Irrigation Water Requirement

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

About Irrigation Water Requirement

Converts an agronomic water depth into controller runtimes. Net demand is grossed up for distribution uniformity - no sprinkler system waters evenly, so you must over-apply to give the driest area enough - then divided by system flow and split across the cycles you schedule.

Formulas

Gross water required

Litres = area m2 x depth mm / distribution uniformity

Runtime per cycle

Minutes = gross litres / flow rate / cycles per week

Worked example

With the default values below, Irrigation Water Requirement returns Runtime per watering cycle of 98.8. Change any input above and every figure here is recalculated in your browser.

Inputs used

Irrigated zone areaminimum 0.1 m2: 400 m2
Water depth required per weekminimum 0.1 mm: 25 mm
System flow rateminimum 0.1 L/min: 45 L/min
Distribution uniformityaccepts 30–100 %: 75 %
Watering cycles per weekaccepts 1–21: 3

Results

Runtime per watering cycle: 98.8 min
Water applied per week: 13,333 L
Total runtime per week: 296.3 min
System precipitation rate: 6.75 mm/h

Step by step

1Values used
2Gross water required
3Runtime per cycle
4Runtime per watering cycle= 98.8 min
5Water applied per week= 13,333 L
6Total runtime per week= 296.3 min
7System precipitation rate= 6.75 mm/h

Assumptions and limits

The result is derived only from the 5 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 2 decimal places, 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

What is a good distribution uniformity?

Above 80% for a well-designed drip system, 70-75% for good rotor sprinklers and often under 60% for mismatched spray heads. Improving uniformity saves more water than reducing runtime.

Why does precipitation rate matter?

If it exceeds the soil's infiltration rate, water runs off instead of soaking in. Heavy clay may accept only 5-8 mm per hour, which forces short, repeated cycles.