Chapter 11: Sprinkle Irrigation - NRCS Irrigation ToolBox Home Page

Gun and boom sprinklers can be used on most
crops, but they produce relatively high application
rates and large water drops that tend to compact
the soil surface and create runoff problems. Therefore,
these sprinklers arc most suitable for coarsetextured
soils with high infiltration rates and for
relatively mature crops that need only supplemental
irrigation. Gun and boom sprinklsrs are not recommended
for use in extremely windy areas because
their distribution patterns become too distorted.
Large gun sprinklers are usually trailer or skid
mounted and like boom sprinklers are towed from
one position to another by a tractor. Boom sprinklers
are unstable and can tip over when being
towed over rolling or steep topography.
Figure 11-7,-Outer
cnd of center-pivot lateral in operation.
Contimous-Move Lateral
Center-Pivot
The center-pivot system sprinkles water f om a
continuously moving lateral pipeline. The lateral is
fixed at one end and rotates to irrigate a large
circular area. The fixed end of the lateral, called the
"pivot point," is connected to the water supply.
The lateral consists of a series of spans ranging in
length from 90 to 250 R and carried about 10 ft
above the ground by "drive units," which consist of
an "A-frame" supported on motor driven wheels
(fig. 11-7).
Devices are installed at each drive unit to keep
the lateral in a line between the pivot and end-drive
unit; the end-drive unit is set to control the speed of
rotation. The most common center-pivot lateral
uses 6-in pipe, is a quarter mile long (1,320 ft), and
irrigates the circular portion (126 acres plus 2 to 10
acres more depending on the range of the end sprinklers)
of a quarter section (I 60 acres). However,
laterals as short as 220 ft and as long as a half mile
are available.
The moving lateral pipeline is fitted with impact,
spinner, or spray-nozzle sprinklers to spread the water
evenIy over the circular field. The area to be irrigated
by each sprinkler set at a uniform sprinkler
spacing along the lateral becomes progressively
larger toward the moving end. Therefore, to provide
uniform application the sprinklers must be designed
to have progressively greater discharges, closer
spacings, or both, toward the moving end. Typically,
the application rate near the moving end is
about 1.0 iph. This exceeds the intake rate of many
soils except for the first few minutes at the beginning
of each irrigation. To minimize surface
ponding and runoff, the Iaterals are usually rotated
every 10 to 72 hours depending on the soil's
infiltration characteristics, the system's capacity,
and the maximum desired soil moisture deficit.
Five types of power units commonly used to drive
the wheels on center pivots are electric motors,
water pistons, water spinners and turbines, hydraulic
oil motors, and air pistons. The first pivots
were powered by water pistons; however, electric
motors are most common today because of their
speed, reliability, and ability to run backwards and
forwards.
Self-propelled, center-pivot sprinkler systems are
suitable for almost all field crops but require fields
free from any obstructions above ground such as
telephone lines, electric power poles, buildings, and
trees in the irrigated area. They are best adapted
for use on soils having high intake rates, and on
uniform topography. When used on soils with low
intake rate and irregular topography, the resulting
runoff causes erosion and puddles that may interfere
with the uniform movement of the lateral
around the pivot point. If these systems are used
on square subdivisions, some means of irrigating
the four corners must be provided, or other uses
made of the area not irrigated. In a 160-acre quarter-section
subdivision, about 30 acres are not irrigated
by the centerpivot system unless the pivot is
provided with a special corner irrigating apparatus,
With some corner systems only about 8 acres are
left rxnirrigated.
Most pivot systems are permamently installed in
a given field. But in supplemental irrigation areas