In this article we will discuss about:- 1. Introduction to Sprinkler Irrigation 2. Components of a Sprinkler System 3. Types 4. Atmospheric Factors 5. Evaluation.
Contents:
- Introduction to Sprinkler Irrigation
- Components of a Sprinkler System
- Types of Sprinkler Systems
- Atmospheric Factors in Sprinkler Operation
- Evaluation of Sprinkler Irrigation Systems
1. Introduction to Sprinkler Irrigation:
The method of applying water above the ground surface somewhat resembling rainfall is known as sprinkler irrigation. The spray is obtained by the flow of water under pressure through small orifices or nozzles referred to as sprinklers.
A pump is used for developing the required pressure. In some situations when the source of water is high enough above the area to be irrigated, the required pressure may be developed by gravity alone.
The sprinkler method of irrigation can advantageously be used under one or more of the following conditions:
1. Land unsuitable or uneconomical for levelling.
2. Soils too porous, highly erodible or relatively impermeable, so that it is difficult to irrigate them by other methods.
3. Rate of flow available is too small to distribute water efficiently by surface irrigation methods.
4. Where frequent light applications are needed.
Sprinkler method has the following advantages over the surface irrigation methods:
1. Elimination of the channels for conveyance, their maintenance and water losses in conveyance.
2. Closer control of water application convenient for giving light and frequent irrigation and higher application efficiency.
3. Areas located at a higher elevation than the source can be irrigated.
The sprinkler method has, however, the following limitations:
1. Uneven water distribution caused due to high winds.
2. Evaporation losses when operating under high temperatures. This becomes more important when the irrigation water has larger amounts of dissolved salts.
3. Initial investment and continued operating costs are much higher than in case of surface irrigation methods.
2. Components of a Sprinkler System:
A typical sprinkler system consists of a pumping unit, main line, laterals, risers and sprinkler heads. Additional devices consist of debris screens, desilting devices, flow regulators and fertilizer applicators.
The type of pumping unit is decided depending on the source. Both vertical turbine pumps and centrifugal pumps can be used. In order to develop required pressure, booster pumps may sometimes be needed. The pumps can be driven either by electric motors or internal combustion engines. Main lines may be permanent or portable depending upon the situation.
In case of permanent mains, they could be buried underground at appropriate locations along the farm boundaries. Underground concrete pipelines on the farm can act as supply points to the sprinkler systems rather than as main lines because of the pressure limitations.
Lateral lines are usually made portable so that they can be moved after each setting. Lightweight aluminium pipes can conveniently be used both for the main and laterals. Quick setting couplers enable the movement of the laterals quickly during irrigation.
The rotating sprinkler head is the most important component in the system. Its operating characteristics determine the rate, amount and distribution of water application. Most of the sprinklers used for field crops are of the slowly rotating type with either one or two nozzles.
The most commonly used types are the small to medium sprinklers with capacities ranging from 7.5 to 75 l/min and which operate at pressures from 1.4 to 4.2 kg/cm2. They cover an area of 10 to 40 m in diameter.
These sprinklers rotate around a vertical axis. The rotation is caused either by the torque produced by the reaction of the water leaving a nozzle or by the impact of a spring-loaded arm that periodically interrupts the jet from one of the nozzles. Some rotating sprinklers can be adjusted to irrigate a particular segment of a circle. Sprinkler heads commonly used have two nozzles, one to cover the farther area from the sprinkler and the other to cover the area near the sprinkler. Single nozzles are used for low application rates.
There are several variations in the design and operating characteristics of the rotating sprinkler to suit different conditions.
Important among them are:
(i) Popup sprinkler head,
(ii) Part- circle medium sized agricultural sprinkler, and
(iii) Large sized sprinklers.
Popup sprinklers are used for irrigating lawns. These have a flat cover and are installed in flush with the ground. During the period of operation, the cover opens by means of a spring.
The full circle high application rate sprinklers are generally used when higher application rates are desired as in orchards or in cases where closer spacing of the sprinklers is not possible due to plantations.
Debris basins and desilting basins are needed to remove any foreign materials and sediment entering the sprinkler system. They will especially be needed if water from surface runoff is used through the sprinklers.
Sprinkler systems can be used to apply fertilizers along with water. The fertilizer is applied through the suction side of the pump. This could, however, cause corrosion of the pump parts because of the chemical nature of the fertilizers.
Another method to add the fertilizer solution to the sprinkler system is a Venturi is fixed in the sprinkler line creating differential pressures and allowing the fertilizer solution to flow into the water line.
3. Types of Sprinkler Systems:
The sprinkler system consists of the water source, pumping unit, main line, laterals, risers and the sprinkling units.
Sprinkler systems are broadly in the following major types:
(i) Nozzle-line systems,
(ii) Perforated portable pipe systems, and
(iii) Revolving sprinkler systems.
In the nozzle-line systems, small bore nozzles are placed in water pipe at uniform intervals along its length. A rectangular strip is usually irrigated. This is the earlier form of the sprinkler systems and its use now is limited to nurseries and small areas.
