Perhaps the most overlooked problem with landscape
irrigation system performance is pressure. A healthy balance
of pressure is good for the performance and longevity of your
irrigation system, and thus the aesthetics of your landscape.
Keeping some fundamental hydraulics in mind, minimizing the impacts
of too much or too little pressure in an irrigation system is
simple, quick and effective.
Static and Working Pressure
is the point when water is motionless. In a closed level system,
pressure is the same at every point. If the system is not level,
there will be slight differences in pressure caused by elevation
Dynamic, or working, pressure is pressure at any point when water
us flowing. This pressure is always less than static because
the flow of water results in a loss of pressure due to friction
in the pipe and through any pipe fittings. Pressure loss in a
system equals static pressure minus working pressure. Now that
the basics have been reviewed, the following troubleshooting
guidelines can be considered.
Pressure Problems and Solutions
is a primary cause of poor turf performance at many sites. Signs
are doughnut patterns, runoff and large water drops in the stream
of the sprinklers. The results are poor uniformity and possibly
sprinkler rotation problems for gear drive and impact heads,
which result in increased dry areas. The best techniques to overcome
inadequate pressure are:
- Increase pressure.
- Use low-pressure nozzles.
They have barbs on them so that the diameter of throw can't change,
but more water will be placed along the stream and closer to
- Use shorter laterals, particularly
if there are differences between pressure at the first and last
sprinkler on a zone. If pressure is too low along the entire
lateral, add a valve and split the lateral into two separate
- Reduce the nozzle sizes on
all heads on the zone by one nozzle size. Remember that you are
reducing the amount of water being applied overall, but it can
improve uniformity and you might not have to adjust your scheduling.
- Consult a designer or manufacturer
before making large retrofit changes.
Too much pressure causes misting
and drift of your irrigation water and that can potentially damage
impact heads from rapid rotation. Some valves have pressure-regulating
features (not to be confused with flow control), and can be field
If the site has no regulator, one can be installed behind the
valve. To adjust the pressure regulator, place a pressure
gauge on the most distant sprinkler. Adjust the regulator
or flow-control valve so that the sprinkler pressure is at the
middle of its pressure range. Pressures can vary, and this pressure
will be inadequate during low pressure and too high during high
pressure. A regulator may be required in these instances.
systems rely exclusively on delivered pressure. When this fluctuates,
so will the distance of throw and precipitation rates of the
sprinklers. A 10 percent pressure difference between laterals
can cause a 5 percent difference in precipitation rates. A 30
percent difference can cause a 14 percent difference in the precipitation
rate. A 15 percent differential should be the maximum.
Once the ratio is set, the difference between sprinklers will
be almost constant. Precipitation will increase if pressure increases
because of the increase in flow through the nozzle. This differential
is controlled by the size of the pipe.
Many sites have pressure regulators to set flow and pressure
rates. Oftentimes these devices fail or are set at a pressure
too low during installation. They might have to be set to a higher
pressure or replaced altogether. Consider installing a regulator
with a higher range, for increased pressure downline of the regulator.
Flow and Pressure Control
Using flow control
for pressure control almost always results in uneven pressure
and poor uniformity. Most valves come with a flow control knob
or dial on top of the valve, which maintenance personnel tend
to use as a pressure regulator when the pressure is too great.
Unfortunately, this device responds to changes in pressure and does not
compensate for them. Always use a pressure regulator designed
to regulate pressure. A low-pressure problem might also be attributable
to partially closed valves. Check the valve stems to ensure that
they are completely open.
Altering the System
Adding new heads
or laterals to an existing valve will absolutely affect pressure
in the zone. Although the intent might be to increase coverage
or reduce hot spots, it actually increases problems during the
peak stress months of summer. As with any design, additions to
an existing system should consider both static and dynamic pressure,
pipe size, sprinkler head type and flow, and distances between
existing heads and laterals. Try changing nozzle sizes or heads
to improve coverage.
Changing a sprinkler's nozzle size can be a help or a hindrance,
depending on system pressure and the head's precipitation rate.
Low-flow or flow-control nozzles are used to improve uniformity,
and can be life savers on systems with small pipes or areas with
Properly designed flow nozzles deliver the same flow rate from
each sprinkler, even though each sprinkler has a different pressure.
A flexible orifice is used in the nozzle to deflect the stream
of water. With high pressure, the orifice is small, at low pressure
it's large. This helps to improve flow rate and precipitation.
Another method, matched precipitation, is a nozzle configuration
used on different arcs of heads. As an example, a 45 degree head
would not have the same nozzle size as a 90 degree because of
the difference in coverage. If they did, the 45 degree would
put out twice the water as the 90 degree, resulting in higher
precipitation and poor uniformity. Always use matched nozzles
when working with pressure problems.
Unfortunately, head-to-head coverage too often is a luxury, rather
than a standard. Less than a 75 percent head-to-head throw will
result in hot spots and low precipitation. If pressure is low,
these problems are amplified. If pressure has dropped, you might
have to move heads or change to a brand with better throw with
lower pressure. It is always better to have excessive pressure
than inadequate pressure, since pressure regulation costs less
than trying to overcome a low-pressure problem.
The performance of your irrigation system relies on the integrity
of the design, installation and maintenance. When any of these
three components is compromised, so is the hydraulic performance.
Maintaining the system's designed pressure results in water conservation,
healthy plant material and overall system durability.