Irrigation

Irrigation Points : Irrigation, hydraulic and irrigations, Necessity of Irrigation, Less Rainfall, Scope of Irrigation Science, Storage Diversion, or Lifting of Water, Conveyance of Water to the Agricultural Fields, Application of Water to Agricultural Fields, Drainage and, Relieving Water-Logging, Development of Water Power, Sources of Water Irrigation Irrigation may be defined as the process of artificially supplying water to soil for raising crops. it is a science of planning and designing an efficient, low-cost, economic irrigation 8ystern tailored to fit natu.ral conditions. It is the engineering of controlling and harnessing the various natural sources of water, by the construction of dams and reservoirs, canals and head works and finally distributing the water to the agricultural fields. Irrigation engineering includes the study and design of works in connection with river control, drainage of water logged areas, and generation of hydroelectric power, Necessity Basically an agricultural county, and all its resources depend on the agricultural output. Water is evidently the most vital element in the plant life. Water is normally supplied to the plants by nature through rains. However, the total rainfall in a particular area may be either insufficient, or ill-timed. In order to get the maximum yield, it is essential to supply the optimum quantity of water, and to maintain correct timing of water, This Is possible only through a systematic Irrigation system—by collecting water during the periods of excess rainfall and releasing it to the crop as and when it is needed Thus, the necessity of irrigation can be summarized in the following four points: Less Rainfall When the total rainfall is less than needed for the crop, artificial supply is necessary. In such a case, irrigation work may be constructed at a place where more water is available, and then to convey the water to the area where there is deficiency of water. Scope of Irrigation Science The scope of irrigation is not limited to the application of water to the soil. It deal! with all aspects and problems extending from the watershed to the agricultural farms. It deals with the design and construction of all works, such as dams, weirs, head regulators etc. in connection with the storage or diversion of water, as well as the problems of subsoil drainage, soil reclamation and water-soil crop relationships. An irrigation engineer is also required to have the knowledge of cultivation of various crops, their maturing and protection from pests. Briefly speaking, the scope of irrigation can be divided into two heads:
(a) Engineering aspect
(b) Agricultural aspect. (a) Engineering aspect 1. Storage Diversion, or Lifting of Water This is the first phase of irrigation engineering. By the construction of a dam across the river, a suitable reservoir can be created arid water can be stored. Alternatively, if river is perennial and carries sufficient discharge, suitable diversion works, such as a weir, barrage and bandhara can be constructed across the river and water can be diverted to the canal. Places where ground water table is high, suitable wells can be dug and water can be lifted and fed to small channels or pipes. 2. Conveyance of Water to the Agricultural Fields The stored or diverted water is conveyed to the agricultural fields through some suitable distribution system. If the project is big, the distribution system will consist of two or more main canals and a number of distributaries, and minors. From the canal, the water is led to the field through water course’ or field channel. These field channels take off from the distributaries or minors, through modular or non-modular outlets, so that water supply can be regulated. Thus, this second phase includes the design and construction of suitable canal system, along with various regulatory works such as head regulators. cross regulators, falls etc for the efficient working of the canal, In addition to these, suitable cross drainage works, such as aqueducts, super passages, level crossings, bridges, etc. will have to be designed and constructed at the places where the canal crosses a natural drain or a road. 3. Application of Water to Agricultural Fields The water is applied to the crop by flooding, furrows, corrugations, subsoil irrigation or by sprinkling. A detailed account of these methods has been given in the next chapter. 4. Drainage and Relieving Water-Logging Proper disposition of excess water by suitable drainage methods is very important. Due to the conveyance losses, and excess application of water to the crops the sub-soil water level is raised, and sometimes the soil is water-logged and becomes useless from production point of view. Hence the design of surface and subs surface drainage system is of vital importance in maintaining the high productivity of irrigated lands. 5. Development of Water Power Most of the irrigation projects are associated with the generation of the hydroelectric power either at the dam site or at canal falls. (b) Agricultural Aspect The agricultural aspect deals with the thorough study of the following points:
1. Proper depths of water necessary in single application of water for various crops.
2. Distribution of water uniformly and periodically.
3. Capacities of different soils for irrigation water, and the flow of water in soils
4. Reclamation of waste and alkaline lands, where this can be carried out through the agency of water. A multipurpose river valley project includes the following aspects:
1. Irrigation
2. Water supply for public health and sanitation
3. Generation of hydroelectric power
4. Flood control and river training
5. Soil conservation
6. Inland navigation
7. Fish culture. A multipurpose river valley scheme embodies the study of the following sciences:
1. Agronomy and animal husbandry
2. Soil science and mechanics
3. Engineering: Civil, mechanical, electrical and agricultural
4. Hydrograph and hydrology
S. Hydraulics
6. Geology
7. Sociology
8. Economics
9. Infrastructure
10. Administration Sources of Water The sources of water, may be classified as surface sources and sub-surface sources. The surface sources of water are the sources in which the water flows over the earth’s surface. These include rivers, lakes, streams, natural ponds, storage or impounded reservoirs. The water from these sources is known as surface water. The sub-surface or ground water sources are these which, supply water from below the earth’s surface. These include springs, infiltration galleries, wells and porous pipe galleries.

The quantity of surface water depends mainly upon rainfall. The topography of the catchment area is important in addition to the rainfall and run-off in case of impounded reservoirs. There are two types of impurities in water supply i.e. suspended and dissolved. The surface waters have suspended impurities. The suspended matter contains the pathogenic or disease producing bacteria. Therefore, surface waters are not considered safe, for water supply without the necessary treatment.

The sub-surface sources are the important sources of water supply. The water of ground water sources does not require any treatment and its temperature remains uniform throughout the year. The rain that falls on earth’s surface percolates partly through it and becomes the sub-surface water or ground water. The earth is formed of different layers of materials like sand, gravel and lime stone etc. The layers which allow the water to pass through them” and, contain quantities of water are known as pervious layer or aquifers or water bearing strata i.e. layers of sand, gravel etc. The layers such as limestone and sand stone which do not allow the water to pass through them are called impervious strata or a Aquacade. The top surface of water in the soil is called underground water table or simply water table. The portion of the soil through which lateral movement of water takes place, is called zone of saturation.

The natural outflow of sub surface or ground water at earth’s surface is called a spring. This is also called out ropping of the water table. Some springs discharge hot water due to the presence of sulphur and other materials. The spring may be gravity springs (surface spring and shallow spring) and artesian or deep seated spring. When a acquifer is exposed in a valley against a vertical cut, a surface spring is formed.

In shallow springs, the ground water is collected in the form of reservoir and forces the water to overflow at the surface of the ground.

An artesian or deep seated spring is formed when pervious or water bearing stratum is closed between two impervious stratums and have too much hydraulic gradient and water flows out automatically.

The horizontal tunnels constructed at shallow depths along the banks of a river to intercept the ground water table are called Infiltration galleries. The quantity of water available from infiltration gallery depends upon the yield of the source, nature of soil and size of gallery. The yield of the gallery may be about 15000 litres per metre length of the gallery. The infiltration galleries are most suitable for increasing the surface source supply in dry season.

The vertical wells provided along the banks of a river to draw ground water in dry season and percolated water in rainy season, are called infiltration wells. These are also called rainy wells.

A vertical hole dug out or drilled into the ground to get sub-surface or ground water is known as a well. The wells may be gravity wells, artesian wells, shallow wells, deep wells, open or dug wells, tube well and driven wells.

A tube or pipe fitted at the bottom with a filter sunk into the ground to tap the underground water is known as tube well. Now-a-days tube wells are becoming more and more popular for supplying water. The quality of tube well water is good as compared to surface water.