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Essay on Sprinkler Irrigation
Essay Contents:
- Essay on the Definition of Sprinkler Irrigation
- Essay on the Components of Sprinkler Irrigation
- Essay on the Objectives of Sprinkler Irrigation
- Essay on the Types of Sprinkler Irrigation Systems
- Essay on the Elements of Sprinkler Irrigation System
- Essay on the Capacity of Sprinkler Irrigation Systems
- Essay on the Selection of Sprinkler Irrigation Systems
- Essay on the Maintenance of Sprinkler Irrigation Systems
Essay # 1. Definition of Sprinkler Irrigation:
Sprinkler irrigation or overhead irrigation is a method of applying irrigation water which is similar to natural rainfall. Water is distributed through a system of pipes usually by pumping. It is then sprayed into the air through sprinklers so that it breaks up into small water drops which fall to the ground.
In contrast to surface irrigation, sprinkler systems are designed to deliver water to the field without depending on the soil surface for water conveyance or distribution. Sprinklers are designed and arranged to apply water at rates lower than soil in filterability to prevent ponding and surface runoff. The pump supply system, sprinklers and operating conditions must be designed to enable a uniform application of water.
Essay # 2. Components of Sprinkler Irrigation:
A typical sprinkler irrigation system (Fig 8.22) consists of the following components:
1. Pump unit
2. Mainline and sometimes sub-mainlines
3. Laterals
4. Sprinklers
5. Pressure regulators
6. Fertiliser applier
7. Filters
8. Flow control valves
9. Vacuum valve
10. Booster pump.
The pump unit is usually a centrifugal pump which takes water from the source and provides adequate pressure for delivery into the pipe system.
The mainline – and sub-mainlines – are pipes which deliver water from the pump to the laterals. In some cases these pipelines are permanent and are laid on the soil surface or buried below ground. In other cases they are temporary and can be moved from field to field. The main pipe materials used include asbestos cement, plastic or aluminium alloy.
The laterals deliver water from mainlines or submainlines to the sprinklers. They can be permanent but more often they are portable and made of aluminium alloy or plastic so that they can be moved easily.
The most common type of sprinkler system layout is shown in Fig 8.22. It consists of a system of lightweight aluminium or plastic pipes which are moved by hand. The rotary sprinklers are usually spaced 9-24 m apart along the lateral which is normally 5-12.5 cm in dia. The lateral pipe is located in the field until the irrigation is complete.
The pump is then switched off and the lateral is disconnected from the mainline and moved to the next location. It is reassembled and connected to the mainline and the irrigation begins again. The lateral can be moved one to four times a day. It is gradually moved around the field until the whole field is irrigated. This is the simplest of all systems. Some use more than one lateral to irrigate larger areas.
A common problem with sprinkler irrigation is the large labour force needed to move the pipes and sprinklers around the field. In some places such labour may not be available and may also be costly. To overcome this problem many mobile systems have been developed such as the hose reel raingun and the centre pivot.
Essay # 3. Objectives of Sprinkler Irrigation:
The main objective of a sprinkler system is to apply water as uniformly as possible to fill the root zone of the crop with water.
i. Wetting Patterns:
The wetting pattern from a single rotary sprinkler is not very uniform (Fig 8.23). Normally the area wetted is circular (top view). The heaviest wetting is close to the sprinkler (side view). 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 per cent 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 out 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.
ii. Application Rate:
This is the average rate at which water is sprayed onto the crops (mm h-1). 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.
iii. 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. To avoid crop and soil damage use small diameter nozzles operating at or above the normal recommended operating pressure.
