Evapotranspiration Calculator
Estimate daily evapotranspiration (ET) for your garden or farm.
Calculate how much water plants lose to heat, wind, and humidity each day.
Evapotranspiration (ET) is the combined water loss from soil evaporation and plant transpiration. It tells you how much water your plants and soil are losing to the atmosphere each day — and therefore how much you need to replace through irrigation or rainfall.
Reference ET (ET₀) is calculated for a reference grass crop under ideal conditions. Actual crop ET is then estimated by multiplying ET₀ by a crop coefficient (Kc) that varies by plant type and growth stage.
Simplified Hargreaves equation (temperature-based): ET₀ ≈ 0.0023 × (Tmean + 17.8) × (Tmax − Tmin)^0.5 × Ra
Where:
- Tmean = mean daily temperature (°C)
- Tmax, Tmin = daily max/min temperature (°C)
- Ra = extraterrestrial radiation (MJ/m²/day) — approximated by latitude and day of year
Factors that increase ET:
- High temperature
- Low humidity
- Strong wind
- Clear skies and high solar radiation
- Large leaf area (dense crops, big trees)
Typical ET₀ values:
- Cool overcast day: 1–2 mm/day
- Mild sunny day: 3–4 mm/day
- Hot sunny summer day: 6–9 mm/day
- Extreme heat wave: 10–14 mm/day
Crop coefficients (Kc) — sample values:
- Lawn grass: 0.8–1.0
- Vegetables (mid-season): 1.0–1.2
- Maize (mid-season): 1.15–1.20
- Tomatoes (mid-season): 1.15
- Trees (full canopy): 0.7–1.0
- Bare soil: 0.10–0.15
Irrigation planning: If ET₀ = 5 mm/day and your crop Kc = 1.15, your crop needs 5 × 1.15 = 5.75 mm/day. Over 7 days: 40.25 mm of water — that is about 40 liters per square meter per week.