General Water resources engineering is an important branch of engineering for all living beings. Water is the best gift of nature to all living beings. There is no life without water. All the plants, creatures and humans survive on the earth due to water only. Irrigation is the process of artificially supplying water to the soil as per crop requirement for raising crops. In the absence of required quantity of moisture in the soil, the plants and crops cannot grow. The main function of irrigation is to supplement the rain fall. Natural rainfall, snowfall, hail etc are the primary sources of water on and below the earth`s surface In tropical areas, the quantity of rainfall is insufficient to meet the minimum requirements of moisture, required by the crops and plants for their growth. In such areas for proper growth of crops it is necessary to supply the required quantity of water artificially. As far as tropical countries are concerned, no tropical country can develop without proper irrigation. In this chapter the aspects of water resources engineering are discussed as regards to necessity, advantages, disadvantages of irrigation, different types of dam watershed management and ground water recharge.Necessity of Irrigation Irrigation is necessary because of following reasons1) Non uniform rainfall: The rainfall in a particular area may not be uniform over the crop period. Sometimes during the crop period, there is lack of water. Due to this either the yield may be less or the crop may die. To avoid this, during the period of excess of rainfall, extra water is stored in a reservoir and it is supplied as and when required.2) Less rainfall: When the total rainfall is less as compared to that required for growth of crops, and then we have to supply water to that area from other areas where more quantity of water is available.3) Raising cash crops: For raising cash crops like sugarcane, cotton and fruits etc extra water is required than available rainfall, then irrigation is must.4) Controlled water supply: By supplying water to crops at proper time yield of crop is increased.Benefits of Advantages of Irrigation1) Increase in food production: Due to the controlled and timely supply of water to the crop, the yield increases.2) Protection from famine: Due to construction of irrigation works, employment is created to people and thus they get relief against famine.3) Cultivation of cash crops: Irrigation makes it possible to grow cash crops such as sugarcane, cotton, tobacco etc.4) Addition to the wealth of country: Revenue of the state increases, crop produced makes country self sufficient in food requirement.5) Increase in prosperity of people: Due to more yield from crops, the income and living standard of farmers is improved.6) Generation of hydro electric power: From major projects and waterfalls electricity is generated, which is free from pollution.7) Domestic and industrial supply: The water from reservoir and canal network can be used for domestic and industrial supply of water.8) Inland Navigation: economical mode of transportation is in large canals inland navigation is possible.9) Improvement of communication: Irrigation channels and their side roads improve communication facilities.10) Canal plantation: Along the sides of canal damp and wet condition prevails, where plantation is possible.11) Improvement in the ground water storage: Due to constant percolation of water ground water table is raised.Disadvantages of IrrigationFollowing are the disadvantages of excessive irrigation1) Water logging: Due to over-irrigation the water table rises up to ground level causing water logging and salt efflorescence in the area.2) Spreading of mosquitoes: Due to excessive irrigation ponds and depressions are filled with water, causing breeding places for mosquitoes.3) Costly structures: Costly cross drainage works are required at crossing of canal and natural steams. For crossing roads and railway lines costly bridges, culverts are to be constructed.4) Damp climate: The areas which are already damp and cold become damper and colder due to irrigation.Multipurpose Irrigation Project Dams are constructed across the rivers and streams to create an artificial lake or reservoir behind it. Dams and reservoirs are the most important elements in multipurpose irrigation projects. They require very careful planning, designing and operation.Storage works are constructed to survey many purposes, such as,i) Storage and control of water for irrigation.ii) Storage and diversion of water for domestic use.iii) Water supplies for industrial use.iv) Generation of hydroelectric powerv) Increasing water depth for navigationvi) Storage space for flood control.vii) Preservation and cultivation of useful aquatic life andviii) RecreationDepending upon the purpose served, reservoirs may be classified as under.1) Storage or conservation reservoir: Storage reservoirs are primarily used for water supplies for irrigation, hydroelectric power generation, domestic and industrial water supply. A storage reservoir is constructed to store excess water during the period of high inflow and release it gradually as and when required.2) Flood control reservoir: Floods protection reservoirs are those which store water during floods and release it gradually at a safe rate when the flood recedes. These are constructed to protect the area lying on its downstream side from the damages due to floods.3) Distribution reservoir: A distribution reservoir