Evapotranspiration Calculators

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Evapotranspiration (ET) is the combined water loss from Earth's surface to the atmosphere — encompassing evaporation from soil, water bodies, and plant surfaces, plus transpiration through plant stomata. It is the largest water loss term in most terrestrial water balances, returning 60–70% of precipitation back to the atmosphere in humid regions. Reference evapotranspiration (ET₀) is calculated from weather data for a standardized reference grass crop using the FAO Penman-Monteith equation. Actual ET depends on crop type (via crop coefficient Kc) and soil moisture availability. ET measurement and modeling are essential for irrigation scheduling, hydrological modeling, and climate science.

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FAO Penman-Monteith Equation (ET₀)

ET₀ = [0.408Δ(Rn−G) + γ(900/(T+273))u₂(es−ea)] / [Δ + γ(1+0.34u₂)]

Where: Δ = slope of saturation vapor pressure curve; Rn = net radiation (MJ/m²/day); G = soil heat flux; γ = psychrometric constant; T = mean air temperature (°C); u₂ = wind speed at 2 m height; es = saturation vapor pressure; ea = actual vapor pressure. ET₀ units: mm/day.

Actual vs. Reference ET

  • ET₀ (reference): Calculated for hypothetical well-watered short grass; represents atmospheric evaporative demand
  • ETc (crop): ETc = Kc × ET₀; accounts for crop-specific physiology via crop coefficient
  • ETa (actual): Real ET under field conditions, which may be less than ETc if soil moisture is limiting

Measuring ET

  • Lysimeters: Weighing containers with known soil volume; most direct field measurement
  • Eddy covariance: Measures turbulent water vapor flux; used at ecosystem scale
  • Water balance: ETa = P − Q − ΔS (precipitation − runoff − storage change)
  • Remote sensing: MODIS and Landsat-based ET products for regional scales

Typical ET₀ Values

Arid summer: 8–12 mm/day. Temperate summer: 4–7 mm/day. Cool/humid winter: 0.5–2 mm/day. Annual ET₀: 400 mm/yr (humid temperate) to 2,500 mm/yr (hot arid zones).

Glossary

Evapotranspiration (ET)
Combined water loss from soil evaporation and plant transpiration; the largest water loss term in most terrestrial water balances; 60–70% of precipitation returns to atmosphere as ET.
Reference Evapotranspiration (ET₀)
ET from a hypothetical well-watered reference grass; calculated by FAO Penman-Monteith from weather data; represents atmospheric evaporative demand; multiplied by Kc to get crop ET.
Vapor Pressure Deficit (VPD)
The difference between saturated and actual water vapor pressure (kPa); the primary atmospheric driver of plant transpiration and stomatal responses; high VPD = high transpiration demand.

Frequently Asked Questions

ET₀ is the evapotranspiration from a hypothetical well-watered short grass, calculated from weather data using the Penman-Monteith equation — it represents atmospheric evaporative demand. ETc = Kc × ET₀ is the potential ET of a specific crop at its current growth stage, assuming adequate soil moisture. ETa (actual ET) is what actually occurs in the field — it equals ETc when water is not limiting, but is less than ETc when soil moisture is below the threshold for full water supply. Irrigation is applied when ETa threatens to fall below ETc during critical growth stages.

The main drivers of ET: (1) Net radiation (Rn) — the primary energy source for evaporation; accounts for ~60–80% of ET in most conditions. (2) Vapor pressure deficit (VPD = es − ea) — the difference between saturated and actual air vapor pressure; high VPD drives high stomatal conductance and transpiration. (3) Wind speed — transports water vapor away from the surface. (4) Temperature — influences saturation vapor pressure and plant physiology. (5) Stomatal conductance — plants regulate water loss through stomatal aperture. High Rn + high VPD + low humidity + strong wind produce the highest ET rates.

Direct methods: (1) Weighing lysimeters — soil-filled containers on load cells measuring total water balance directly; most accurate but expensive. (2) Eddy covariance towers — measure vertical turbulent fluxes of water vapor using fast sensors (10–20 Hz); standard method for ecosystem-scale ET; requires processing and corrections. Indirect methods: (3) Water balance — ET = precipitation − runoff − soil water change (residual method); simple but accumulates errors. (4) Remote sensing — MODIS, Landsat, and Sentinel satellites used to estimate ET over large areas using surface energy balance algorithms.

The FAO Penman-Monteith equation calculates ET₀ by combining the energy balance (radiation) and aerodynamic (vapor transport) approaches. It requires: minimum and maximum temperature, relative humidity (or dew point), solar radiation (or sunshine hours), and wind speed. It was adopted by FAO as the universal standard in 1998 because it gives consistent results across climates without local calibration. The combination method is physically based — both energy supply and atmospheric demand are incorporated, unlike simpler temperature-only methods (Thornthwaite, Hargreaves) that work only in the climate they were developed in.