott coefficients as=0.25 and bs=0.50. Temperature-range estimation uses kRs=0.16 inland or 0.19 coastal and is capped at clear-sky radiation Rso.
Wind: Wind measured at another height is converted to 2 m using FAO-56 Eq. 47.
Reference: FAO Irrigation and Drainage Paper 56 — Crop Evapotranspiration
Explore our new tool, ETo Tracker, to know the Reference Evapotranspiration value specific to your location.