is small storage reservoir used for water supply in a city.4) Multipurpose reservoir: A reservoir planned and constructed to serve various purposes together is called multipurpose reservoir. A reservoir designed and constructed for one purpose and incidentally serving other purposes is not considered as multipurpose reservoir. Such reservoir will be called multipurpose reservoir only when designed to serve those purpose also in addition to its main purpose. A reservoir designed to protect downstream area from floods and also to store water for power supply, irrigation, industrial needs, hydroelectric power generation etc. is called a multipurpose reservoir. Since in most of the cases single purpose reservoirs are not economically feasible usually multipurpose reservoirs are constructed.Dams A dam is a hydraulic structure constructed across a river to store water on upstream side. So necessarily, dam should be an impervious or fairly impervious barrier to form reservoir.Classification of dams Dams may be classified into different categories depending upon the purpose they serve on the basis of material used for construction.1) Classification according to use[i] Storage dam: A storage dam is constructed to store water on its upstream side during the period of excess supply and use it in period of deficiency.[ii] Diversion dam: A diversion dam raises water level slightly in the river, and thus water is diverted into canal system. A diversion dam is always of smaller height and no reservoir is formed on its upstream to store water.[iii] Detention dam: A detention dam is constructed to store water during floods and release it gradually as at a safe rate when the flood recedes. By the provision of detention dam, flood damage downstream is reduced.2) Classification according to material[i] Rigid dam: Rigid dams are those which are constructed of rigid materials such as masonry, concrete, steel or timber.[ii] Non rigid dam: Non rigid dams are those which are constructed of non rigid materials such as earth and/or rock fill.3) Classification according to flow condition[i] Overflow dam: If water is allowed to flow over the top of dam it is known as overflow dam. These are also known as spill ways. Gates are normally provided over spill ways for allowing the water to pass to downstream during floods and for extra storage of water after the floods.[ii] Non overflow dam: If the water is not allowed to flow over the top of the dam, it is known as non overflow dam.All the dams are generally a combination of overflow and non overflow damsSelection of site for DamThe following factors should be considered for the selection of site for various types of dams.1) Suitable foundations should be available at the site selected for particular type of dam. Foundation should be free from open pockets or fault planes. No dam should be built across an active fault.2) The length of dam should be as small as possible, and for a particular height should store maximum volume of water. It means the river valley at the dam site should be narrow, but should be wider on upstream.3) The bed level at the dam site should be preferably higher than that of the river basin.4) Good site for the location of separate spillway is essential in the case of earth or rock fill dam.5) Materials required for a particular type of dam should be locally available.6) The reservoir basin should be fairly water tight.7) The sight should be such that a deep reservoir is formed and the value of land and minimum property should be submerged by the proposed dam 8) The dam site should be easily accessible for transportation of cement, labour, machinery, food and other equipments.9) Healthy environment should be available near the site for construction of labour and staff colonies. Gravity Dam (Rigid Dams) A gravity dam is one in which the external forces such as water pressure, silt pressure, uplift pressure, wave pressure etc are resisted by the self weight of dam. Thus weight of the dam or gravity forces maintains stability of the dam. A gravity dam may be constructed either of masonry or concrete. The different terms of importance are as follows1) Maximum water level or full reservoir level: The maximum level to which water rise during the worst flood is known as the maximum water level or full reservoir level.2) Minimum poor level: The lowest water surface elevation up to which water in reservoir can be used is called minimum pool level.3) Normal pool level: It is the maximum elevation to which the reservoir water surface will rise during normal operation conditions.4) Useful and dead storage: The volume of water stored in the reservoir between the minimum pool level and normal pool level is called useful storage. The volume of water stored in the reservoir below minimum pool level is known as dead storage.5) Free board: The margin between the maximum water level and top of the dam is known as free board. Free board must be provided to avoid the possibility of water spilling over the top of the dam due to wave action.6) Drainage gallery: A gallery provided near the foundation to drain off the water which seeps through the foundation and the body of dam is called drainage gallery.Earthen Dams and Rock fill Dams (Non Rigid Dams) Earth dams are constructed using locally available soils and gravels; they can be used up to moderate height only.Earth dams may be classified as, 1) Homogeneous type dams,2) Zoned type dams,3) Diaphragm type dams. A