Nozzle problem or pressure problem: reading the dry spot under a pivot

You catch a dry streak under a pivot and the first question is always the same: is this a nozzle, or is this pressure. Guess wrong and you spend an afternoon pulling sprinkler heads that were fine, or you chase a pressure problem that was really a worn-out nozzle package the whole time. The good news is the dry pattern itself usually tells you which one you're looking at, if you know what to read.

Nozzle problems stay put on the machine, not the field

A clogged, worn, or missing nozzle is a fixed point on the pivot. It lives at the same tower, same span, same distance from the pivot point, every time the circuit comes around. That's the tell. Walk the field over a few days or check it across more than one pass, and a nozzle problem shows up as a narrow wedge or streak that repeats at the identical radius no matter where the pivot happens to be pointed that day. Sharp edges on both sides, usually no more than one or two sprinkler spacings wide. If you can draw a straight line from the pivot point through the dry spot and it lands on the same tower section every single time, that's a nozzle, a plugged screen, a cracked gasket, or a regulator that quit regulating.

Walk that tower. Pull the nozzle and check it against the chart for that position, debris, wear on the orifice, a regulator that's stuck or spitting water instead of a steady pattern. End guns and corner arms are worth a second look here too; a failed end gun solenoid leaves the same kind of fixed, repeatable dry arc, just at the outer edge of the circle instead of mid-span.

Pressure problems fade in from the end, they don't jump sectors

A pressure problem reads differently. Instead of a tight wedge, you get a gradient, full coverage near the pivot point that tapers off and goes dry toward the last two or three towers. That's classic low system pressure: a pump that's lost head, a mainline restriction, a filter screen loading up, or just more pivots running off the same lateral than the system was sized for. Because the whole end of the lateral is starved, every nozzle out there underperforms a little at once, and that spreads the dry zone into something wide and soft-edged instead of a clean line.

Elevation plays into this too. If part of your circuit climbs a rise, watch for the dry spot to show up only on that high ground and disappear once the machine swings back down to grade. That pattern points to elevation head eating your pressure, a purely hydraulic effect with no broken part behind it. A pressure gauge at the pivot point and another at the last tower will confirm it fast. A far-end reading well under what your design sheet calls for puts the problem squarely on pressure.

One more pattern worth knowing: if the dry spot moves around with time of day rather than staying fixed to a radius, suspect a pump that's cycling or a well that's drawing down, not the sprinkler package at all.

What this means for your rounds

None of this requires new equipment, just knowing what you're looking at before you drive out. A fixed, sharp-edged streak at the same radius every pass means pull that tower's nozzle and regulator. A soft-edged gradient that worsens toward the far end or on high ground means check pressure at the pivot and at the last tower before you touch a single sprinkler head.

The harder part on a district-sized circuit load is catching the pattern early, before the crop stress makes it obvious from the road and before a full-season yield hit is already baked in. Irrigation Monitor compares each pivot's weekly pass against its own geometry so a dry wedge at a fixed radius versus a fading gradient toward the end tower shows up as a sector flag instead of something you only notice driving the circuit by eye. If you'd rather see the pattern on the report than find it in the field, that's exactly the gap it's built to close.