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.

Description

Reference evapotranspiration describes atmospheric demand over a reference surface.

Reference evapotranspiration describes atmospheric demand over a reference surface. Hargreaves estimates it from daily temperature range and extraterrestrial radiation when the fuller weather inputs for other methods are unavailable.^{[1, 2]}

When to use Reference Evapotranspiration Hargreaves

Use this tool to 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.

Prepare the climate observations and independently chosen model parameters. Confirm units, daily coverage, chronological order and the intended seasonal window.

How to use Reference Evapotranspiration Hargreaves and understand its results

Prepare the climate observations and independently chosen model parameters. Confirm units, daily coverage, chronological order and the intended seasonal window.
Enter dailyMinimumTemperaturesC, dailyMaximumTemperaturesC, dailyExtraterrestrialRadiationMjPerM2, callerHargreavesCoefficient, callerTemperatureOffsetC using the field table and calculation rules, then run the tool.
Read hargreavesReferenceEtMmPerDay, totalHargreavesReferenceEtMm. 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
dailyMinimumTemperaturesC	Daily minimum temperatures c	list of number	Required
dailyMaximumTemperaturesC	Daily maximum temperatures c	list of number	Required
dailyExtraterrestrialRadiationMjPerM2	Daily extraterrestrial radiation mj per m2	list of number	Required
callerHargreavesCoefficient	Caller hargreaves coefficient	number	Required
callerTemperatureOffsetC	Caller temperature offset c	number	Required

Pair daily minimum and maximum temperatures in °C with nonnegative extraterrestrial radiation in MJ/m²/day, and supply a positive coefficient and finite temperature offset. Maximum must be at least minimum, and mean temperature plus offset must be nonnegative. The factor 0.408 converts radiation to equivalent evaporation. Read daily estimates in mm/day and their total over the supplied days.

Paired lists must be nonempty and have equal lengths.

ET0_i = coefficient(Tmean_i + offset)√(Tmax_i−Tmin_i)Ra_i × 0.408

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
hargreavesReferenceEtMmPerDay	Hargreaves reference et mm per day	list of number
totalHargreavesReferenceEtMm	Total hargreaves reference et mm	number

json

{
  "input": {
    "dailyMinimumTemperaturesC": [
      10
    ],
    "dailyMaximumTemperaturesC": [
      20
    ],
    "dailyExtraterrestrialRadiationMjPerM2": [
      30
    ],
    "callerHargreavesCoefficient": 0.0023,
    "callerTemperatureOffsetC": 17.8
  },
  "output": {
    "hargreavesReferenceEtMmPerDay": [
      2.920001654601175
    ],
    "totalHargreavesReferenceEtMm": 2.920001654601175
  }
}

Limitations and assumptions

The coefficient is supplied, not regionally calibrated here. The temperature-based method omits measured wind and humidity and does not give actual crop water use. The input radiation unit matters; evaporation-equivalent radiation must not be converted a second time.

Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Numeric lists are limited to 100,000 entries; the rules above may require smaller limits. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Extraterrestrial Radiation Daily: Integrate geometric daily extraterrestrial solar radiation from independently supplied declination/distance correction, handling polar day/night without apparent sunrise/refraction or cloud attenuation. For a complementary task, use 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.

Further background: Meteorological data and solar radiation equations.

References

Reference Evapotranspiration Hargreaves — tool.io
Meteorological data and solar radiation equations — FAO

Similar or alternative tools

Extraterrestrial Radiation Daily
Integrate geometric daily extraterrestrial solar radiation from independently supplied declination/distance correction, handling polar day/night without apparent sunrise/refraction or cloud attenuation.
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.