Reference Evapotranspiration (ET₀) Calculator

About the Reference Evapotranspiration (ET₀) Calculator

Reference evapotranspiration (ET₀) quantifies the atmospheric demand for water from a hypothetical grass surface under well-watered conditions. It serves as the baseline against which all crop water requirements are measured. The FAO-56 Penman-Monteith equation is the internationally accepted standard for computing ET₀, combining energy balance and aerodynamic approaches.

This page uses a simplified version of the Penman-Monteith method suitable for field-level planning. You provide daily maximum and minimum temperatures, average relative humidity, wind speed, and solar radiation, and the tool returns an ET₀ estimate in inches per day.

Regular ET₀ monitoring — whether from on-farm weather stations, state networks, or satellite-derived data — underpins accurate irrigation scheduling. This page turns common weather inputs into a planning-level ET₀ estimate that can then feed crop coefficients and irrigation schedules.

When This Page Helps

ET₀ is not useful as an abstract climate number. It matters because other water-budget steps depend on it. This page provides that baseline.

How to Use the Inputs

1. Enter the daily maximum temperature in °F.
2. Enter the daily minimum temperature in °F.
3. Enter the average relative humidity as a percentage.
4. Enter the average wind speed in miles per hour.
5. Enter solar radiation in MJ/m²/day (typically 15–30).
6. Read the estimated ET₀ in inches per day.

Example Calculation

Tips & Best Practices

• Use local weather station data for the most accurate inputs.
• Solar radiation gauges or estimates from sunshine hours improve accuracy.
• In arid windy areas, ET₀ can exceed 0.40 in/day in midsummer.
• In humid coastal regions, ET₀ may stay below 0.15 in/day even in summer.
• Re-estimate ET₀ weekly during the irrigation season.
• Compare your calculated ET₀ with published values from state extension services.

The Full Penman-Monteith Equation

The FAO-56 Penman-Monteith equation is a complex formula requiring net radiation, soil heat flux, vapor pressure deficit, psychrometric constant, and aerodynamic resistance. This calculator uses a simplified approach to make estimation accessible without specialized instruments.

When to Use Measured ET₀

If your state maintains an agricultural weather network, prefer their published ET₀ over calculated values. Networks use calibrated sensors and quality-controlled algorithms that outperform simplified field estimates.

ET₀ and Irrigation Scheduling

Irrigation scheduling links ET₀ to a checkbook method: start with full soil water, subtract daily ETc (= ET₀ × Kc), add rainfall, and irrigate when the cumulative deficit reaches a threshold (typically 50% of available water capacity in the root zone).

Frequently Asked Questions

• What is reference evapotranspiration?

  ET₀ is the rate of evapotranspiration from a standard grass surface that is 12 cm tall, well-watered, and actively growing. It represents the atmospheric demand for water independent of crop type.

• How does ET₀ differ from ETc?

  ET₀ is the baseline; ETc is the actual crop water use. ETc = ET₀ × Kc, where Kc is the crop coefficient that varies by crop species and growth stage.

• What units is ET₀ typically reported in?

  In the U.S., ET₀ is often reported in inches per day. Internationally, millimeters per day is standard. 1 inch = 25.4 mm.

• Why is the Penman-Monteith method preferred?

  It accounts for both radiation and aerodynamic components of evapotranspiration, making it accurate across a wide range of climates. FAO adopted it as the sole standard in 1998.

• Can I use this in winter?

  Yes, but ET₀ is very low in winter. If temperatures are below freezing, evapotranspiration is negligible and the formula may not apply well.

• What is a typical range for ET₀?

  In the U.S., ET₀ ranges from near 0 in winter to about 0.35–0.45 in/day in peak summer. Desert areas like southern Arizona can exceed 0.50 in/day.