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 lose to the atmosphere each day, and therefore how much has to come back 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.

Hargreaves equation (temperature-based): ET₀ (mm/day) ≈ 0.0023 × (Tmean + 17.8) × (Tmax − Tmin)^0.5 × Ra / 2.45

Where:

• Tmean = mean daily temperature (°C)
• Tmax, Tmin = daily max and min temperature (°C)
• Ra = extraterrestrial radiation (MJ/m²/day)
• 2.45 = latent heat of vaporisation (MJ/kg), which turns energy into millimetres of water

The (Tmax − Tmin) term is doing something clever. A big daily swing means clear skies and strong sun; a narrow one means cloud cover holding heat in overnight. So the temperature range stands in for solar radiation, which is why this method needs no pyranometer.

Two simplifications you should know about. Ra is fixed here at 25 MJ/m²/day, a mid-latitude annual average, rather than derived from your latitude and the date. Real Ra runs from about 15 in midwinter at 45° north to over 40 in midsummer, so this reads high in winter and low at the height of summer. Use it in the growing season and treat the shoulders with suspicion.

The humidity and wind inputs are not part of the published Hargreaves equation, which takes temperature alone. They are adjustment factors added here because both genuinely move ET and gardeners usually know them. They are reasonable, not validated.

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, which is about 40 litres per square metre per week.

Why this calculator uses Hargreaves, not Penman-Monteith. The FAO has recommended Penman-Monteith since 1998 and it is the international standard. The catch is what it needs: net radiation, vapour pressure, the slope of the saturation vapour curve and the psychrometric constant. Most gardeners and small farms have none of that. Hargreaves gets you a usable ET₀ from two thermometer readings, and for deciding whether the beds need watering on Thursday that is the right trade. Published comparisons put it within roughly 10 to 20% of Penman-Monteith in temperate climates, worse in very humid or very windy places. Commercial agronomy and anything involving water rights still uses the full form.