Water Application Methods - Practices of Irrigation and On-farm Water Management - page 47

exceptions) designed to apply water at a lower rate than the soil infiltration rate
so that the amount of water infiltrated at any point depends upon the application rate
and time of application but not the soil infiltration rate.
Sprinkler irrigation systems are normally used under more favorable operational
conditions than surface systems because farmers may control the discharge rates,
duration, and frequency. Many sprinkler systems have independent water supply or
are connected to networks which may be operated on demand. However, the pres-
sure from the hydrants or farm pumps is often not appropriate resulting in lower
(or higher) discharges than those envisaged during the design phase. Pressure head
(and discharge) variations at the hydrant should be identified by the user when
appropriate equipment is available.
Sprinklers can be moved manually to ensure an even distribution of water over
the ground, but a series of small fixed sprinklers are commonly used in an irrigation
system. The “throw” of a sprinkler is the area of land which receives water from it,
and sprinklers are placed “head to head,” meaning that they are placed sufficiently
close together so that there are no gaps of dry land between them.
2.3.4.2 Types of Sprinkler Systems
Many types of sprinkler devices and sprinkler systems are available. Sprinkler irri-
gation systems exist in various shapes, sizes, costs, and capabilities. Descriptions of
the more common types are given below.
Portable (or Hand-Move) Sprinkler System
These systems employ a lateral pipeline with sprinklers installed at regular intervals.
The lateral pipe is often made of aluminum, with 20-, 30-, or 40-feet sections, and
special quick-coupling connections at each pipe joint. The sprinkler is installed on
a pipe riser so that it may operate above the crop being grown (in orchards, the riser
may be short so that these types of sprinklers operate under the tree canopy). The
risers are connected to the lateral at the pipe coupling, with the length of pipe section
chosen to correspond to the desired sprinkler spacing. The sprinkler lateral is placed
in one location and operated until the desired water application has been made. Then
the lateral line is disassembled and moved to the next position to be irrigated. This
type of sprinkler system has a low initial cost but a high labor requirement. It can be
used on most crops, though with some, such as corn, the laterals become difficult to
move as the crop reaches maturity.
Solid Set and Permanent Systems
Sprinklers irrigate at a fixed position. Solid set systems are similar in concept to
the hand-move lateral sprinkler system, except that enough laterals are placed in
the field so that it is not necessary to move the pipe during the season. The laterals
are controlled by valves, which direct the water into the laterals irrigating at any
particular moment. The pipe laterals for the solid set system are moved into the