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What is Irrigation? Explain Traditional and Modern Methods of Irrigation
All the crop plants need water for their growth. The crop plants absorb water from the soil. The amount of water in the soil is not constant throughout the year. Water from the soil is lost constantly by evaporation and percolation to lower depths of the ground. It is, therefore, necessary to supply water to the crop plants in the fields, periodically.
The process of supplying water to crop plants in the fields is called irrigation. Just as we cannot survive without water for a long time, in the same way, plants also cannot survive without water for a long time. For example, if we stop watering plants grown in our home for a considerable time, the plants become pale, wilt and ultimately die. Water is absorbed by the roots of the plants. Alongwith water, minerals and fertilisers are also absorbed by crop plants. Plants contain nearly 90 per cent water.
Why is Irrigation Necessary
- Irrigation before ploughing the fields makes the soil soft due to which the ploughing of fields becomes easier.
- Irrigation is necessary to provide moisture for the germination of seeds. This is because seeds do not grow in dry soil.
- Irrigation is necessary to maintain the moisture of soil for healthy crop growth so as to get good yield.
- Irrigation is necessary for the absorption of nutrient elements by the plants from the soil. The irrigation water dissolves the nutrients present in the soil to form a solution. This solution of nutrients is then absorbed by the roots for the development of plants.
- Water supplied to the crops during irrigation protects the crop plants from hot air currents as well as frost.
Factors Affecting Irrigation Requirements of Crops
The irrigation requirements (or water requirements) of crops depend on three factors:
- Nature of the crop,
- Nature of the soil, and
Each crop needs a specific amount of water during the various stages of its growth and ripening. Some crops need more water whereas others require less water. For example, paddy crop (rice crop) is transplanted in standing water and requires continuous irrigation whereas other crops like wheat, gram (chana) and cotton, etc., do not require so much water.
For cereal crops like wheat, irrigation is needed at only three stages: before ploughing the field ; at the time of flowering ; and at the time of development of grain.
There are two important types of soils in which the crops are grown ; Sandy soil and Clayey soil. The crops grown in a sandy soil need irrigation more frequently whereas the frequency of irrigation for the crops grown in a clayey soil is comparatively less.
This can be explained as follows : Sandy soil is highly porous having high permeability. So, when we irrigate the’ crops standing in a sandy soil, then water quickly percolates down the soil and the crop plants are not able to absorb adequate amount of water. So, due to the poor water retaining capacity of the sandy soil, the crops cultivated in sandy soil need more frequent irrigation.
On the other hand, clayey soil is much less permeable than sandy soil due to which it can retain water for a much longer time. So, when the crop grown in a clayey soil is irrigated, the water remains in the soil for a longer time, and hence the plants can absorb this water in adequate amount. So, due to better water retaining capacity of the clayey soil, the crops cultivated in clayey soil need irrigation less frequently.
The frequency of irrigation of crops also varies from season to season. For example, the frequency of irrigation (or watering) of the crops is higher in summer season. This is because during the hot days of summer, the rate of evaporation of water from the soil and the leaves of crop plants is increased. On the other hand, the frequency of irrigation (or watering) of the crops is comparatively lower in the colder winter season.
Sources of Irrigation
Crops are supplied water for irrigation from different sources like : Rivers, Canals, Wells, Tube-wells, Dams (Reservoirs), Ponds and Lakes. Even rain is a source of irrigation of crops. The water available in wells, lakes and canals is lifted up by different methods in different regions for taking it to the fields.
Traditional Methods of Irrigation
The various traditional methods of irrigation are :
- Moat (Pulley system),
- Chain pump,
- Dhekli, and
- Rabat (Lever system).
In the moat system of irrigation, water is drawn out from a well by using a big container tied to a long rope which moves over a pulley fixed at one edge of the well (see Figure). The rope tied to container is usually pulled by animals such as bullock (buffalo or camel). When the rope is pulled at the free end, the container filled with water (tied at the other end of rope) comes out of the well (see Figure).
The farmer pours out water from the container into the fields and lowers empty container back into the well to get it refilled. The water-filled container again comes out of the well when the bullock pulls the rope and this process is repeated to get a continuous supply of water for irrigation.
CHAIN PUMP :
Chain pump is an arrangement to lift water from a source of water like a stream, pond or lake (which is at a lower level than the fields) so as to provide irrigation in the fields. A chain pump consists of two large wheels, one fixed at the lower level of water source and the other fixed at the higher level above the fields (see Figure).
