Matched Precipitation on a Rotor Zone

Why this matters

The lawn with soggy, mossy corners and a brown patch in the middle is usually not a controller problem and not a broken head. It is a zone where every rotor carries the same nozzle, so the corner heads put down four times the water of the head in the middle. No runtime fixes that, because any runtime that waters the middle drowns the corners. The fix is a nozzle choice made head by head, and it is one of the few irrigation repairs that costs a handful of parts and changes how a whole lawn looks within a month.

How to measure what a zone actually does (catch cans, distribution uniformity, runtime math) belongs to 'Irrigation Startup and Audit'. Setting a rotor's arc and radius belongs to 'How to Adjust Arc and Radius on a Gear-Drive Rotor'. This card is the reason the nozzles have to differ.

Precipitation rate, and why arc matters

Precipitation rate is the depth of water a head or a zone puts down per hour, in inches per hour. For any area:

inches per hour = 96.25 x gallons per minute / square feet watered.

The 96.25 converts gallons per minute over square feet into inches per hour (one gallon spread over one square foot is about 1.6 inches deep, and there are 60 minutes in an hour; 1.604 x 60 is 96.25).

A rotor throws water over an arc. A full-circle head waters a full circle; a half-circle head at the same radius waters half of it; a quarter-circle head waters a quarter. If all three carried the same nozzle and passed the same gallons per minute, the quarter would put that water on a quarter of the area, so its depth per hour would be four times the full circle's.

Matched precipitation means each head's flow is in proportion to its arc, so every part of the zone gets the same depth per hour. At the same radius and spacing:

• a half-circle head needs twice the flow of a quarter,
• a full-circle head needs four times the flow of a quarter,
• and any odd arc gets a flow in proportion (a 270 degree head needs three times a quarter).

Spray heads with fixed-arc nozzles are matched by design: a quarter nozzle is built to pass about a quarter of the full-circle nozzle's flow. Rotors are not. Most ship with one nozzle installed, often a mid-size, and carry a tree of others in the cap or the box. Matching a rotor zone is the installer's job, and on many installed systems it was never done.

A matched zone, laid out

A square lawn area 70 by 70 feet, watered by nine rotors set head to head at 35 foot spacing (each head throws to its neighbors). The corner heads turn a quarter circle, the edge heads a half, the center head a full circle. Flows are illustrative; real nozzle flows come from the rotor maker's chart at the zone's operating pressure.

 MATCHED ROTOR ZONE, 70 ft x 70 ft, heads at 35 ft
 Q = quarter (90 deg)  H = half (180)  F = full (360)

  Q 1.0 gpm -------- H 2.0 gpm -------- Q 1.0 gpm
     |                  |                  |
     |                  |                  |
  H 2.0 gpm -------- F 4.0 gpm -------- H 2.0 gpm
     |                  |                  |
     |                  |                  |
  Q 1.0 gpm -------- H 2.0 gpm -------- Q 1.0 gpm
  |<----- 35 ft ----->|<----- 35 ft ----->|

  zone flow: 4 x 1.0 + 4 x 2.0 + 1 x 4.0 = 16 gpm

Zone precipitation rate. 96.25 x 16 gpm / 4,900 sq ft = 1,540 / 4,900 = 0.31 in per hour.

Check it head by head. In a square head-to-head layout, a full-circle head's share of the lawn is one spacing square, 35 x 35 = 1,225 sq ft. A half-circle head's share is half of that (612.5 sq ft) and a quarter's is a quarter (306.25 sq ft).

• Full: 96.25 x 4.0 / 1,225 = 0.31 in per hour
• Half: 96.25 x 2.0 / 612.5 = 0.31 in per hour
• Quarter: 96.25 x 1.0 / 306.25 = 0.31 in per hour

Every head lands the same depth per hour, and the zone figure agrees with each head. That agreement is what matched means.

These are averages over each share. Real heads throw more water at some distances than others, and overlap from neighbors is what evens it out; that is why head-to-head spacing matters as much as the nozzles. How evenly the water actually lands is the catch-can audit's question.

