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· 4 min read
How to Set Irrigation Run Times Correctly
Manesh Jayawardhana
CIO & Co-founder
The controller is set to twenty minutes a zone because that’s what it was set to when you moved in. Whether that delivers five millimetres or twenty-five depends on the sprinkler heads, the water pressure and how worn the nozzles are — none of which anyone checked.
Irrigation timing is a two-step calculation, and both steps need a number you have to measure rather than look up.
Step one: measure your application rate
How many millimetres of water your system puts down per hour. Not the manufacturer’s figure — yours.
Put several straight-sided containers around the zone. Tuna tins work well. Run the zone for fifteen minutes, then measure the depth in each and average it. Multiply by four for the hourly rate.
Two things come out of that test. The rate, which is what the calculation needs. And uniformity — if one container has 8 mm and another has 3 mm, part of the zone is being over-watered while part is under-watered, and no run time fixes that. It’s a head spacing, pressure or nozzle problem, and it matters more than timing.
run time = target depth ÷ application rate
25 mm a week at 12 mm/hour is 2 hours 5 minutes a week.
Step two: check against infiltration rate
Soil absorbs water at a limited rate, and it varies enormously by texture.
Sandy soil takes water quickly — often 20 mm an hour or more. Clay is far slower, sometimes only 5 mm an hour, and compacted clay slower still.
If your application rate exceeds the soil’s infiltration rate, the excess runs off. It doesn’t matter how long you run the zone; water arriving faster than the soil accepts it goes down the drive.
The fix is cycle and soak: split the run into shorter cycles with time between them for the water to absorb. Three cycles of 42 minutes with an hour between beats one run of 2 hours 5 minutes on clay, and delivers the same depth with none wasted.
| Soil | Rough infiltration | If your rate is higher |
|---|---|---|
| Sandy | 20+ mm/hour | Usually fine in one run |
| Loam | 10-20 mm/hour | May need splitting |
| Clay | 5 mm/hour or less | Split into cycles |
| Slope, any soil | Effectively lower | Split regardless |
Deep and infrequent
For established plants, watering deeply and less often generally encourages deeper rooting, which makes them more resilient in dry spells. Frequent shallow watering keeps roots near the surface, where they dry out first.
The exceptions are seedlings and shallow-rooted crops, which need lighter and more frequent watering because their roots simply aren’t deep enough to reach stored moisture.
Common mistakes to avoid
- Using the manufacturer’s application rate instead of measuring — nozzle wear and pressure both change it.
- Ignoring poor uniformity revealed by the catch test.
- Running long on clay and watching half of it run off.
- Watering at midday, when evaporation loss is highest.
- Never re-measuring after changing heads or adding a zone, which changes pressure across the whole system.
How to do it with Irrigation Requirement Calculator
The Irrigation Requirement Calculator converts depth to run time and handles cycle splitting.
- Measure your application rate with catch cans rather than trusting the specification.
- Divide the target depth by that rate for total run time.
- Split into cycles if your rate exceeds the soil’s infiltration rate.
- Re-measure after any change to heads, nozzles or pressure.
Other garden and irrigation tools are in the tools directory.
Frequently asked questions
How do I measure application rate?
Put straight-sided containers around the zone, run it fifteen minutes, and measure the depth collected. Multiply by four for the hourly rate. It’s usually different from the specification, and it reveals uneven coverage too.
Why split the run into cycles?
Because soil absorbs at a limited rate. Applying faster produces runoff, so several shorter cycles with soak time deliver the same depth with none wasted.
Is deep and infrequent better than daily?
For established plants, generally yes — it encourages deeper roots. Seedlings and shallow-rooted crops need lighter, more frequent watering.
Final thought
Do the catch-can test once. It gives you the number every other irrigation decision depends on, and it usually reveals a coverage problem nobody knew about.