SPRINKLER IRRIGATION

INTRODUCTION
An irrigation sprinkler (also known as a water sprinkler or simply
a sprinkler) is a device used to irrigate agricultural crops, lawns, landscapes,
golf courses, and other areas. They are also used for cooling and for the control
of airborne dust. Sprinkler irrigation is the method of applying water in a
controlled manner in way similar to rainfall. The water is distributed through a
network that may consist of pumps, valves, pipes, and sprinklers.
Irrigation sprinklers can be used for residential, industrial, and agricultural usage.
It is useful on uneven land where sufficient water is not available as well as on
sandy soil. The perpendicular pipes, having rotating nozzles on top, are joined to
the main pipeline at regular intervals of time. When water is allowed to flow
through the main pipe under pressure with the help of pump it, escapes from the
rotating nozzles. It gets sprinkled on the crop. With careful selection of nozzle
sizes, operating pressure and sprinkler spacing the amount of irrigation water
required to refill the crop root zone can be applied nearly uniform at the rate to
suit the infiltration rate of soil.In sprinkler or overhead irrigation, water is piped
to one more central locations within the field and distributed by overhead high
pressure sprinklers or guns.

Sprinkler layout
 A typical sprinkler irrigation system consists of the following
components:
 Pump unit
 Mainline and sometimes sub-mainlines
 Laterals
 Sprinklers
 Other accessories such as valves, bends, plugs and risers.

Wetting patterns in sprinklers

The wetting pattern from a single rotary sprinkler is not very uniform. Normally
the area wetted is circular. The heaviest wetting is close to the sprinkler. For
good uniformity several sprinklers must be operated close together so that their
patterns overlap. For good uniformity the overlap should be at least 65% of the
wetted diameter. This determines the maximum spacing between sprinklers.
The uniformity of sprinkler applications can be affected by wind and water
pressure. Spray from sprinklers is easily blown about by even a gentle breeze
and this can seriously reduce uniformity. To reduce the effects of wind the
sprinklers can be positioned more closely together.
Sprinklers will only work well at the right operating pressure recommended by
the manufacturer. If the pressure is above or below this then the distribution will
be affected. The most common problem is when the pressure is too low. This
happens when pumps and pipes wear. Friction increases and so pressure at the
sprinkler reduces. The result is that the water jet does not break up and all the
water tends to fall in one area towards the outside of the wetted circle. If the
pressure is too high then the distribution will also be poor. A fine spray
develops which falls close to the sprinkler.

Application rate: This is the average rate at which water is sprayed onto the
crops and is measured in mm/hour. The application rate depends on the size of
sprinkler nozzles, the operating pressure and the distance between sprinklers.
When selecting a sprinkler system, it is important to make sure that the average
application rate is less than the basic infiltration rate of the soil. In this way all
the water applied will be readily absorbed by the soil and there should be no
runoff.
Sprinkler drop sizes: As water sprays from a sprinkler it breaks up into small
drops between 0.5 and 4.0 mm in size. The small drops fall close to the
sprinkler whereas the larger ones fall close to the edge of the wetted circle.
Large drops can damage delicate crops and soils and so in such conditions it is
best to use the smaller sprinklers. Drop size is also controlled by pressure and
nozzle size. When the pressure is low, drops tend to be much larger as the water
jet does not break up easily. So, to avoid crop and soil damage use small
diameter nozzles operating at or above the normal recommended operating
pressure.

General classification of different types of sprinkler systems

Sprinkler systems are classified into the following two major types on the basis
of the arrangement for spraying irrigation water:
1. Rotating head or revolving sprinkler system
2. Perforated pipe system.
1) Rotating head: Small size nozzles are placed on riser pipes fixed at uniform
intervals along the length of the lateral pipe and the lateral pipes are usually laid
on the ground surface. They may also be mounted on posts above the crop
height and rotated through 90 degree, to irrigate a rectangular strip. In rotating
type sprinklers, the most common device to rotate the sprinkler heads is with a
small hammer activated by the thrust of water striking against a vane connected
to it.
2) Perforated pipe system: This method consists of drilled holes or nozzles
along their length through which water is sprayed under pressure. This system is
usually designed for relatively low pressure (1 kg/cm2). The application rate
ranges from 1.25 to 5 cm per hour for various pressure and spacing.
Based on the portability, sprinkler systems are classified into the following
types:
(i) Portable system: A portable system has portable main lines, laterals
and pumping plant.
(ii) Semi portable system: A semi portable system is similar to a portable
system except that the location of water source and pumping plant is
fixed.
(iii) Semi-permanent system: A semi-permanent system has portable
lateral lines, permanent main lines and sub mains and a stationery
water source and pumping plant.
(iv) Solid set system: A solid set system has enough laterals to eliminate
their movement. The laterals are positions in the field early in the crop
season and remain for the season.
(v) Permanent system: A fully permanent system consists of permanently
laid mains, sub mains and laterals and a stationery water source and
pumping plant. The water is pumped under pressure to the fields. The
pressure forces the water through sprinklers or through perforations or
nozzles in pipelines and then forms a spray.
(vi) A high speed centrifugal or turbine pump can be used for operating
sprinkler irrigation for individual fields. Centrifugal pump is used
when the distance from the pump inlet to the water surface is less than
eight meters. For pumping water from deep wells or more than eight
meters, a turbine pump is suggested. The driving unit may be either an
electric motor or an internal combustion engine.
 Types of Sprinklers Based on Precipitation

Type of Sprinkler Precipitation Rate

(mm per hour)

Low Volume Sprinkler Less than 13

Medium Volume Sprinkler 13 – 25

Large Volume Sprinkler (Rain gun) Above 25

Response of Different Crops to Sprinkler Irrigation System

Crops Water Saving (%) Yield increase (%)

Chilli 33 24

Cotton 36 50

Fenugreek 29 35

Gram 69 57

Jowar 55 34

Maize 41 36

Onion 33 23

Sunflower 33 20

Wheat 35 24
Advantages
 Eliminates water conveyance channels, thereby reducing conveyance
loss.
 Suitable in all types of soil except heavy clay.
 Water saving up to 30% - 50 %.
 Suitable for irrigation where the plant population per unit area is very
high.
 Helps to increase yield.
 Reduces soil compaction.
 Mobility of system helps system operation easy.
 Suitable for undulating land.
 Saves land as no bunds required.
 Soluble fertilizers and chemicals use are possible.
 Provides frost protection & helps in alteration of micro climate.
 Reduces labour cost.
 It is most suitable for oil seeds and other cereal and vegetable crops
 Closer control of water application convenient for giving light and
frequent irrigation and higher water application efficiency
 Influences greater conducive micro-climate
 Areas located at a higher elevation than the source can be irrigated
 Less problem of clogging of sprinkler nozzles due to sediment laden
water.

Disadvantages of Sprinkler Irrigation System:

 High operating cost.

 Water will drift when there is a lot of wind.
 A stable water supply is needed.
 Saline water may cause problem.
 Water must be free from sand, debris and large amount of salt.
 Use of common garden hoses in combination with spray nozzles may
generate aerosols containing droplets smaller than 10 μm, which can be
inhaled by nearby people.
 Water stagnating in a hose between uses, especially when warmed by the
sun, can host the growth and interaction of Legionella and free-living
amoebae (FLA) as biofilms on the inner surface of the hose.