Agriculture Reference Evapotranspiration Penman Monteith
Evaluate named FAO-56 daily grass reference Penman–Monteith with caller radiation, atmospheric coefficients, 2 m wind and VPD; retains signed condensation scenarios, not hourly constants, weather derivation or actual crop ET.
Description
The FAO-56 daily Penman–Monteith relation combines radiation and aerodynamic demand for a grass reference surface.
The FAO-56 daily Penman–Monteith relation combines radiation and aerodynamic demand for a grass reference surface. Its daily constants and two-meter wind convention matter when supplying meteorological inputs.1, 2
Evaluate named FAO-56 daily grass reference Penman–Monteith with caller radiation, atmospheric coefficients, 2 m wind and VPD; retains signed condensation scenarios, not hourly constants, weather derivation or actual crop ET.
When to use Agriculture Reference Evapotranspiration Penman Monteith
Use this tool to evaluate named FAO-56 daily grass reference Penman–Monteith with caller radiation, atmospheric coefficients, 2 m wind and VPD; retains signed condensation scenarios, not hourly constants, weather derivation or actual crop ET.
Prepare the field, crop, soil or material measurements for the intended period. Confirm units, dry/wet bases and supplied parameters; keep paired observations aligned.
How to use Agriculture Reference Evapotranspiration Penman Monteith and understand its results
- Prepare the field, crop, soil or material measurements for the intended period. Confirm units, dry/wet bases and supplied parameters; keep paired observations aligned.
- Enter meanAirTemperatureC, netRadiationMjPerM2Day, soilHeatFluxMjPerM2Day, saturationSlopeKpaPerC, psychrometricConstantKpaPerC, windAtTwoMetersMPerSecond, vaporPressureDeficitKpa using the field table and calculation rules, then run the tool.
- Read signedFao56ReferenceEtMmPerDay. The explanation below defines the units, signs, ordering and any flags or placeholder values.
- If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.
| Input field | Meaning | Value type; see rules below | Requirement |
|---|---|---|---|
| meanAirTemperatureC | Mean air temperature c | number | Required |
| netRadiationMjPerM2Day | Net radiation mj per m2 day | number | Required |
| soilHeatFluxMjPerM2Day | Soil heat flux mj per m2 day | number | Required |
| saturationSlopeKpaPerC | Saturation slope kpa per c | number | Required |
| psychrometricConstantKpaPerC | Psychrometric constant kpa per c | number | Required |
| windAtTwoMetersMPerSecond | Wind at two meters m per second | number | Required |
| vaporPressureDeficitKpa | Vapor pressure deficit kpa | number | Required |
Enter finite net radiation and soil heat flux in MJ/m²/day, temperature with T+273>0, positive saturation slope and psychrometric constant in kPa/°C, nonnegative two-meter wind in m/s and VPD in kPa. Apply the displayed daily equation and return signed mm/day. Negative modeled condensation is retained.
ET0 = [0.408Δ(Rn−G) + γ(900/(T+273))u2 VPD]/[Δ+γ(1+0.34u2)] Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.
| Output field | Meaning | Value type |
|---|---|---|
| signedFao56ReferenceEtMmPerDay | Signed fao56 reference et mm per day | number |
{
"input": {
"meanAirTemperatureC": 27,
"netRadiationMjPerM2Day": 10,
"soilHeatFluxMjPerM2Day": 0,
"saturationSlopeKpaPerC": 0.1,
"psychrometricConstantKpaPerC": 0.05,
"windAtTwoMetersMPerSecond": 2,
"vaporPressureDeficitKpa": 1
},
"output": {
"signedFao56ReferenceEtMmPerDay": 3.847826086956521
}
}Limitations and assumptions
Weather-derived coefficients must be supplied independently. The equation uses T+273 exactly as implemented and is not the hourly form or actual crop ET. Reference-surface assumptions and measurement comparability remain necessary.
Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.
Alternative or Complementary methods
Use Reference Evapotranspiration Hargreaves: Evaluate calibrated daily Hargreaves reference ET with explicit MJ/m²/day radiation converted to equivalent evaporation by 0.408; differs from Penman–Monteith weather inputs and is not actual crop ET. For a complementary task, use Agriculture Crop Evapotranspiration: Integrate reference ET×caller crop coefficients across unequal stage durations, keeping modeled daily rates; no mutable crop calendar or universal Kc lookup.
Further background: FAO Penman–Monteith equation and reference surface.
References
Similar or alternative tools
- Reference Evapotranspiration Hargreaves
Evaluate calibrated daily Hargreaves reference ET with explicit MJ/m²/day radiation converted to equivalent evaporation by 0.408; differs from Penman–Monteith weather inputs and is not actual crop ET.
- Agriculture Crop Evapotranspiration
Integrate reference ET×caller crop coefficients across unequal stage durations, keeping modeled daily rates; no mutable crop calendar or universal Kc lookup.