Agriculture is the backbone of Indian economy. Agricultural growth plays an important role in achieving certain national goals, such as reducing rural poverty, providing food and nutritional security, supplying raw materials to major industries. The cornerstone of India’s agricultural revolution has been the availability of improved varieties of cereals, oilseeds, and pulses etc., breeds of livestock including poultry and fisheries, horticultural plant materials, and improved management practices for increasing the productivity, sustainability and stability of various crops and livestock enterprises. After independence, India has made remarkable development in increasing food production and productivity, credit goes to concerted efforts made under various Agri revolutions. For agriculture, Land and Water are two most important resources, of which, water (irrigation) becomes lifeline of agriculture. It is a truth in agriculture "if we fail in irrigation, we will fail in agriculture. Water is required for agriculture as well as for other sectors and the demand of the water is increasing alarmingly. At present level, agriculture consumes over 80 per cent of total water consumption in India. The country is endowed with many perennial and seasonal rivers. The river system constitutes 36 per cent of geographic area. Most of agricultural fields are irrigated by use of underground water for assured irrigation. Sprinkler irrigation system is one the micro irrigation system for use of underground water to agriculture.
Sprinkler 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. The pump supply system, sprinklers and operating conditions must be designed to enable a uniform application of water. Sprinkler irrigation system allows application of water under high pressure with the help of a pump. It releases water similar to rainfall through a small diameter nozzle placed in the pipes. Water is distributed through a system of pipes, sprayed into air and irrigates in most of the soil type due to wide range of discharge capacity. Sprinkler irrigation is suited for most row, field and tree crops and water can be sprayed over or under the crop canopy. However, large sprinklers are not recommended for irrigation of delicate crops such as lettuce because the large water drops produced by the sprinklers may damage the crop. Sprinkler irrigation is adaptable to any farmable slope, whether uniform or undulating. The lateral pipes supplying water to the sprinklers should always be laid out along the land contour whenever possible. This will minimize the pressure changes at the sprinklers and provide a uniform irrigation.
Sprinklers are best suited to sandy soils with high infiltration rates although they are adaptable to most soils. The average application rate from the sprinklers (in mm/hour) is always chosen to be less than the basic infiltration rate of the soil so that surface ponding and runoff can be avoided. Sprinklers are not suitable for soils which easily form a crust. If sprinkler irrigation is the only method available, then light fine sprays should be used. The larger sprinklers producing larger water droplets are to be avoided. A good clean supply of water, free of suspended sediments, is required to avoid problems of sprinkler nozzle blockage and spoiling the crop by coating it with sediment. 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. The uniformity of sprinkler applications can be affected by wind and water pressure.
Sprinkler Irrigation System consists of the following components:
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Pump unit
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Mainline and sometimes sub-mainlines
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Laterals
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Sprinklers
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 the mainlines or sub mainlines 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. 1. 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 diameter. This is so it can be carried easily. 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 re-assembled 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.
Operating Sprinkler Systems:
Wetting patterns: The wetting pattern from a single rotary sprinkler is not very uniform. Normally the area wetted is circular (see topview). The heaviest wetting is close to the sprinkler (see sideview). 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. 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 (see Annex 2). 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.
Advantages
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Eliminates water conveyance channels, thereby reducing conveyance loss.
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Suitable in all types of soil except heavy clay.
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Water saving up to 30% - 50 %.
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Suitable for irrigation where the plant population per unit area is very high.
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Helps to increase yield.
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Reduces soil compaction.
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Mobility of system helps system operation easy.
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Suitable for undulating land.
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Saves land as no bunds required.
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Soluble fertilizers and chemicals use are possible.
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Provides frost protection & helps in alteration of micro climate.
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Reduces labour cost.
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