Essay # 4. Types of Sprinkler Irrigation System:
Sprinkler irrigation systems are grouped as indicated below:
(i) On the basis of method of water application:
1. Rotating sprinklers (impact sprinkler, gear driven, reaction type, and fixed head sprinkler)
2. Perforated pipe system.
(ii) On the basis of portability:
1. Portable system
2. Semi-portable system
3. Semi-permanent system
4. Permanent system.
(iii) On the basis of precipitation rates:
1. Low volume sprinkler system
2. Medium volume sprinkler system
3. High volume sprinkler system.
(iv) On the basis of principle of operation:
1. Whirling sprinkler
2. Turbo hammer sprinkler
3. Propeller sprinkler
4. Mini-sprinkler.
(v) On the basis of movement:
1. Set move irrigation system (hand move, tow move, side roll, gun type)
2. Solid set system
3. Continuous move system (center pivot system, liner move system, traveller system).
Essay # 5. Elements of Sprinkler Irrigation System:
Design of sprinkler system for a given situation needs consideration of soil, crop, climate and topography besides the equipment availability as indicated below:
1. Inventory of resources
2. Quality of irrigation water
3. Capacity of the system
4. Optimum water application rate
5. Selection of sprinklers
6. Design of laterals
7. Design of main line
8. Selection of pump and power unit
9. General consideration for layout.
Designing of sprinkler and drip irrigation systems is an engineering job. Major steps involved, which may serve as a guide, are briefly discussed.
Essay # 6. Capacity of Sprinkler Irrigation Systems:
Capacity of the pump to be used depends on the area to be irrigated, application depth at each irrigation, irrigation frequency and irrigation efficiency.
Capacity is given by:
where, Q = Capacity of the pump (lps)
A = Area to be irrigated (ha)
d = Depth of application (cm)
F = Number of days allowed for completing one irrigation
H = Operating hours per day
E = Irrigation efficiency
Optimum water application rate:
It depends on soil type, crop cover and land slope. It is the application rate without puddling or surface runoff. Suggested maximum application rates of sprinklers for average conditions are given in Table 8.7.
TABLE 8.7: Suggested maximum application rates (cm h-1) for Sprinklers for average soil, slope and tilth
Essay # 7. Selection of Sprinkler Irrigation Systems:
Sprinkler spacing along the laterals and spacing of the laterals can be decided using the formula:
where, q = Required average discharge of individual sprinklers (lps)
I = Determined rate of application (cm hr-1)
S1 = Spacing between sprinklers along the laterals (m)
Sm = Spacing between positions of the laterals (m).
Spacing of sprinklers, nozzle pressure, speed of rotation and wind velocity affect the distribution pattern of sprinklers. Most commonly used sprinklers spread the water over a circular area at a rate that diminishes with radial distance from the sprinkler. For uniform distribution over the field, sprinkle circles must be overlapped.
Alternative geometric pattern (square, rectangular or triangular) can be used to obtain optimum overlap among adjacent units. The uniformity of distribution with a sprinkler can be experimentally determined. In finally deciding the discharge from an individual sprinkler, the pattern efficiency should be considered.
Discharge from an individual sprinkler is given by:
where, q = Nozzle discharge ( m3 s-1)
a = Cross-sectional area of nozzles or orifice (m2)
h = Pressure head at nozzle (m)
g = Acceleration due to gravity (m s-2)
C = Coefficient of discharge, which is a function of friction and contraction losses (good nozzle has a coefficient between 0.95 to 0.96)
The type of sprinkler to be selected is interrelated with the operating pressure of the sprinkler, nozzle discharge and sprinkler spacing. Sprinklers are designed to operate under different pressures. Higher the operating pressures, finer will be the spray and larger coverage.
Design of laterals:
As the flow goes along the laterals, its volume decreases because of the discharge through sprinklers. A uniform dia of the lateral is adopted. The rate of flow entering the lateral is calculated and a trial dia of the pipe is selected.
Assuming that the flow is through the entire length without sprinklers, the frictional loss in the lateral is calculated using Scobey’s formula:
where, Hf = Total friction loss (ft)
Ks = Coefficient of retardation
L = Length of pipe (ft)
Q = Total discharge (gpm)
D = Inside dia of pipe (ft)
Standard tables, based on the above formula, are available for the purpose. Frictional loss is multiplied using a correction factor in standard tables corresponding to the number of sprinklers on the lateral. To the frictional loss thus calculated, the elevation is added if the lateral goes uphill or the drop is subtracted if the lateral goes down hill.
