2.5 Pressurized Irrigation Systems 2.5.1 Sprinkler
irrigation systems
Sprinkler irrigation is a versatile means of applying water to
any crop, soil, and topographic condition (Schwab et al., 1993; Fonteh
and Assoumou, 1996). Sprinkler systems can be efficient on soils and topography
that is not suitable or efficient for surface irrigation methods. In general,
systems are described according to the method of moving the lateral lines on
which various types of sprinklers are attached. Laterals may be stationary or
movable. Sprinkler systems are highly efficient but there are general concerns
about the maintenance and investment costs for these systems.
Hand-moved laterals have the lowest investment cost but the
highest labour requirement. These systems are only suitable for short growing
crops.
The side roll lateral system uses the irrigation pipe as the
axle of large diameter wheels that are spaced about 12 m apart. These laterals
are moved by a gasoline powered motor and thus require less labor than
hand-move systems. Side rolls should be used for crops that will not interfere
with the movement of the lateral or sprinkler pattern.
Centre pivots consist of radial pipelines that rotate around a
central pivot by water pressure, electric motors, or oil hydraulic motors
(Schwab et al., 1993). A variety of nozzle types, nozzle heights, and
application rates can be used in centre pivot systems. Sprinkler systems are
selected according to the field conditions for the most efficient operation.
Linear moved laterals use hardware similar to that of a centre
pivot, but move in a straight line across the field. Solid-set systems have
sprinklers that are placed over the entire field, where all or some of the
sprinklers may operate at the same time. Sprinkler heads vary greatly from
older impact heads to more modern spray heads that have an assortment of
application and placement modes (Howell, 2003).
2.5.2 Micro irrigation systems
Micro irrigation is a method for delivering slow, frequent
applications of water to the soil using a low pressure, low volume distribution
system and special flow-control outlets (Schwab et al., 1993). If
managed properly, micro irrigation can increase yields and decrease water,
fertilizer, and labor requirements. Micro irrigation includes: micro
sprinklers, drip irrigation, and subsurface drip irrigation (SDI).
Micro sprinklers, often referred to as micro sprayers or
misters, typically consist of small emitters placed on short risers above the
soil surface. Water is conveyed through the air, but travels only a short
distance before reaching the soil surface. The wetted area of emitters in these
systems is small, can be controlled fairly easily, and has different shapes to
match desired distribution patterns. The advantages of micro sprinkler
irrigation systems are the potential for controlling frost, greater flexibility
in applying water, and lower susceptibility to clogging.
Drip systems deliver water directly to the soil surface or
subsurface (SDI) and allow water to dissipate under low pressure in a
predetermined pattern. These systems are advantageous because water is applied
directly to or just above the root zone of the plant, thereby minimizing deep
percolation losses, reducing or eliminating the wetted area from which water
can evaporate, and eliminating losses associated with runoff. These systems are
also advantageous because they reduce water consumption by weeds, while
operating at a lower pressure.
Micro irrigation systems apply water on a high-frequency basis
and create near optimal soil moisture conditions for the crop. Under proper
management, micro irrigation systems save water because only the plant`s root
zone is supplied with water and little, if any, is lost to deep percolation,
consumption by non beneficial plants, or soil surface evaporation. In addition
to being highly efficient, these systems also require relatively little labour
input if designed properly. Yields of some crops have been shown to increase
under these systems because the high temporal soil water level needed to meet
transpiration requirements is maintained (Colaizzi et al., 2003).
The major disadvantages of micro irrigation systems are high
initial cost and potential for the emitters clogging. In some cases, labor
inputs may be quite high if rodents
burrow and chew system components. Proper design, operation, and
maintenance can overcome many of these issues.
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