The same zone, unmatched

Now the same nine heads exactly as they often come out of the box: every rotor carrying the same 2.0 gpm nozzle.

• Quarter heads: 96.25 x 2.0 / 306.25 = 0.63 in per hour
• Half heads: 96.25 x 2.0 / 612.5 = 0.31 in per hour
• Full head: 96.25 x 2.0 / 1,225 = 0.16 in per hour

The corners now get four times the center (0.63 against 0.16) and twice the edges. The zone total is 9 x 2.0 = 18 gpm, so the zone's average works out to 96.25 x 18 / 4,900, about 0.35 in per hour, a number that tells you nothing about any part of the lawn.

What it looks like on the lawn. Set the controller to put half an inch on the middle and the corners get two inches: soggy turf, moss, algae on the walk, mower ruts. Set it for the corners and the middle gets an eighth of an inch: drought stress and a brown patch in the center that the customer will blame on grubs or fertilizer. The usual compromise runtime gives both at once.

What the fix is. Re-nozzle the corners down and the center up until flows run in the 1 : 2 : 4 proportion the arcs require, keeping all heads near the radius the spacing needs. A nozzle swap on a rotor is a cap-off, nozzle-out, nozzle-in job on each head, with the arc and radius re-checked afterward. Then the runtime gets set from the matched rate, and the audit confirms it.

Why sprays and rotors never share a zone

Fixed spray heads commonly put down 1.5 inches per hour or more. Rotors, like the matched zone above, commonly run well under 1 inch per hour. On the same valve, they run for the same minutes.

At the matched rotor rate of 0.31 in per hour, half an inch takes 0.5 / 0.31 = 1.6 hours, about 97 minutes. A spray head at 1.5 in per hour running those same 97 minutes puts down 1.5 x 1.6, about 2.4 inches. The spray-watered strip floods and runs off while the rotor area gets its half inch. Shorten the runtime to suit the sprays and the rotor area gets a fraction of what it needs. Mixing them guarantees one area is wrong.

The same logic separates rotors from sprays carrying multi-stream rotary nozzles. Those nozzles sit on spray bodies but put down a much lower rate than a fixed spray, closer to a rotor's. They do not match fixed sprays, and whether they match a given rotor zone is a chart comparison, not an assumption. Mixed rotor models on one zone are a similar problem: different nozzle families and rotation speeds make matching a guess.

What changes the match

Pressure. Every flow figure on a nozzle chart is at a stated pressure. A zone running well below the chart pressure passes less flow and throws shorter, so heads stop reaching their neighbors and the center of each spacing square goes dry first. Check pressure at a head before re-nozzling a zone whose coverage looks wrong everywhere.

Radius reduction. Turning a rotor's radius screw down shortens the throw by breaking up the stream, without cutting the flow much. A head reduced well below its nozzle's natural radius puts the same gallons on less area and its rate climbs. Most makers recommend a smaller nozzle rather than heavy screw reduction; their guide gives the limit.

Irregular layouts. Real lawns are not 70 foot squares. Where spacing differs from head to head, or an edge head turns 120 degrees instead of 180, the head's share of the area is no longer a clean fraction of a square, and matching means sizing each nozzle to its own arc and spacing from the chart. The principle does not change: flow in proportion to the area each head is responsible for.

Slopes and soils. Matched precipitation evens the depth put down. It does not make clay take water faster. A matched zone on a clay slope still needs cycle and soak to keep the water where it lands; see 'Irrigation Controller Programming' and 'Soil Texture by Feel for Lawn Techs'.

Scope

Re-nozzling and adjusting heads is routine service work. Changing zone layout, adding heads or valves, and anything touching the backflow preventer may be licensed work where your state or locality licenses irrigation contracting, and backflow testing is done by a certified tester. Route those at the point they come up.

References

• Rotor and spray manufacturers' nozzle performance charts (flow, radius and precipitation rate at stated pressure; recommended radius reduction limits)
• See related: Irrigation Startup and Audit; How to Adjust Arc and Radius on a Gear-Drive Rotor
• See related: Irrigation Controller Programming; The Slope Zone That Flooded the Curb and Browned the Crest