Crops use different amounts of water at different stages: a young tomato uses little, a full-canopy plant in fruit uses more than a lawn. The standard method multiplies reference evapotranspiration (ET₀) by a crop coefficient (Kc).
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Choose a crop and stage, and enter ET₀. The calculator gives the daily and weekly water use and what is left to irrigate after rain.
ETc = Kc × ET₀ weekly use = ETc × 7 net irrigation = weekly use − effective rain volume = depth × area
Kc values come from the FAO-56 guidelines and assume a healthy, well-watered crop.
A tomato crop at full canopy (Kc 1.15) with ET₀ of 5 mm uses about 5.8 mm a day: 57.5 L a day on 10 m², 403 L a week, or 303 L after 10 mm of rain.
A factor that adjusts reference evapotranspiration for a particular crop and growth stage.
Typically 4–7 mm a day, or 4–7 litres per square metre (0.1–0.2 gallons per square foot).
Reference evapotranspiration: the water used by a well-watered grass surface under local weather.
Larger canopies intercept more sun and transpire more water than young plants.