·
Sprinklers. In the early 1900s,
urban areas had pressurized water
systems and used new sprinkler
heads to water lawns in town.
Gradually, plant nurseries adapted
the sprinklers to their use and then
farmers combined sprinklers with
high pressure pumps and pipes. In
1946, sprinklers irrigated less then
250,000 acres of farmland. By 1959,
3.4 million acres were under
sprinklers.
To achieve uniform application, center pivots
require an even emitter ow rate across the
radius of the machine. Since the outer-most
spans (or towers) travel farther in a given
time period than the innermost spans, nozzle
sizes are smallest at the inner spans and
increase with distance from the pivot point.
Aerial views show elds of circles created by
the watery tracings of "quarter- or half-mile
of the center-pivot irrigation pipe," created by
center pivot irrigators which use "hundreds
and sometimes thousands of gallons a
minute."
Most center pivot systems now have drops
hanging from a u-shaped pipe called a
gooseneck attached at the top of the pipewith
sprinkler heads that are positioned a few feet
(at most) above the crop, thus limiting
evaporative losses and wind dri. ere are
many different nozzle con gurations
available including static plate, moving plate
and part circle. Pressure regulators are
typically installed upstream of each nozzle to
ensure each is operating at the correct design
pressure.
Drops can also be used with drag hoses or
bubblers that deposit the water directly on
the ground between crops. is type of
system is known as LEPA (Low Energy
Precision Application) and is oen
associated with the construction of small
dams along the furrow length (termed
furrow diking/dyking). Crops may be
planted in straight rows or are sometimes
planted in circles to conform to the travel of
the irrigation system
Originally, most center pivots were waterpowered. ese were replaced by hydraulic
systems and electric motor-driven systems.
Most systems today are driven by an electric
motor mounted at each tower.
For a center pivot to be used, the terrain
needs to be reasonably at; but one major
advantage of center pivots over alternative
systems is the ability to function in
undulating country. is advantage has
resulted in increased irrigated acreage and
water use in some areas.
Center pivot irrigation typically uses less
water compared to many surface irrigation
and furrow irrigation techniques, which
reduces the expenditure of and conserves
water. It also helps to reduce labor costs
compared to some ground irrigation
techniques, which are oen more labourintensive.
Some ground irrigation techniques involve
the digging of channels on the land for the
water to ow, whereas the use of center-pivot
irrigation can reduce the amount of soil tillage
that occurs and helps to reduce water runoff
and soil erosion that can occur with ground
irrigation. Less tillage encourages more
organic materials and crop residue to
decompose back into the soil, and reduces soil
compaction.
Most center pivot machines are electrically
powered, using either a generator or a public
power source. Pivots use both 120 and 480
volts of alternating current (VAC) to operate.
120 VAC is used as the control circuit,
powering the safety circuit, the forward and
reverse movement of the pivot, and, more
precisely, the movement of the Last Regular
Drive Unit (LRDU). e 480 VAC is the power
circuit and supplies the needed energy for the
drive units to move.
www.watex.co.za
www.farmersreviewafrica.com
[25] FARMERS
REVIEW AFRICA
November - December 2016