Why Do I Have Dry Spots Even Though My Sprinklers Run?

Dry Spots Even Though the Sprinklers Run? Why

Quick answer

Dry spots on a system that appears to be running normally are usually a coverage problem rather than a water-quantity problem. Sprinkler zones are designed so each head’s spray reaches its neighbour, and when a head is misaligned, blocked, worn or under-pressured, the overlap disappears and gaps open up. Running longer rarely fixes it, because the gap is receiving little water no matter how long the cycle lasts.

Why overlap matters more than run time

Sprinkler heads do not apply water evenly across their radius. They deliver more close in and less at the outer edge of their throw, so a single head leaves its own perimeter under-watered.

Irrigation design compensates with head-to-head coverage: heads are spaced so each one’s spray reaches the next head. The weak outer edge of one pattern is filled in by the weak outer edge of its neighbour, and the result across the zone is reasonably uniform.

Break that overlap and you get a gap that receives only the thin outer edge of one pattern. Doubling the run time doubles a very small number, so the dry spot stays dry while everything around it is overwatered. This is the single most common reason people conclude their system needs more capacity when it actually needs correction.

What destroys the overlap

A head knocked out of alignment, tilted by roots, or reset wrongly after a repair.
Arcs reduced to keep water off a sidewalk, without re-checking what that did to the coverage.
A blocked pattern — a shrub that has grown into the spray path, a fence, a new raised bed, mature tree trunks. The head works; the water does not arrive.
A head buried by mulch or raised grade so it cannot clear the turf canopy.
Worn or eroded nozzles delivering a shorter, distorted throw.
Mixed nozzle types on one zone, so application rates differ from head to head and no single run time suits all of them.
Low pressure at the head, from elevation, a leak, or too many heads on the zone, reducing every radius at once.
A rotor that stalls partway through its arc, leaving the remainder of its sweep unwatered.

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When the problem is the soil, not the system

Piedmont clay is the other half of this. Clay accepts water slowly, and most irrigation systems apply water faster than clay can absorb it. Past that point everything extra runs off the surface, following the grade to the low ground.

The result on a sloped North Georgia lot is a reliable pattern: dry high ground, soggy low ground, and a homeowner increasing run times because the top is dry, which sends still more water to the bottom. The high ground never improves because the extra water leaves before it can infiltrate.

The correction is cycle-and-soak — splitting a single long run into two or three shorter cycles with soak periods between, so the same total volume is applied at a rate the soil can actually take. This is the most consistently useful scheduling change available in this region, and it costs nothing.

Compaction does the same thing locally. A patch where vehicles parked, where construction traffic ran, or where foot traffic is heavy will shed water regardless of coverage, and no scheduling change fixes compaction. That is an aeration job.

Diagnosing it properly

Run the zone in daylight and watch it. Most of these faults are visible in one cycle: blocked spray, stalled rotors, misaligned heads, short throws. Watching is the highest-value ten minutes available.
Check the dry spot against head positions. Is it between two heads — an overlap gap? Or at the far edge of the zone — a radius or pressure problem? Or at the top of a slope — elevation and runoff?
Run a catch-cup test if it is not obvious. Identical containers placed across the zone, run for a set time, then compared. It measures what each area actually receives and turns an argument into data.
Check for shade and root competition. Mature canopy both blocks spray and competes for soil moisture. On older Roswell and Duluth lots this is frequently the whole answer.
Rule out a leak on that zone. A lateral break starves everything downstream of it, which looks like a coverage problem but is not.
Correct, then re-time. Fix alignment, nozzles, blockages and pressure first. Reassess run times afterwards, because a corrected zone delivers more per minute than a broken one.

Terms used on this page

Head-to-head coverage. Spacing heads so each one’s spray reaches the next, so their weak outer edges overlap and coverage stays uniform.
Cycle and soak. Splitting one long run time into several shorter cycles with soak periods between, so slow-absorbing soil can infiltrate instead of shedding water.
Catch-cup test. Placing identical containers across a zone and running it for a set time to measure how evenly water is actually applied.

Services that fix this

Irrigation System Inspection

Every zone run, every head watched, every fault written down — before it becomes an expensive repair.

Sprinkler Head Repair

Geysering heads, heads that will not pop up, spray on the driveway and the dry patches they cause.

Low Sprinkler Pressure

Heads that barely rise, spray falling short, one zone weaker than the rest — and what actually causes it.

Where we work

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.