It is recommended that the total pressure variation in the lateral should not be more than 20 per cent value and if it is within it, the selected dia is adopted; otherwise the procedure is repeated till a satisfactory value of dia is obtained.
Assuming that the frictional loss Hf in a lateral is within 20 per cent of the average pressure, the average pressure head in a lateral can be expressed by
Ha = H0+ 1/4 Hf
where, H0 is the pressure at the sprinkler on the farthest end. When the lateral is on nearly level land or on contour, the pressure head at the main due to the lateral (Hn) is given by
Hn = H0 + Hf
Using the equation (Ha = H0 + 1/4 Hf) for the value of Ha and substituting the same in the equation (Hn = H0 + Hf) and by making allowances for the differences in elevation along the lateral and for the riser height, the head at the main line due to lateral can be expressed as
Hn = Ha + 3/4 Hf ± 3/4 He + Hr
where, Hn= Head at main line
Ha= Average head in the sprinkler lateral
Hf = Friction loss in a sprinkler lateral
He= Maximum difference in elevation between the first and the last sprinkler on the lateral
Hr = Raiser height.
The terms 3/4 Hf and 3/4 He are approximations (3/4 He will be positive or negative depending upon the lateral is uphill or down slope respectively).
Design of main line:
Tables are available for determining friction losses in portable aluminium pipe, RCC and PVC pipes. The larger the pipe size the lesser will be the frictional losses, but the cost of larger size dia pipe is more than a pipe of smaller dia.
Selection of pump and power unit:
In selecting a suitable pump, it is necessary to determine the maximum total head against which the pump is working.
This may be determined by:
Ht = Hn + Hm + Hj + Hs
where, Ht = Total design head against which the pump is working (m)
Hn = Maximum head required at the main to operate the sprinklers on the lateral at required average pressure (m)
Hm = Maximum friction loss in the main and suction line (m)
Hj = Elevation difference between the pump and the junction of the lateral and the main (m)
Hs = Elevation difference between the pump and the source of water after draw down (m).
The amount of water that will be required is determined by multiplying the number of sprinklers by the capacity of each. When the total head and the rate of pumping are known, the pump may be selected from rating curves or tables furnished by the manufacturer.
Technical Specifications for a Typical Sprinkler System:
Technical specifications of a typical sprinkler irrigation system for two farms of 4 and 2 ha, which may serve as a guide, are given in Table 8.8.
TABLE 8.8: Technical specification of a typical sprinkler irrigation system
Essay # 8. Maintenance of Sprinkler Irrigation System:
1. Operational Problems:
i. Operating pressure of sprinkler
ii. Blockages
iii. Leaking of couplers, saddles
iv. Sprinkler is not operating properly.
2. Maintenance of System:
i. Coupler joints
ii. Rubber gaskets
iii. Sprinkler parts
iv. Storage of pipes and fittings
v. Maintenance of pump
vi. Operating pressure of sprinkler.
Operating pressure of the sprinkler is important to achieve the desired uniformity of more than 80 per cent in the field. Setting of optimum pressure can be achieved by proper selection of pump, providing of the bye pass arrangement, use of booster pumps etc.
a. Blockages – use of the end flush valve assembly on the lateral end
b. Keeping the in between pipes open – during installation keeping riser pipe open
c. Stones/vegetative material chocking the spreader and range nozzle – provide wire mesh to foot valve suction area
d. Leaking of couplers and saddles – gaskets are not properly installed, check the welded joint. Clamp and latches, saddles nut and bolts are not properly fixed
e. Sprinkler is not operating properly – check spring tension, rubber and teflon gaskets, swing arm alignment, check pivot pin etc.