The perforated pipe system consists of drilled holes in portable thin wall tubing to irrigate about 3 to 15 m wide strip in a fairly uniform distribution-pattern. The system is designed for relatively low operating pressure (about 1 kg/cm2).
The application rates are usually 1.25 to 5 cm per hour. Because of its limited coverage in one setting, a large number of movements are needed to irrigate even moderately sized areas.
The revolving sprinkler system consists basically of rotating sprinklers either installed on a pipeline, or on portable quick coupling lateral pipes, or with some type of moving device. The system is versatile either for small or large areas.
This system can further be classified as:
(i) Hand move systems,
(ii) Mechanical move systems, and
(iii) Solid set systems.
In the hand move systems, the lateral lines are manually moved across the irrigated areas. The mechanical move systems adopt a variety of devices for moving the lateral lines across the irrigated areas. In the solid set system, the entire irrigated area is equipped by a set of sprinklers operated either simultaneously or in sequence by automatic timed or manual controls.
Sprinkler system may also be classified as permanent, portable, semi-permanent and semi-portable depending upon the makeup of the components.
4. Atmospheric Factors in Sprinkler Operation:
Sprinkler operation is affected by atmospheric conditions. Atmospheric conditions affect the evaporation losses from the spray and from the wet foliage surfaces. Evaporation losses are influenced by temperature, relative humidity and wind velocity. Wind distorts the application pattern and the higher the wind velocity greater the destortion.
Fig. 15.19 shows the distribution pattern with the influence of wind. To improve distribution under windy conditions, the sprinkler laterals may be moved only one-half the normal spacing. Irrigating at night when wind velocities are low can also be done.
5. Evaluation of Sprinkler Irrigation Systems:
A sprinkler system even though designed as per the procedure should be evaluated to know its performance. To evaluate the system the pressure at the sprinkler; water distribution pattern and the average depth of application are observed.
Soil moisture observations can also be taken to calculate the account of soil moisture to be added. The pressure at the sprinkler nozzle can be observed using a pressure gauge with Pitot tube attachment. The pressure should be as per the prescribed pressure for the sprinkler under consideration.
To study the water distribution patterns, a sprinkler lateral with three sprinklers is selected. Open cans are set around the sprinklers in a grid pattern (Fig. 15.20). The cans are spaced about 1.5 m apart when the sprinklers are less than 10 m about 3 m apart when the sprinklers are 10 m or more apart.
The sprinklers are run till more than 1 cm depth is collected in all the cans. The depth of water collected in each can and the time of operation are noted. The depth in each can is calculated by taking the volumetric measurements of water and then converting the same to depth units rather than directly measuring the depth. To obtain the depth of application under different sprinkler settings, the data collected can be suitably superimposed as in Example 1.
The gauge depths can be used to characterise the uniformity of distribution and the pattern efficiency. To characterise the uniformity of distribution, the most commonly used concept is the coefficient of uniformity. Christiansen’s coefficient of uniformity is given by the expression-
An absolutely uniform application would give an uniformity coefficient of 1.0. Uniformity coefficients of 0.85 or more are considered to be satisfactory.
The pattern efficiency (also known as distribution efficiency) can be calculated after obtaining, the total depths at each of the grid point. The minimum depth is calculated considering average of the lowest one fourth of the cans used in a particular test. Pattern efficiency is given by –
The pattern efficiency is useful in calculating the average depth to be applied for a certain minimum depth. For example – if a minimum depth of 4 cm is to be applied and the pattern efficiency is 80 per cent, then an average depth of 5 cm is to be applied. The pattern efficiency is influenced by the wind conditions.
The application efficiency of a lateral is calculated by dividing the minimum depth of catch with the computed depth for the average discharge of all the sprinklers on the lateral. Since sprinkler discharge is a function of pressure, differences of pressure along the lateral effect the lateral efficiencies. If lateral pressures can be maintained about the same for all settings, lateral efficiency and system efficiency can be considered to be same.
Example:
In a test with 3 sprinklers located on a lateral gauge cans were located at 3 m grids. The sprinklers were run for 2 hours and the volumes collected in the cans are given in Fig. 15.20. Assuming the inside base area of the cans to be 10 cm2, calculate distribution patterns for 18 m moves. Also, calculate distribution efficiency, and the uniformity coefficient. If the average discharge of the sprinklers is 18 lpm, calculate the application efficiency.
Solution:
(a) For a move of 18 m the depths collected at each of the grid points will be as follows –
The depths collected at each of the grid points are calculated from the data given in Fig. 15.20. The volumes are converted to depths by dividing with the inside area of the can (10 cm2).
The depth at each point is calculated by superimposing the depth that will result by setting the sprinklers at the prescribed distance. For example – the depths in the bottom line are obtained as follows.
The other values are obtained similarly by super imposition.
(b) When the move distance is 9 m, the depths collected at each of the grid point will be as follows –
