Evapotranspiration Calculator
Hot and still, or cool and windy — same total, different fix.
Work out Evapotranspiration. Hot and still, or cool and windy — same total, different fix. Counts the thing everyone forgets to count.
Crop water use per day
5.94 mm
Reference ET₀ of 5.94 mm × Kc 1 · 5.94 mm to apply
FAO-56 Penman-Monteith is the reference method, and it splits into a radiation term and an aerodynamic term. Reporting both separately is useful because a hot still day and a cool windy one can reach the same total for entirely different reasons — and the response differs accordingly. The crop coefficient converts reference demand into a particular crop's demand, and it changes through the season: low at establishment, peaking at full canopy, falling again at senescence. A single figure across a whole season is a rough approximation. Only part of rainfall is effective. Runoff and drainage below the root zone take a share, and heavy rain on dry ground contributes far less than the gauge suggests — which is why irrigation scheduling from a rain total alone over-waters after storms and under-waters after drizzle.
How the Evapotranspiration Calculator works
Reference evapotranspiration by FAO-56 Penman-Monteith, with the radiation and aerodynamic terms reported separately, a crop coefficient applied, and irrigation need after effective rainfall.
Also known as: how much to water my crop · penman monteith et0 · irrigation scheduling calculator · crop water requirement per day
Frequently asked questions
What is evapotranspiration?
Water lost from a surface by evaporation from soil plus transpiration through plants. Reference ET₀ is the demand over a standard short grass surface, which crop coefficients then scale.
Why report the radiation and aerodynamic terms separately?
Because a hot still day and a cool windy one can reach the same total for entirely different reasons — and shade helps one while a windbreak helps the other.
What is a crop coefficient?
The ratio of a crop's water use to the reference. It changes through the season — low at establishment, peaking at full canopy, falling at senescence — so a single figure is a rough approximation.
How much irrigation do I need?
Crop ET minus effective rainfall. One millimetre is exactly one litre per square metre, which makes the conversion from depth to volume trivial.
Why is only some rainfall effective?
Runoff and drainage below the root zone take a share. Heavy rain on dry ground contributes far less than the gauge suggests, which is why scheduling from a rain total over-waters after storms.
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