Evapotranspiration Calculator

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.

Why this calculator uses Hargreaves, not Penman-Monteith. The FAO has formally recommended the Penman-Monteith equation since 1998 as the most accurate reference-ET method, and it is the international scientific standard. The catch: Penman-Monteith needs net radiation, vapor pressure, slope of the saturation vapor curve, and the psychrometric constant — data most gardeners and small farms simply don’t have. The Hargreaves equation used here trades a few percent of precision for the ability to estimate ET₀ from just two temperatures plus humidity and wind. For irrigation scheduling at a backyard or hobby-farm scale that trade-off is the right one. Commercial agronomy and water-rights work still uses the full Penman-Monteith form.