Splitting irrigation into zones does more than divide a farm up. Done right, it is what lets a modest main pipe serve a large area. Done wrong, it is why the far end of every block underperforms.
What a main can carry
Water in a main should travel at about 1.5 m/s. Push past about 2 m/s and friction losses climb steeply, and closing a valve starts producing water hammer, which cracks fittings over a season.
| Pipe | Approx. bore | Capacity |
|---|---|---|
| HDPE 32 mm | ~28 mm | ~3,325 L/h |
| PVC 1¼ in | ~35 mm | ~5,195 L/h |
The two differ by more than 50 %, which is why pipe material and bore are measured at the site survey rather than assumed. A working design figure of 4,000 L/h sits between them.
Zones must irrigate one at a time
With one valve open at a time, the demand on the main is one zone’s worth — never the whole farm. Open two together and you overrun the pipe: pressure falls, the emitters furthest from the manifold run starved, and the plants at that end quietly underperform all season without anything appearing to break.
Sizing a zone
Each zone must fit within the main’s ceiling on its own. At 2.5 plants/m² with one 2 L/h dripper per plant, each square metre draws 5 L/h, so a 4,000 L/h main supports about 800 m² per zone.
| House floor area | Per zone | Peak flow, one zone | Verdict |
|---|---|---|---|
| 800 m² | 200 m² | 1,000 L/h | Within capacity |
| 1,600 m² | 400 m² | 2,000 L/h | Within capacity |
| 2,400 m² | 600 m² | 3,000 L/h | Within capacity |
| 3,200 m² | 800 m² | 4,000 L/h | At the ceiling |
| 4,000 m² | 1,000 m² | 5,000 L/h | Over — needs 5–6 zones |
Above the ceiling the answer is more zones, not a bigger pipe. At a peak demand of about 5 L/m² per day, each zone needs roughly one hour a day, so four zones need about four hours — there is ample room in the day, however large the house.
An automatic controller makes “one zone at a time” a rule rather than a habit, so it cannot be broken by accident at 6 am.