Half the ground, the same nozzle
The form opens with a fairly ordinary zone: six full circle rotors at 3.0 GPM, eight half circles at 2.0, four quarter circles at 1.5, on 30 ft triangular spacing. A full circle head owns 779 sq ft on that grid, a half circle owns 390, a quarter owns 195.
Divide the flow by the ground and the three groups land at 0.37, 0.494 and 0.741 inches an hour. The corners are getting water exactly twice as fast as the middle of the lawn. Nothing about that is subtle once it is written down, and nothing about it is visible when you are standing in the garden watching the zone run, because all three groups look like they are working.
One valve, one clock
Thirty minutes on that zone puts 0.185 in on the full circle ground, 0.247 on the half circles and 0.37 on the corners. If the target were half an inch, the full circles need 81 minutes and the corners need 40. Set the clock for the full circles and the corners take a full inch. Set it for the corners and most of the lawn takes a quarter of what it wanted.
The reason this cannot be scheduled away is that the controller has one output per valve. It can decide how long the valve is open. It cannot decide how long each head is open, and there is no seasonal adjustment, cycle-and-soak setting or smart controller feature that separates heads sharing a solenoid.
Matching, and what it costs
The page prints the nozzle flow each arc would need to land on the reference rate. Matched to the full circles, the half circles want 1.5 GPM instead of 2.0 and the quarters want 0.75 instead of 1.5, and the whole valve then draws 33 GPM instead of 40 — 17 percent less flow on the same zone, which is sometimes enough to let a zone that was split for flow reasons be recombined.
Matching downwards like that is the cheap direction, because smaller nozzles are the same price as larger ones and the zone gets easier to feed. Matching upwards is not free: it raises the flow the valve and the lateral have to carry, and a zone already near the limit of its supply cannot absorb it. Choosing which group to match to is the actual decision, which is why the form offers the first group, the flow-weighted average and the slowest group rather than picking one for you.
What the grid share is not
The area behind every number here is the head share of a regular grid — the definition matched precipitation is written against. A real corner head throws into a bed, over a path, and against a fence that bounces some of it back, and the ground it actually wets is not a tidy quarter of 779 square feet. So treat the ratio as a design check on the nozzle selection, not as a measurement of what falls on the grass. Cups on the ground settle the second question and often disagree with the first, and where they disagree the cups are right.
Questions people ask
What does matched precipitation rate actually mean?
That every head on one valve applies the same depth per minute over the ground it is responsible for. Since a half circle covers half the ground of a full circle at the same spacing, it has to carry half the nozzle flow to match. When the arcs are matched, one run time serves the whole zone. When they are not, the run time is a compromise that leaves some ground wet and some dry no matter what you set.
Can I fix mismatched arcs with the controller?
No. Every head on a valve opens and closes together, so one run time applies to all of them. Cycle-and-soak, seasonal adjustment and weather-based scheduling all scale the whole zone by the same factor and leave the ratio between the groups exactly where it was. The fixes are matched nozzles or separate valves.
Should I match up to the fastest group or down to the slowest?
Downwards costs less flow and usually less money, because it means smaller nozzles on the short-arc heads and a lower total draw through the valve — on the defaults here, 33 GPM instead of 40. Matching upwards raises the flow the supply has to deliver and can push a zone past what it can be fed. The form lets you set the reference either way and prints the resulting zone flow so you can see what each choice costs.
Where do I get the nozzle flow figure?
The nozzle performance chart for that exact nozzle, read at the pressure that reaches the head rather than the pressure at the valve. Flow rises with pressure, so a chart read at the wrong row is wrong in the same direction for every head and quietly shifts the whole comparison. Nothing on this page supplies a nozzle flow.
Does this account for overspray onto paths and beds?
No. It uses the head share of a regular grid, which is how matched precipitation is defined, and a real perimeter head throws over boundaries and into things. That is the difference between a design check and a measurement. Cups on the ground measure what actually lands and are the arbiter when the two disagree.