Crop Water Requirement Calculator: Daily Water Use by Crop
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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What this calculator does
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.
How to use the crop water requirement calculator
- Choose a crop to load its coefficients, or enter your own.
- Choose the current growth stage.
- Enter today's or the average ET₀.
- Enter effective rain for the week.
- Irrigate the net amount shown.
How the calculation works
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.
Worked example
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.
Assumptions and limitations
- Kc values are for field conditions; small gardens with walls or heat islands can use more.
- Effective rain is less than total rain on hot or heavy storms.
- Add a little extra for irrigation inefficiency.
Frequently asked questions
What is a crop coefficient?
A factor that adjusts reference evapotranspiration for a particular crop and growth stage.
How much water does a vegetable garden use per day in summer?
Typically 4–7 mm a day, or 4–7 litres per square metre (0.1–0.2 gallons per square foot).
What is ET₀?
Reference evapotranspiration: the water used by a well-watered grass surface under local weather.
Why does water use change with growth stage?
Larger canopies intercept more sun and transpire more water than young plants.