homogeneous type earth dam is constructed from a single kind of material. A zoned type dam is made up of two or more different materials. In such dams impervious zone in the middle is supported and enclosed from both sides by impervious materials. Impervious core may be inclined or vertical. In diaphragm type dam a thin central impervious core is supported by thick impervious cores. In zoned type and diaphragm type earth dam impervious and pervious cores are separated by transition filters. Cross section of earth dam involves following1) Crest width: It is determined by the requirement of roadway at top. Generally it is between 6 to 12 m but in no case should be less than 3 m.2) Side slopes: Side slopes of earth dam usually vary in the range between 2 horizontal to 1 vertical to 1 vertical depending on the foundation soil and the material used for construction of dam.3) Free board: Sufficient free board must be provided in earthen dam to avoid overtopping of dam. It is about 1.5 times the wave height plus some safety factor. Usually free board is about 3 m.4) Horizontal drainage blanket: It is provided at the base of dam, which extends from downstream toe of the dam towards the centre of dam. Its thickness should not be less than 1 m. Its length is about 25 to 100% of half of the base width of dam. It facilitates easy drainage of seeping water.A rock fill dam is constructed by using rubbles or rock pieces of various sizes to provide stability and an impervious membrane to provide water tightness. Advantages of earth dams1. Earth dams can be constructed on any type of foundation.2. They can be constructed rapidly with relatively unskilled labour and locally available materials.3. They are generally economical. Disadvantage of earth dams1. They fail suddenly without any warning.2. Separate spillways are required to drain extra water during floods.3. They require constant maintenance and supervision.Impervious layer, core and shell prevent water to drain through cross section of dam. The water tightness to earth and rock fill dam is due to these impervious elements only.Important Multipurpose Irrigation Projects Some important multipurpose irrigation projects of India are,1) Bhakra dam, 2) Nagarjuna Sagar dam, 3) Jayakwadi project.1) Bhakra dam Bhakra dam is a 226 meter high concrete gravity dam. The dam is about 520 meter long at the top and only 100 meter at the bottom. The thickness of dam at foundation is about 400 meter and it tapers to 9 meter at the top which acts as road surface. Bhakra dam was the highest concrete gravity dam of the world is Grand Dixon Dam in Switzerland (284 meter high). Bhakra dam is situated near Bhakra village in Himachal Pradesh, state of India. It has been constructed on Satlaj River coming from Himalaya. Satlaj is a perennial river which carries huge quantity of water during floods and rains. The various functions served by this dam arei) Flood control ii) Irrigationii) Electricity and iv) Fish developmentBhakra dam project consists of the followingi) 226 meter high Bhakra dam.ii) Nagal Hydel channel.iii) Ganguwal and Kotta power houses.iv) Bhakra canal system.v) About 28 meter high Nagal dam.2) Nagarjuna Sagar dam The multipurpose Nagarjuna Sagar dam is located across Krishna River, near nandikonda village in Nalgonda district of Andhra Pradesh. It is named after Acharya Nagarjuna, who lived at the spot about 2000 years ago. The dam is a 125 meter high masonry dam and 1450 meter long. The maximum base width of dam is 98 meter and top width is 8.5 meter. The water-shed area at the dam site 83087 sq miles and observed maximum flood discharge is 1.1710^6 cusecs. The dam irrigates in Guntur, Kurnool, Nalgonda, Khammam and Krishna districts. Its irrigation potential is above 3.510^6 acres of land and electrical potential is 1 lakh kilowatt hours of firm power and 4.6 lakh kilowatt hours of seasonal power.3) Jayakwadi project The Jayakwadi project is the largest irrigation project in Marathwada region of Maharashtra state. The main dam of this project is constructed across Godavari River at Paithan for storing 2909 million cubic meter of water. The dam is 10210 meter long and consists of 610 m long masonry overflow dam in the river gorge, with earthen portion of dam on both sides. Maximum height of earth dam above ground level is 37 m while deepest foundation is 27 m below ground level, in the part of the earth portion. Maximum height of masonry dam is 37 m. The reservoir created behind Paithan dam, which is named as Nathsagar, has submerged 35000 hectares of land covering 94 villages. These villages have been shifted and rehab lined at safer places. This has affected a population of 77000 persons.Economic benefits of Jayakwadi projectI. Direct benefits1) Increased agricultural productions It is observed that due to commissioning of the project, production of crop has steadily increased.2) Fisheries The production of the fish is of order of 650 million tons per year which is worth Rs. 130.00 lakh.3) Water supply for drinking purposes the requirements of municipal water supply to important adjacent towns is dependent on this dam only.4) Industrial development Jayakwadi project is supplying water to Paithan and Aurangabad industrial area.5) Generation of hydroelectric power For 5 hours a day, with a discharge of 50 m^3/sec, it is generating 12 M W power.II. Indirect benefits1) Flood protection 2) Support to sugar factories2) Employment generation 4) Tourism development