When pressure drops suddenly across the whole system, the cause is something shared: the water supply itself, a mainline leak, a partly closed shutoff or master valve, or a clogged filter or backflow device. When it drops on one zone only, the cause is on that zone. That single distinction — everything versus one zone — splits the diagnosis in half before anyone measures anything.
Pressure and flow are not the same thing
This distinction is worth getting straight, because almost every low-pressure complaint is actually about one or the other and the fixes differ completely.
Pressure is force, measured in pounds per square inch. It is what makes a rotor turn and throws spray to its design radius. Flow is volume over time, measured in gallons per minute. It is what determines how many heads a zone can operate at once.
A system can have plenty of pressure and not enough flow, in which case each head sprays with decent force but adding heads makes everything worse. It can have plenty of flow and not enough pressure, in which case water arrives in volume but nothing throws properly. And a leak reduces both, because escaping water consumes flow and drops the pressure available downstream.
When someone says "low pressure", what they have usually observed is heads not throwing as far as they used to, which is a pressure symptom. Heads that throw fine individually but not when several run together is a flow symptom.
Whole-system causes
On well-fed and lake-fed properties — common around Lake Lanier, and on acreage in Milton, Braselton and Dawson County — the pump side is the first place to look rather than the last. A waterlogged pressure tank or a failing pressure switch produces exactly the symptoms people describe as an irrigation problem, and no amount of work on the valves and heads will touch it.
| Cause | How it is identified |
|---|---|
| Reduced supply pressure from the provider or well | Measure static pressure at a hose bib with nothing running and compare against what the system was designed for |
| Mainline leak | Meter turns with the controller off; often a soggy area unrelated to any head |
| Irrigation shutoff or master valve partly closed | Visual check — a gate valve backed off a few turns, or a ball valve not fully open |
| Clogged filter or screen on the supply | Inspection; common on lake-fed and well-fed systems |
| Failing backflow assembly or a fouled check | Pressure measured upstream and downstream of the device |
| Pump or pressure tank fault on well and lake-fed systems | Pressure switch, tank charge and pump behaviour checked directly |
| A pressure-reducing valve that has drifted or failed | Measured pressure well below supply pressure with no leak present |
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What changed, and when
- Establish whether it is sudden or gradual. Sudden means something broke or something was closed. Gradual over seasons suggests wear, scale, filter fouling, or heads added over time.
- Establish whether it is every zone or one. Run each zone and watch. One weak zone among healthy ones points at that zone’s valve, lateral or head count, not at the supply.
- Check the meter with the system off. A turning meter with the controller off means a mainline leak or a valve not sealing — either of which steals pressure from everything.
- Check the obvious valves. Irrigation shutoff, master valve, and any flow control stems that a previous repair may have left closed down. This costs nothing and resolves a surprising share of cases.
- Consider recent work anywhere on the property. Plumbing work, a new pressure-reducing valve at the house, landscaping, fencing. Anything that touched the supply or the pipe.
- Measure rather than estimate. Static pressure with nothing running, then operating pressure with a zone running, tells you whether the system has a pressure problem, a flow problem, or a leak.
Why a booster pump is usually the wrong answer
People reach for more pressure, and occasionally that is correct — a genuinely under-supplied property on a long service line may need it. Far more often it treats a symptom and creates new problems.
If the real cause is a mainline leak, a booster pump pushes more water out of the break. If it is a partly closed valve, the pump fights the restriction. If it is a zone with more heads than the supply can serve, the pump raises pressure into a flow-limited zone, which stresses pipe and fittings, drives misting at the nozzles, and shortens the life of every component.
Diagnose first. If the measurements show the supply is genuinely adequate and the loss is inside the system, the fix is inside the system.
Terms used on this page
- Static pressure. Pressure in the line with no water flowing. Measured at a hose bib with everything off.
- Operating pressure. Pressure with a zone running. Always lower than static, and the number the heads actually experience.
- Flow. Volume of water per unit time, in gallons per minute. Determines how many heads a zone can run.
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Irrigation Medics repairs irrigation systems across Hall County and the Lake Lanier communities, the Gwinnett corridor through Duluth, Suwanee and Buford, and North Fulton and Forsyth — Johns Creek, Alpharetta, Milton, Roswell and Cumming. Same rates throughout. See the service area.