What causes a dry wedge pattern under a center pivot
You're looking at the field from the road and there it is: a pie-slice of stressed or stunted crop, wider at the pivot point and narrowing out toward the end tower, or the reverse. It reads like someone drew a wedge on the circle with a protractor. That shape isn't random, and it isn't disease pressure or a soil pocket either, most of the time. A true wedge almost always points back to the machine.
The three usual suspects
A stuck or misaligned sprinkler head. If one or two drops on a span stopped rotating or are pointed the wrong way, they keep watering the same few degrees of arc every pass instead of spreading it across the full sweep. You get a dry wedge where they should be hitting and, sometimes, a wet streak right next to it where the water's landing instead.
A pressure drop along the lateral. Pivots are designed to deliver roughly even pressure at every nozzle from the pivot point to the end tower, but a partially closed valve, a kink, or a leak upstream changes that math. Nozzles past the problem spot put out less than they're rated for, and because a pivot's outer sectors already need more water per revolution to cover more ground, a pressure shortfall shows up worst at the far end first. That's the classic "wedge that gets drier as you move outboard" shape.
End gun or corner-arm timing. If you're running an end gun or a swing arm and the timing is off relative to where the pivot is on its circuit (GPS alignment drifts, a sensor gets bumped, a timer relay sticks), you end up applying water to the wrong arc of the circle. The wedge shows up exactly where the end gun should have fired and didn't, or where it kept firing past the point it should have shut off.
Less often, a wedge traces to a water right restriction on one part of the field, a soil texture change that happens to run radially instead of in patches, or a sector that was always undersized on the original system design. Worth ruling out, but check the equipment first. Pivots fail mechanically far more often than fields fail geologically in a pie-slice shape.
Why the wedge is easy to miss until it's expensive
A dry wedge doesn't show stress on day one. Depending on crop and growth stage, it can take a week or two of underwatering before the canopy color or height difference is visible from the cab, and by then you've already lost uniformity for that cycle. Catching it by driving the circuit means somebody has to be looking at the right angle on the right day, and a single pass around a quarter-mile pivot doesn't always put you at the spot where the wedge is widest.
This is the gap between what a pivot's controller tells you (it's running, it's on schedule, pressure's nominal at the panel) and what's actually landing on the ground at each sector. The controller can be right and the wedge can still exist. A stuck nozzle thirty towers out doesn't trip an alarm.
Telling the causes apart from the pattern itself
A wedge that's roughly the same width from pivot point to end tower points at a stuck sprinkler or a mistimed end gun, something localized to one arc of rotation. One that's narrow near the pivot and widens toward the last span points at pressure loss along the lateral. When the dry patch only shows up in the outer third, right where the corner arm or end gun operates, that's a timing or alignment problem.
Knowing which shape you're looking at saves a service call. You send the crew to check nozzle wear and alignment for the first pattern, pressure gauges and valves for the second, and the end gun's GPS or timer for the third, instead of walking the whole span guessing.
Irrigation Monitor checks each pivot's sectors weekly against its own geometry and flags where the pattern doesn't match what the circle should be getting, so a wedge gets caught before it costs a cutting. You can see what that weekly sector report looks like on the Irrigation Monitor homepage.
If you've got a pivot throwing an odd wedge right now, that weekly check is worth a look before you send a crew out to walk the whole span.