Estimate reference evapotranspiration ET₀ using a simplified Penman-Monteith approach. Input weather data to calculate daily ET₀ in inches.
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 calculator 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. A few tenths of an inch per day error in ET₀ can accumulate to several inches over a growing season, significantly affecting irrigation volumes and crop performance. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation.
ET₀ is the foundation of every science-based irrigation schedule. Without it, you cannot accurately determine crop water requirements, schedule irrigations, or evaluate whether your water supply is sufficient for the season. This calculator gives you a quick, physics-based ET₀ estimate from readily available weather data. Having a precise figure at your fingertips empowers better planning and more confident decisions.
Simplified Hargreaves-Samani (when full data limited): ET₀ (mm/day) ≈ 0.0023 × (T_mean + 17.8) × (T_max − T_min)^0.5 × Ra Where: T_mean = (T_max + T_min) / 2 (°C) Ra = extraterrestrial radiation (mm/day equivalent) Result converted: ET₀ (in/day) = ET₀ (mm/day) / 25.4
Result: ET₀ ≈ 0.30 in/day
With T_max 95°F (35°C), T_min 65°F (18.3°C), T_mean 26.7°C, and Ra approximated from solar radiation, the simplified method yields roughly 7.6 mm/day or about 0.30 inches per day, typical for a hot, moderately dry summer day.
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.
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.
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).
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.
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.
In the U.S., ET₀ is often reported in inches per day. Internationally, millimeters per day is standard. 1 inch = 25.4 mm.
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.
Yes, but ET₀ is very low in winter. If temperatures are below freezing, evapotranspiration is negligible and the formula may not apply well.
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.