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Crop Water Requirement

Free Crop Water Requirement calculator with clear step-by-step results.

About Crop Water Requirement

Applies the standard FAO approach: reference evapotranspiration multiplied by a crop coefficient gives actual crop water use, effective rainfall is subtracted to get the net requirement, and dividing by system efficiency gives the gross depth you must actually pump. One millimetre over a hectare is ten cubic metres.

Formulas

Crop water use

ETc = ET0 x Kc x days

Gross irrigation

Gross mm = (ETc - effective rainfall) / irrigation efficiency

Worked example

With the default values below, Crop Water Requirement returns Irrigation depth to apply of 249.8. Change any input above and every figure here is recalculated in your browser.

Inputs used

Reference evapotranspirationminimum 0.1 mm/day
5.5 mm/day
Crop coefficient (Kc)accepts 0.1–1.5
1.05
Length of the growth stageaccepts 1–365 days
45 days
Effective rainfall over the stageminimum 0 mm
60 mm
Irrigation system efficiencyaccepts 20–100 %
80 %
Field areaminimum 0.01 ha
5 ha

Results

Irrigation depth to apply
249.8 mm
Crop water use over the stage
259.9 mm
Net irrigation requirement
199.9 mm
Water volume for the field
12,492.2 m3

Step by step

  1. 1Values used
  2. 2Crop water use
  3. 3Gross irrigation
  4. 4Irrigation depth to apply= 249.8 mm
  5. 5Crop water use over the stage= 259.9 mm
  6. 6Net irrigation requirement= 199.9 mm
  7. 7Water volume for the field= 12,492.2 m3

Assumptions and limits

  • The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Where do I find Kc for my crop?

FAO Irrigation and Drainage Paper 56 tabulates it by crop and growth stage. Values run from about 0.3 at emergence to 1.15-1.2 at full canopy for many field crops.

What is effective rainfall?

Only the fraction that infiltrates and stays in the root zone. On heavy soils or steep ground, well under half of a heavy storm may be effective.

Related Calculators

Crop Water Requirement – Calculate Irrigation depth to apply