Evapotranspiration Calculator (Hargreaves)
Estimates how much water a reference grass crop loses each day when all you have is temperature.
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The formula
ET₀ = 0.0023 × Ra × (Tmean + 17.8) × √(Tmax − Tmin)
This is the Hargreaves-Samani equation, with temperatures in °C and Ra the radiation at the top of the atmosphere expressed in mm/day. Ra comes from the latitude and date, using the FAO-56 equations.
What to watch out for
The daily temperature range stands in for cloud and humidity, since clear days tend to have wide ranges. Multiply ET₀ by a crop coefficient to get the use for a particular crop, then take it to the irrigation water requirement calculator.
Frequently Asked Questions
What is ET₀?
Reference evapotranspiration: the water used by a well-watered, short grass surface. Crops are compared with it through a crop coefficient.
What does the equation give in a summer example?
At 52° N on 21 June, with 22 °C and 11 °C, about 4.5 mm/day.