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Rispri LabsENGINEERING A CIRCULAR FUTURE

Free tool · Water conservation

Irrigation Water Requirement Calculator

Crop water need is reference evapotranspiration (ET₀) × crop coefficient (Kc). Subtract effective rainfall, then divide by irrigation efficiency to get the water you must apply. One acre of wheat at mid season (ET₀ 5 mm/day, Kc 1.15) needs about 26,000 litres a day by drip.

Your inputs

Results update as you type. Example values are filled in.

Mid season Kc values from FAO-56 Table 12. Early growth stages use lower Kc.

mm/day

North India: about 2 to 3 in winter, 6 to 9 in May and June.

mm/day
L/s

Optional. Used to estimate pump hours per day.

How to use this calculator

  1. Enter the area and pick acre, hectare or m².
  2. Choose the crop to load its mid season crop coefficient, or type your own Kc.
  3. Enter ET₀ for your location and season, and any effective rainfall.
  4. Choose the irrigation method to apply its efficiency.

How is irrigation water requirement calculated?

ETc = ET₀ × Kc

Gross irrigation (mm/day) = (ETc − effective rain) ÷ efficiency

Volume (L/day) = gross depth (mm) × area (m²)

This is the FAO-56 single crop coefficient method. ET₀ comes from weather data using the Penman-Monteith equation; IMD and state agriculture departments publish typical values.

Worked example

With irrigated area 1 acre, crop: Wheat (Kc 1.15), crop coefficient kc 1.15, reference evapotranspiration et₀ 5 mm/day, effective rainfall 0 mm/day, irrigation method: Drip (90%), days in the period 30, pump or supply flow 5 L/s:

Irrigation water per day: 25,855 litres

The calculator also gives crop water use etc 5.75 mm/day; net irrigation need 5.75 mm/day; gross water to apply 6.39 mm/day.

Assumptions and limits

  • Uses one Kc for the whole period; real Kc rises from planting to mid season and falls near harvest.
  • Does not include leaching for saline water or water for land preparation (puddling for rice can add 150 to 250 mm).
  • Efficiency values are typical field figures.

Frequently asked questions

How much water does wheat need per acre?

Over a season wheat uses about 400 to 450 mm of water. At mid season with ET₀ of 5 mm/day it uses about 5.75 mm/day, which is 23,000 litres per acre per day before losses.

What is ET₀?

ET₀ is reference evapotranspiration: water lost from a well watered grass surface under local weather. Multiplying it by a crop coefficient gives the crop water use.

How much water does drip irrigation save?

Drip delivers water to the root zone at about 90% efficiency compared with about 60% for flood irrigation, so it typically cuts applied water by 30 to 50% for the same crop.

References

  • FAO Irrigation and Drainage Paper 56, Crop Evapotranspiration (Allen et al., 1998)
  • ICAR, crop water requirement guidelines

Built and maintained by the Rispri Labs engineering team. Last updated .