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, “In the absence of rainfall, supply of water to land by artificial means for growth of plants and for increasing productivity is irrigation.”
Man has been using water for irrigation since ancient times. Important civilizations of the world have developed on the basis of irrigation management. Normally, groundwater and surface water are used for irrigation and when water available in these sources is taken away artificially by flowing it for supplying water in required quantity to crops, it is called irrigation.
This is done at such places where rainfall is insufficient. Man has discovered many methods to fight with nature and irrigation is one such means. According to Peter Wales, “It is an artificial means of watering the crops or plants or an art of supplying water to the crops”.
According to Dudley Stamp, “In the absence of rainfall, supply of water to land by artificial means for growth of plants and for increasing productivity is irrigation.” For optimum development of crops, balanced irrigation has been considered essential, because if irrigation is done beyond a limited quantity, the water would go downwards through seepage due to gravitational force of the earth.
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This leads to not only leaching of nutritive elements of soil but a state of water logging is also created in the soil. Similarly, less than necessary quantity of irrigation, is also harmful because in that case water does not reach the lithosphere.
Through balanced irrigation, quantity of water received by plants from the soil is properly recharged. Plants from their root zone, lose that water through evaporation and transpiration. Instead of irrigating very deeply and for a long time, it should be done less but at short intervals.
While irrigating through different methods of irrigation, all the water does not reach the soil but some water goes waste by infiltration, transpiration, flow etc., and some water is filled in drains and lower parts of the fields. Efficiency of irrigation depends on soil texture, method of irrigation and quantity of water stream. Efficiency of irrigation is possible through the following formula (Surajbhan 1993):
After estimation of efficiency of irrigation, the interval between irrigation is most important. This is affected by season and nature of crops.
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Need for Irrigation:
In any geographical region, equal distribution of natural substances is not necessary. Somewhere there is dry climate and at some other places, it is humid. Similarly, on the surface of the earth also, supply of water is not in the same proportion. In such circumstances, human beings have to arrange supply of water for crops through artificial means for fulfilling their primary needs.
Thus, irrigation becomes necessary on account of the following reasons:
1. Nature of Rainfall:
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Due to irregular, uncertain and limited rainfall, scarcity of water is caused. Consequently, the need for irrigation arises. Sometimes, it can rain heavily and for a long duration, whereas sometimes it can be light and for a short duration. If there is torrential rain, it is harmful for agriculture whereas at other places it might rain at short intervals in a balanced way.
2. Nature of Soil:
There is greater need of irrigation in sandy soil whereas in clay or alluvial soil the need for irrigation is lower.
3. Probability of Drought:
At places having irregular rainfall, since there is probability of drought, irrigation is needed.
4. Need of Irrigation in Dry Areas:
In dry areas, where rainfall ranges between 40 to 50 cm per year, the need arises for irrigation.
5. More Need of Water for Special Crops:
Crops like rice, jute, sugarcane, etc. need more water, which can be fulfilled only through irrigation.
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6. More Need of Water for Improved Seed Varieties:
New and high- yielding seeds need additional water through irrigation for higher productivity. This was felt after the green revolution.
7. For Pasture Development:
Along with agriculture, it is essential to develop pastures for cattle development and dairy development, which also need water.
8. Population Increase:
Population of the world is multiplying fast, and it needs additional food production. This can be possible only through irrigation.
Factors Affecting Irrigation Development:
Different systems of irrigation are found in different geographical contexts. Factors affecting irrigation development are as follows:
1. Topography and Slope of Soil:
Irrigation development is done according to structure of the ground level. If ground level is plain, canals or wells would be constructed, whereas in stony areas, the tanks would be suitable. Similarly, slope of ground level also affects irrigation development along with agriculture. Canal system was developed according to the slope. Ground level plays an important role in construction of wells. If ground level is hard and rocky, deeper wells cannot be constructed.
2. Structure, Depth and Permeability of Soil:
Water can be absorbed in the soil according to the structure of the soil. If there is lesser depth, it affects permeability and it can create a condition of water logging.
3. Depth of Groundwater:
Quantity of groundwater and its depth also affects irrigation development.
4. Climate:
Quantity and duration of rainfall affect irrigation development. On this basis, rain water flow and seepage is affected. Evaporation and transpiration occurs according to temperature. Speed of air affects evaporation. Thus, climate exercises an important effect on irrigation development.
5. Social Factors:
Social reasons also affect irrigation development. Labour is an important factor. Scattered fields or their consolidation directly affect the irrigation development.
6. Economic Factors:
Among economic factors, means of transport apart from capital also affect irrigation development.
7. Resettlement:
Problems of resettlement also affect some big or small irrigation development projects.
Means of Irrigation:
On account of different physical conditions on the earth, different sources of irrigation are found in different places. Differences also occur due to ground level latitude, for example, canal irrigation is developed in plain areas, whereas on stony ground level, tanks, lakes or groundwater are used as means of irrigation.
Surface water is used through canals, water reservoirs, tanks, lakes, ponds, while groundwater is used through wells, tube wells and artesian wells. Canal area development is done according to quantity of water of rivers, whereas development of wells is done according to quantity of groundwater, its depth and as per chemical qualities. Anicuts, check dams and loose stone check dams are used for irrigation on un-leveled ground level.