Phil Busey Agronomy
Consulting Inc.


Upsizing Nozzles, Poor Distribution Uniformity, and Bare Spots in Sports Turf

(Try the "Pump size and irrigation capacity converter" to determine how much landscape area can be irrigated by a particular size of pump running a maximum hours per week.)

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Soil Moisture
Cup catchment test of irrigation system to evaluate irrigation distribution uniformity.


Spectrum FieldScout TDR 300 with Garmin GPS 72H used to measure and georeference volumetric soil moisture at 2-foot intervals along diagonal transect between sprinkler heads that span bare spots.


Variation in soil moisture along a diagonal transect between sprinkler heads, and spanning bare spots in the center.


When I tried to solve bare spots in a newly renovated sports field, I suspected irrigation problems, but it wasn't a tight case. Evidence supporting irrigation as a cause of problems was that the surface had been capped using 4 inches depth of sand rootzone mix with no organic matter and high infiltration rate. Secondly, around the bare spots, when I put a pressure gauge with a pitot tube into the Rain Bird Falcon 6504 sprinkler head nozzles with the zone running, I measured only 33 to 38 psi (pounds per square inch) pressure at the nozzles, less than the manufacturer's minimum specification of 40 psi for sufficient overlap, as published in their catalog.

Measure pressure at the nozzles not the pump

Despite the reasons to suspect irrigation as the problem, the irrigation contractor said it was impossible that the pressure was only 33 to 38 psi because the sprinklers could not pop up at so a low pressure. When he measured the pressure at the pump it was over 55 psi. That was true but it didn't tell what was happening at the nozzles where water is emitted. In fact, most of the zones had sufficient pressure at the nozzles, about 50 psi. But the two zones around the bare spots had only 33 to 38 psi measured at the nozzles when they were running. Something was greatly reducing the pressure in the pipes supplying the zones around the bare spots, but the contractor was not buying my argument. The one clear difference between the area with bare spots and areas without bare spots was the difference in irrigation nozzle pressure. That should have settled it, but it wasn't the smoking gun, and I wanted details before arguing the irrigation case more strongly.

Harmful changes to irrigation system

A little history was that during years of irrigation problems, the contractor had modified the irrigation from the original design, upsizing nozzles to "get better coverage," and extending the irrigated area by adding more heads to the same zones. When nozzle size is increased, sprinkler radius increases a little and flow rate increases a lot. Based on the manufacturer's catalog, at 50 psi, when nozzles are increased from #12 (beige) to #18 (dark blue), radius increases from 55 feet to 59 feet, an increase of only 7%. But increasing nozzle size increases flow from 11.0 gpm (gallons per minute) per head to 15.4 gpm, an increase of 40%. Upsizing had been done sporadically throughout the system and there were at least five different nozzle sizes throughout. Although selective upsizing of nozzles seems like a logical tweaking of an irrigation system, it actually makes irrigation worse by varying the distribution pattern of adjacent heads and by increasing flow which reduces pressure and messes up uniformity even more. It's easy to say, "that's not the right way to do it," but it takes more conviction to prove it.

Friction loss due to excess flow rate

At 50 psi, five heads on a zone with #12 nozzles distribute 55 gpm, but seven heads with #18 nozzles distribute 107.8 gpm. There were no "as-builts" for this irrigation system, to tell us the actual pipe size in main lines and laterals. But increasing the flow always has a negative effect on pressure due to friction loss in pipes. In 2-inch PVC Schedule 40 pipe, 55 gpm results in 2.06 psi pressure loss per 100 feet of pipe, while 107.8 gpm from seven heads results in 7.2 psi pressure loss per 100 feet. If the main line was 300 feet, the increase in nozzle sizes and the expansion of zones by adding heads could explain a 15-psi difference in nozzle pressure between zones.

Cup catchment test and irrigation distribution uniformity

The cup catchment test showed that Coefficient of Uniformity was 74%, Distribution Uniformity was 60% (values less than 70% are poor), and Scheduling Coefficient was 166%. This says that, because of lack of uniformity, the irrigation system must run 66% longer to distribute a minimal amount of water to dry areas. Despite poor irrigation uniformity by the cup catchment test, I was puzzled that the bare spots were not in an area of lowest precipitaton. There was actually more water captured in the area of the bare spots in the center than near the sprinkler heads.

But I had done the cup catchment test on a windy day, so it is understandable that the actual pattern of water distribution may not have been the same as in the middle of the night, when irrigation was normally scheduled. Irrigation uniformity was not conclusive, so next I did a nematode sample and analysis, and the results were also inconclusive, then I checked chemical and physical properties of the soil, and those results were not conclusive.

Soil moisture

Next, as a check on soil moisture, I used a Spectrum FieldScout TDR 300 Soil Moisture Meter to measure volumetric soil moisture using 3-inch rods, georeferenced with a Garmin GPS 72H, across a diagonal transect from one sprinkler head northwest of the bare spots to a sprinkler head in the southeast, thus spanning through the bare spots. Paired values at 2- foot spacing were averaged, and values presented as a moving average of three adjacent pairs. Each dot in the graph represents the average of six measurements.

There was variation in volumetric soil moisture across the diagonal transect which runs through the bare spots, with the least soil moisture in the middle and more water available nearest the sprinkler heads. The very driest area was offset slightly southeast of the bare spots in the center, probably due to the effect of wind direction during the nights before measurement. While variation in volumetric soil moisture did not prove irrigation nonuniformity alone caused the bare spots, and did not prove that other factors such as soil type, nematodes, and traffic were not involved (they probably were), it convinced me that the bare spots were too dry due to poor irrigation distribution uniformity, due to low nozzle pressure. It also caused me to insist on replacement of irrigation nozzles to make them all the same, and removal of accessory sprinkler heads from the problem zones, either by capping or placement on a new zone.

In this experiment, soil moisture measurement was more representative than irrigation distribution uniformity, possibly because the irrigation system normally came on at night when wind was low. My cup catchment test during the daytime showed a problem of poor irrigation distribution uniformity, but did not pinpoint the area of low coverage to the bare spots.

Despite the obvious failure to maintain the irrigation system in accord with manufacturer specifications, and expected poor distribution uniformity, soil moisture measurements gave me the most convincing evidence that bare spots were caused by bad irrigation.

Take-home lessons:

  1. Design and maintain irrigation systems with knowledge of manufacturer's specifications.
  2. Diagnose irrigation problems by considering layout and nozzle pressure, distribution uniformity, AND soil moisture.
  3. Measure irrigation pressure at the nozzles with the zone running.
  4. Do a cup catchment test to estimate irrigation distribution uniformity, realizing that results vary with wind direction.
  5. Consider but do not be overwhelmed by other factors such as soil type, nematodes, and traffic.
  6. Measure soil moisture in a structured, not random, way. Make enough measurements to show a pattern if one exists.
  7. Remember that everyone is going to have an opinion but careful documentation of evidence is more likely to solve a problem.