Irrigation Methods
Center Pivot Irrigation Method
Center pivots offer the ability to irrigate fields that have
surface slopes that make it impossible or impractical to irrigate with surface
methods. They also offer more water management options than surface irrigation.
The need for good surface drainage still exists with pivot irrigation and should
not be overlooked.
 Pivots
are best suited for large square-, rectangular- or circular-shaped fields free
of obstacles such as trees, fences, roads, power poles, etc. Field ditches are
also a concern if the pivot towers must be able to cross them. Pivots can cover
a range of acreage depending on the allowable length, but the common 1/4-mile
system will cover approximately 130 acres of a 160-acre square field. It is also
possible to tow a pivot from one field to another. It is usually best for a
system not to be towed between more than two points during the season.
Pivots provide the ability to control the irrigation amount
applied by adjusting the system’s speed. This gives the operator advantages for
activating chemicals, watering up a crop and watering small plants. These
advantages are especially important for double-crop soybeans, since they are
more likely to encounter a drought soon after planting. It is also possible to
apply liquid fertilizer and certain pesticides through the system if the
necessary precautions are taken. One of the biggest advantages of pivot
irrigation is the limited labor required for operating the system.
It is recommended that a pivot have a water supply of at
least 5 gpm per acre that is irrigated. At that rate, nearly four days are
required to apply a 1-inch irrigation. A water supply less than this leaves
little room for breakdown time without the risk of getting behind in meeting the
crop water needs. The capacity for a towable system should be greater to account
for the added time needed to move the system. Most new pivots are being equipped
with low-pressure sprinkler packages. Many of these are mounted on drops so that
the water is released closer to the soil surface. This is desirable as long as
the system application rate is matched to the soil and field characteristics so
that excessive runoff is avoided. If a field has a rolling surface and a soil
that tends to crust or seal over, this should be taken into account in the
sprinkler package selection. The application amount can be adjusted to reduce
runoff to some degree, and most producers find that applying approximately 1
inch works best.
The biggest challenge with center pivots is the initial cost.
However, it does offer some advantages that can justify the initial cost,
especially when surface irrigation is not possible and the cost is spread over
an expected service life of at least 15 years.
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