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Irrigation Water Requirement Calculator

Estimates how much irrigation a crop needs for a period and the flow to deliver it.

mm/day

From a local weather service, or the evapotranspiration calculator.

Depends on the crop and its growth stage. FAO Irrigation and Drainage Paper 56 lists values. Local advice is better still.

days

The rain that stays in the root zone, not all that fell.

%

The share of water applied that reaches the root zone. Commonly about 90% for drip, 75% for sprinklers and 60% for surface irrigation.

hours
Water to apply
—

Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.

Net crop need after rain —
Crop water use — mm/day
Volume over the area —
Flow needed —
Reading this —

The formula

ETc = ET₀ × Kc net = ETc × days − useful rain gross = net ÷ efficiency volume = gross × area

This is the FAO crop coefficient approach. One millimetre over one hectare is 10 m³.

What to watch out for

Check that the flow is within what your source and pipework can supply, and split the area into zones if it is not. Apply no more at once than the soil can hold in the root zone.

Frequently Asked Questions

How much water does a crop need in a week?

With ET₀ at 5 mm/day and Kc at 1, it uses 35 mm. After 10 mm of useful rain and at 75% efficiency, apply 33.3 mm.

How many cubic metres is 1 mm on a hectare?

10 m³.

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