How to use it
- Choose the method and enter the date and latitude (negative in the southern hemisphere).
- Enter the day’s maximum and minimum temperatures. For Penman-Monteith also enter the humidity range, the wind speed at 2 m, the hours of bright sunshine and the elevation.
- Read ET₀ and, with a crop coefficient, the crop’s water use.
How it works
Both methods first compute the extraterrestrial radiation Ra from the latitude and the day of the year, along with the day length N (FAO-56 equations 21–25 and 34).
- Hargreaves: ET₀ = 0.0023 × (Tmean + 17.8) × √(Tmax − Tmin) × 0.408 × Ra.
- Penman-Monteith (daily, soil heat flux 0): ET₀ = [0.408 Δ Rn + γ × 900 ÷ (T + 273) × u₂ × (es − ea)] ÷ [Δ + γ (1 + 0.34 u₂)].
Solar radiation comes from sunshine hours (Rs = (0.25 + 0.5 n/N) Ra), the actual vapour pressure from the humidity range, and the psychrometric constant from the air pressure at your elevation.
Example
FAO-56 example 18: 6 July at latitude 50.8° N and 100 m, Tmax 21.5 °C, Tmin 12.3 °C, humidity 63–84%, wind 2.078 m/s and 9.25 hours of sunshine. The calculator gives Ra = 41.1 MJ/m²/d, a day length of 16.1 h, Rn = 13.3 MJ/m²/d, a vapour pressure deficit of 0.59 kPa and ET₀ = 3.9 mm/d, matching the book. The same day by Hargreaves gives 4.1 mm/d.
In US units the defaults (70.7 °F and 54.1 °F, 4.7 mph, 330 ft) give 0.15 in/d, or about 1.07 in over a week of such days.
Tips and common mistakes
- Use sunshine hours, not daylight hours: the value is the time with direct sun, and it cannot exceed the day length.
- Daily ET₀ swings with the weather. Summing several days or using weekly averages is better for planning irrigation.
- Use the irrigation calculator to turn the water depth into a watering time.