Why mixing rotors and spray heads in one zone wastes water

Stand and watch a zone run and it looks fine. Everything is spinning, water is landing, nothing is obviously broken. Then half the zone is dry in August and the other half has fungus, and the run time gets increased, and both problems get worse.

Very often the cause is that the zone contains two kinds of head that put down water at completely different rates.

The numbers behind it

Two families of sprinkler cover a lawn:

• Spray heads — fixed fan pattern, throw eight to fifteen feet, and empty their water fast. Typically somewhere around 1.5 inches per hour.
• Rotors — a stream that sweeps back and forth, throws twenty-five feet or more, and applies water far more slowly. Often in the region of 0.4 to 0.6 inches per hour.

Those are roughly three to one. Put both on the same valve and they get the same run time, because a zone has exactly one run time. Whatever number you choose, one group gets about three times what the other does.

Run long enough for the rotors and the spray areas are drowning: constantly wet leaves, fungus, shallow roots, runoff down the drive. Run short enough for the sprays and the rotor areas never wet below the surface, so the turf browns first in every dry spell. There is no run time that serves both, which is why turning the dial never resolves it.

How to check your own zones

Run one zone and walk it, and note what kind of head each one is — a fan of spray that never moves, versus a stream that sweeps. If a single zone contains both, that zone cannot water evenly.

The can test settles it. Straight-sided cans across the zone, fifteen minutes, measure. In a mixed zone the cans near the spray heads will hold roughly three times what the cans near the rotors do, and you can see the fault in a ruler reading.

What fixing it involves

Honestly: usually re-valving, which means separating the two groups onto different zones. That is real work — new valve, wire, sometimes a new run of pipe — and on a system with several mixed zones it can amount to a partial redesign.

There is a cheaper middle path that works surprisingly often. Some rotor nozzles are available in high-efficiency versions with rates much closer to spray heads, and some spray nozzles come in low-precipitation versions that slow them down. Swapping nozzles on both sides can narrow three-to-one down to something a single run time can live with. Not perfect, but a fraction of the cost, and no trenching.

Why it happens in the first place

Almost never carelessness at install. Usually the lawn changed shape after the system went in — a bed was widened, a patio was added, a tree grew and shaded half a zone — and a head was added to cover the new geometry using whatever was closest at hand. Ten years of small sensible fixes is how a clean design becomes a mixed zone.

Worth asking about before you accept "your lawn just needs more water". More water is what a mixed zone always seems to need, and never the answer. How we zone a system covers what we look at first.