Irrigation water requirement calculator
Irrigation water requirement calculator
The volume a depth of water over a field really costs, once the losses between the source and the root zone are allowed for.
How this is calculated
Net volume = depth x area x 10 cubic metres. Gross = net / (efficiency / 100), and hours = gross / discharge.
- The efficiency divides and does not subtract: at 60 per cent, water that never reached the root zone still had to be lifted and still carried its share of the pumping cost.
- Running time assumes the discharge holds steady for the whole of it. A borehole that drawdowns, a canal whose turn closes or a solar pump whose output follows the sun all give a shorter useful day than this line implies.
A schedule says forty millimetres. The pump has to lift more than that, because water that never reaches the roots still cost money to raise. This page separates the two figures and the running time that follows.
An efficiency divides; it does not subtract
One millimetre of water over one hectare is ten cubic metres — a geometric identity, not a crop constant, which is the only reason this page can convert a millimetre schedule without knowing what is growing.
The efficiency figure is the share of the water applied that is stored in the root zone. At 60 per cent the rest is percolated past the roots, lost from the field surface, or left in a channel — and every litre of it was lifted. Dividing by 0.60 rather than taking 40 per cent off is what puts the loss on the electricity bill where it belongs.
Worked through
Forty millimetres over 1.5 ha is 40 × 1.5 × 10 = 600 m³ of water in the field. At 60 per cent efficiency the source has to supply 600 ÷ 0.6 = 1,000 m³, which is a million litres and 400 m³ that the crop never saw.
A borehole discharging a steady 25 m³ an hour runs for 1,000 ÷ 25 = 40 hours to deliver that. The hours line is the arithmetic; whether the source holds that discharge for forty hours is a question about the aquifer, the canal turn or the sun.
Questions readers ask
What efficiency figure should I enter?
One for the method you are actually using, from a source you can name — a field measurement of the water applied against the water stored, or the design figure for the system as installed. It is a property of the method and its condition, not of the crop, and a value copied from an unrelated page changes every number below it.
Why is the running time longer than the schedule suggests?
Because a schedule in millimetres is usually the amount to be stored in the root zone, and the time is set by the amount that has to leave the source. At 60 per cent the second figure is a sixth more than half the first, and that difference is what a turn, a daylight window or a borehole recovery rate has to fit.
Does this page know how much the pumping costs?
No, deliberately. A tariff is a published rate that varies by state, by season and by whether the feeder is agricultural, and it changes with a regulatory order rather than with your field. Take the volume or the hours to the calculator that holds your own tariff.
My field is not a rectangle. Does the area still work?
It works if the area is right. A depth in millimetres over a hectare is ten cubic metres whatever the shape, so an irregular plot surveyed to 1.5 ha needs no correction here — only a figure you are willing to stand behind.
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