How do I calculate irrigation water requirement for a crop?

Multiply reference evapotranspiration (ETo) by the crop coefficient (Kc) for the growth stage to get ETc, then divide by application efficiency for the gross amount to apply.

How do I convert mm per day to m³ per hectare?

1 mm of water over 1 hectare equals 10 m³, so multiply the depth in mm by 10 and by the area in hectares.

Does this size pumps or pipes?

No. It gives the required flow only; pump head, pipe sizing and zoning need an irrigation engineer's design.

Irrigation water requirement and flow calculator

Answer first: crop water use ETc = Kc × ETo (FAO-56). Gross depth = ETc ÷ application efficiency; daily volume (m³) = gross depth (mm) × 10 × area (ha); required flow = daily volume ÷ irrigation hours.

Method and assumptions

ETo, Kc, efficiency and hours are your inputs: take ETo from a local weather station for the design month and Kc for the crop stage from FAO-56 Table 12 or your agronomist. The result ignores effective rainfall, leaching requirement and soil storage; add a leaching fraction if water is saline.

Formula source: ETc = Kc × ETo — FAO Irrigation and Drainage Paper 56 (Allen et al., 1998), https://www.fao.org/4/x0490e/x0490e00.htm. 1 mm over 1 ha = 10 m³.

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Next steps

Questions

How do I calculate irrigation water requirement for a crop?: Multiply reference evapotranspiration (ETo) by the crop coefficient (Kc) for the growth stage to get ETc, then divide by application efficiency for the gross amount to apply.
How do I convert mm per day to m³ per hectare?: 1 mm of water over 1 hectare equals 10 m³, so multiply the depth in mm by 10 and by the area in hectares.
Does this size pumps or pipes?: No. It gives the required flow only; pump head, pipe sizing and zoning need an irrigation engineer's design.

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