What is evapotranspiration?

Evapotranspiration (ET) is the combined water loss from soil evaporation and plant transpiration. Reference ET (ET0) assumes a well-watered grass surface of defined height and gives a standardized baseline. Actual crop ET is obtained by multiplying ET0 by a crop coefficient (Kc).

What is the Hargreaves-Samani method?

The Hargreaves-Samani (1985) method estimates ET0 using mean temperature, temperature range (Tmax - Tmin), and extraterrestrial radiation (Ra). The formula is: ET0 = 0.0023 × Ra × (Tmean + 17.8) × √(Tmax - Tmin). It is recommended by FAO when humidity and wind data are unavailable.

What is extraterrestrial radiation (Ra)?

Extraterrestrial radiation is the solar radiation received at the top of the atmosphere before atmospheric absorption. It depends only on latitude and day of year. Values typically range from 5 MJ/m²/day in winter at high latitudes to 40+ MJ/m²/day near the equator in summer. Use published tables, FAO Irrigation Paper 56, or this tool's built-in estimator.

How accurate is this calculator?

The Hargreaves-Samani method typically estimates ET0 within 10–20% of the more complex Penman-Monteith equation. Accuracy improves in arid climates. For humid climates, applying a local calibration factor of 0.85 is often recommended. For critical irrigation decisions, validate against local lysimeter or weather-station data.

What is a crop coefficient (Kc)?

A crop coefficient (Kc) scales ET0 to actual crop water use (ETc = Kc × ET0). Kc values vary by crop and growth stage. For example, maize mid-season Kc is about 1.2, wheat is 1.1, and tomatoes are 1.15. FAO Irrigation Paper 56 provides Kc tables for most crops.

What units does this tool use?

ET0 is expressed in mm/day (the standard agronomic unit) and converted to inches/day. Temperature inputs can be in Celsius or Fahrenheit — the tool converts internally. Extraterrestrial radiation is always entered in MJ/m²/day, the FAO standard.

Evapotranspiration Calculator

Name: Evapotranspiration Calculator
Availability: InStock
Author: Nham Vu

Enter daily temperature and solar radiation data to estimate reference evapotranspiration (ET0) using the Hargreaves-Samani method for irrigation scheduling.

Weather Inputs

Temperature Unit

Maximum Temperature (°C)

Minimum Temperature (°C)

Extraterrestrial Radiation Ra (MJ/m²/day)

Typical range: 5–42 MJ/m²/day. Use the estimator below if unknown.

Ra Estimator (by latitude & month)

Latitude (°, negative = South)

Month

Crop Coefficient Kc (optional, default 1.0)

Common values: grass 1.0, maize 1.2, wheat 1.1, tomatoes 1.15.

Enter temperature and Ra values, then click Calculate ET0.

Summary

Enter daily temperature and solar radiation data to estimate reference evapotranspiration (ET0) using the Hargreaves-Samani method for irrigation scheduling.

How it works

Enter the mean, maximum, and minimum daily air temperature in Celsius or Fahrenheit.
Enter the extraterrestrial solar radiation (Ra) in MJ/m²/day for your location and date, or use the built-in latitude estimator.
The Hargreaves-Samani formula calculates ET0 from mean temperature, temperature range, and Ra.
Results are shown in mm/day and inches/day for easy irrigation scheduling.
Crop water requirements can be estimated by multiplying ET0 by a crop coefficient (Kc).

Use cases

Schedule irrigation for gardens, farms, and sports turf.
Estimate crop water demand for different growth stages.
Plan water storage and reservoir management in agriculture.
Cross-check weather station ET data with independent calculations.
Model drought stress risk for field crops or orchards.
Support agronomy research and water-balance studies.
Help homeowners determine lawn watering frequency during summer heat.
Assess seasonal water needs for vineyard and orchard management.

Frequently Asked Questions

Last updated: 2026-06-15 · Reviewed by Nham Vu