The two wheels are connected by a chain passing over them. On the chain are hung small buckets. Below the bottom wheel is the source of water (like a stream, pond or lake) from which the water is to be lifted up to the fields. A handle is attached to the axle of the upper wheel.
When the handle attached to the upper wheel is rotated by the farmer, the wheels connected by the chain start turning. When the lower wheel turns, the buckets attached to chain dip in the stream and get filled with water (see Figure).
The moving chain then lifts these buckets filled with water up to the upper wheel where the buckets tilt, and get emptied in the fields to provide water for irrigation. The moving chain then carries the empty buckets down to the lower wheel to be filled with water again. This process is repeated due to which the water from stream (pond or lake) is continuously lifted up into the fields.
Dhekli is an arrangement to lift water from shallow wells by using the principle of simple lever (The word ‘dhekli’ means ‘lever-beam of well’). In dhekli, a long wooden beam is supported over a forked vertical support fixed in the ground near the well in such a way that its longer arm is towards the well and shorter arm away from it (see Figure).
A bucket is tied to the end of longer lever arm with a rope in such a way that it hangs over the mouth of the well. A weight is tied to the end of shorter lever arm. In order to lift water from the well, the end of longer lever arm carrying the bucket is pulled down by the rope.
When the bucket is lowered into the well, it gets filled with water. On releasing the rope, the weight attached to the end of shorter lever arm comes down and lifts the water- filled bucket out of the well. The farmer then gets hold of the bucket, tilts it and pours out its water into the field. This process is repeated so as to get a large amount of water from the well required for irrigation.
RAHAT (LEVER SYSTEM) :
In the rahat system of irrigation, water is drawn out from a well. In this method, there is a large wheel fixed on an axle above the mouth of the well (see Figure). A long belt with many, many small metal pots is put over the circumference of big wheel which can move over the wheel when the wheel turns. The lower end of the long belt of pots dips in the water of the well.
The big wheel is turned by using a lever system driven by the force of bullocks (or other animals such as buffalo or camel). When the bullocks move the long, horizontal handle of the lever-system by going round and round, the big wheel fixed over the mouth of well starts rotating.
When the wheel rotates, the water filled pots come out of the well one after the other, go over the wheel, come downward, pour water in a channel, get emptied and then go down again into the well to bring out more water (see Figure). The water brought out by the pots connected to a continuously rotating belt is used for irrigation in the fields.
The Use of Pumps For Irrigation
The human labour or cattles are used to lift water in the traditional methods of irrigation. So, the traditional methods of irrigation are cheaper but less efficient. The traditional methods of irrigation are not used much these days. These days, pumps are commonly used for lifting water (from wells, ponds, lakes, streams and rivers). These pumps are run by electricity, diesel, biogas or solar energy. When a pump is used to draw out water from a narrow well, it is called a tube-well.
Nowadays, tube-wells are being used increasingly for lifting underground water to be used for irrigation in agriculture (see Figure). We will now describe some modern methods of irrigation in which though the water is pumped out through tube-wells but used in such a way that its wastage is prevented.
Modern Methods of Irrigation
The modern methods of irrigation help us to use water economically (by preventing its wastage). The two main modern methods of irrigation are :
- Sprinkler system, and
- Drip system.
SPRINKLER SYSTEM :
In the sprinkler system of irrigation, a main pipeline is laid in the fields. Perpendicular pipes having rotating nozzles at the top are joined to the main pipeline at regular intervals. When water from a tube-well is allowed to flow through the main pipeline under pressure with the help of a pump, it escapes from the rotating nozzles (see Figure).
This water gets sprinkled on the crop plants as if it is raining. The sprinkler system of irrigation is more useful for the uneven land where sufficient water is not available. The sprinkler system is also very useful for sandy soil.
DRIP SYSTEM :
In the drip irrigation system, there is a network of narrow pipes (or tubes) with small holes, in the fields (see Figure). When water flows through the narrow pipes, it falls drop by drop at the position of roots of the plants. This water is absorbed by the soil in the root zone of the plants and utilised by the plants.
There is no run-off (or wastage) of irrigation water. Drip system is the best technique for watering (or irrigating) fruit plants, trees and gardens. Drip irrigation system has the following advantages :
- Drip system provides water to plants drop by drop. So, water is not wasted at all. .
- Drip system minimises the use of water in agriculture. So, drip system of irrigation is very useful in those regions where the availability of water is poor.