Irrigation Zoning vs Distribution Uniformity

Frequently asked questions

Why does one dry spot cost water across the whole zone?

Because a zone has one run time and it is set by the worst place in it. Everything on a single valve runs for the same number of minutes, so the controller is set to whatever keeps the driest area alive — nobody sets it to the average, because the average is not what generates the complaint. If the distribution is uneven, that means the areas already receiving more than they need receive proportionally more still, every cycle. So poor uniformity does not present as a dry patch; it presents as a high water bill with a dry patch anyway. Improving the uniformity lets the run time come down toward what the average area needs, which is where the saving comes from, and it is usually a larger saving than any change to the schedule.

How is a catch-can test done?

By placing identical containers in a grid across the zone, running that zone alone for a measured time, and recording the depth collected in each. Straight-sided containers of the same size are used so the depths are comparable, spaced on a regular grid dense enough to capture the variation between heads, and the test is run in still conditions because wind distorts the result substantially. The output is both an average application rate — which converts run time into depth applied, and is what a schedule should actually be built on — and a measure of evenness, usually expressed by comparing the driest quarter of the cans against the overall average. The test takes an hour and is the only way to know what the system is doing rather than what it was designed to do.

Why must head types not be mixed on one valve?

Because they apply water at different rates, and a zone has only one run time. A spray head typically delivers its water much faster than a rotor covering the same area, so a run time long enough for the rotor zone drowns the spray area, and one short enough for the sprays starves the rotors. There is no setting that serves both, which is why mixing them on one valve is one of the few outright errors in irrigation design rather than a matter of degree. The related and subtler requirement is MATCHED PRECIPITATION within a type: nozzles are selected so that a half-circle head delivers half the flow of a full-circle head at the same spacing, because otherwise the corners and edges receive a different depth from the middle.

What spacing gives good uniformity?

Close enough that each head's pattern overlaps its neighbours substantially, because sprinkler output is not even across its own radius — it falls off toward the edge of the throw. The standard practice is head-to-head coverage, where each head's spray reaches the adjacent heads, so that the thin outer edge of one pattern is supplemented by the thin outer edge of the next and the sum is roughly even. Spacing heads at the full radius without overlap produces reliably dry zones between them, which no amount of run time corrects. The spacing also has to be reduced where wind is a regular factor, since wind shortens the effective throw on one side and extends it on the other, degrading a pattern that would be uniform in still air.

How does pressure affect uniformity?

At both ends, and in different ways. Too little pressure and a head does not achieve its design radius or break the stream up properly, so water falls short and close in — which is the commonest cause of dry areas between heads. Too much and the stream atomises into fine droplets that drift on any breeze and evaporate, so a substantial share of the water never reaches the ground where it was aimed. Within a zone, what matters is the VARIATION: pressure drops along a lateral, so heads at the far end can be operating meaningfully below those at the valve, giving a systematic gradient in application across the zone. Pressure-regulating heads or stems address that directly and are one of the most effective uniformity improvements available on an existing system.

What degrades uniformity over time?

Everything physical about the installation. Heads settle and tilt so their arc points partly at the sky or the ground; nozzles clog with grit and their patterns distort; risers fail to retract fully or retract too far; arcs drift out of adjustment so sectors overlap or leave gaps; and growth is the quiet one — a shrub or a maturing grass edge that has reached across a head blocks part of its pattern and creates a dry shadow behind it. None of these is visible from a controller and most are not obvious while the system is running, because a partly blocked pattern still looks like a working sprinkler. Periodic re-testing with catch cans, or at minimum a walk-through with the zone running, is what finds them.

Is drip irrigation more uniform?

Generally yes, and for a structural reason: it applies water at the root zone at a low rate with no throw to be distorted by wind, obstruction or pressure variation in the same way. Pressure-compensating emitters hold their output across a range of pressures, so long runs and slopes do not produce the gradient a sprinkler lateral does, and losses to evaporation and drift are largely removed. Its failure modes are different rather than absent — emitters clog, which is invisible until a plant declines, so filtration and flushing matter; lines are damaged by digging and by rodents; and coverage must actually reach the root zone of each plant rather than a point beside it. For beds, borders and individual plants it is normally the better choice; for turf it is a specialist installation.

Does better uniformity let you water less often?

It lets you water less in total, and the frequency question is answered separately by the soil. Uniformity determines how much of what you apply is useful; the soil's infiltration rate and its water-holding capacity determine how much can be absorbed in one go and how long it lasts. A common and useful pattern is fewer, longer, deeper applications rather than frequent light ones, because that encourages deeper rooting and a plant that tolerates drought better — but a long application on a slow soil runs off, which is what cycle-and-soak scheduling addresses by splitting the run into shorter passes with gaps for infiltration. So the honest sequence is to fix the uniformity, measure the actual application rate with cans, and then set frequency and duration from the soil.