irrigation and water resources created by sajid and farooq

WATER RESOURCES AND IRRIGATION ENGINEERING CED-401

IRRIGATION: The process of artificial application of water to the soil for the growth of agricultural crops is known as irrigation. INTRODUCTION ABOUT IRRIGATION: It is practically a science of planning and designing of the water supply system for the agricultural land to protect the corps from bad effects of drought or low rainfall. It includes the construction of weirs or dams, barrages or canal system for the regular supply of water to the cultivable lands. There are three essential requirements of plant growth, 1. Heat 2. Light 3. Moisture In England the one of three essential requirements is available that is moisture, which means irrigation is not required due to sufficient rainfall. But in Pakistan the first two of the three essential requirements of pant growth that is light and heat is present in large amount but the third (moisture) is required due to insufficient rainfall . Hence Irrigation is supplementary to rainfall. When the rainfall is either deficient or comes irregularly or at unseasonable times in Pakistan. COMPONENTS OF THE IRRIGATION SYSTEM OR IRRIGATION SYSTEM NETWORK: The main components of an irrigation system are listed below, 1. Dam /Barrage 2. Canal Head regulator 3. Main canal 4. Branch Canal 5. Distributory canal 6. Minor canal 7. Water course (W.C) The layout of irrigation network is shown on next page.

LAYOUT OF IRRIGATION NETWORK 1

Compiled Muhammad Farooq Zia By: (2005-CE-44) Muhammad Sajid Nazir


DAM

RIVER

BARRAGE

BRANCH CANAL DISTRIBUTORY

MAIN CANAL

IRRIGATION SYSTEM

W.C

MINOR

W.C FIELD

OUTLET W.C

OFWM

OFWM = ON FARM WATER MANAGEMENT W.C = WATER COURSE

PHASES OF IRRIGATION ENGINEERING: There are four stages of Irrigation Engineering, 1. Storage (Dams, Reservoirs) or Diversion (Barrages) 2



2. Conveyance of irrigation water (Canals) 3. Distribution (water courses ) and Application (Irrigation methods ) of irrigation 4. Drainage of excessive water through drains IMPORTANCE OF IRRIGATION: Question: Why irrigation is required? Answer: We know the water requirements for crops vary from place to place depending upon the nature of crops and site. In some areas there is no need of irrigation because the conditions are fulfilled by the natural resources (rainfall etc). Most of the areas of the earth are situated in arid zones (less than 15'' of mean annual rainfall) and even in humid zones (greater than 30'' of mean annual rainfall). The rainfall is not distributed evenly. So it is only possible to use artificial means (supplying channel) to provide water for more cultivation. In Pakistan the annual rainfall ranges from 75 mm to 800 mm. So irrigation becomes a necessity to provide, (i)

(ii) (iii)

Sufficient amount of water in desirable seasons Period of requirement ( Time when water is required) Desirable amount of water for sufficient type of crops

Thus the necessity of irrigation can be summarized in the following four points, 1. Less rainfall (Need fulfill through artificial supply ) 2. Non-uniform rainfall (through dam , requirement is fulfilled at the need time crop) 3. Commercial crops with additional water ( more water is required for cash crops e.g. Sugar cane , tobacco, rice ) 4. Controlled water supply (By the construction of proper distribution system the yield of crop may be increased)

NECESSITY OF IRRIGATION: Question: What are the factors which govern the necessity of irrigation? Answer: Throughout the crop period adequate quantities of water is required near the root zone of the plants for their growth. At times during the crop period the rainfall may not be adequate to fulfill the water requirement. The intensity of rainfall is practically uncertain and beyond the control of human power and it may not be well distributed throughout the crop season or the culturable area. So, irrigation becomes absolutely necessary to fulfill the water requirement of crops. The following are the factors which govern the necessity of irrigation, (a)Insufficient Rainfall: When the seasonal rainfall is less than the minimum requirement for the satisfactory growth of crops, the irrigation system is essential. (b)Uneven Distribution of Rainfall: 3



When the rainfall is not evenly distributed during the crop period or throughout thre culture able area the irrigation is extremely necessary. (c)Improvement of Perennial Crops: Some crops like sugarcane, cotton, etc require water throughout the major part of the year. But the rainfall may fulfill the water requirement in rainy season only. So for the remaining part of the year, irrigation becomes necessary. (d)Development of Agriculture in Desert Area: In desert area where the rainfall is very scanty, irrigation is required for the development of agriculture.

Merits and demerits of Irrigation: The following are the benefits of Irrigation, Merits of Irrigation 1. Yield of Crops: In the period of low rainfall or drought the yield of crop may be increased by the irrigation system. 2. Protection from famine: The food production of a country can be improved by ensuring the growth of crops by availing the irrigation facilities. This helps a country to prevent famine situation 3. Improvement in cash crops: Irrigation helps to improve the cultivation of cash crops like vegetables, fruits, tobacco, etc. 4. Prosperity of farmers: When the supply of irrigation water is assured, the farmers can grow two or more crops in a year on the same land. Thus the farmers may earn money and improve their living standard.

5. Source of revenue: When, irrigation water is supplied to the cultivators in lieu of some taxes. it helps to earn revenue which may be spent on other development schemes. 6. Hydroelectric Power generation:

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In some river valley projects, multipurpose reservoirs are formed by constructing high dams where hydroelectric power may be generated along with the irrigation system. 7. Water Supply: The irrigation canals may be the source of water supply for domestic and industrial purposes. Demerits of Irrigation 1. Rising of water table: Due to the excessive of water through the bed and banks of the canals, the water table in the surrounding area may be raised which may constantly saturate the root zone of the crops and the soil may develop alkaline property which is harmful to the crops. 2. Formation of marshy land: Excessive seepage and leakage of water from the irrigation canals may lead to formation of marshy lands along the course of the canals. These marshy lands form the colonies of mosquitoes which may be responsible for diseases. 3. Formation of marshy land: The temperature of the commanded area of an irrigation project may be lowered considerably and the area may become damp. Due to dampness, the people residing around the area may suffer from cold, cough and other such diseases originating from dampness. 4. Loss of valuable lands: Valuable land may get submerged when the storage reservoirs are formed by constructing barrages or dams and it also may be lost, while constructing irrigation canals. Resources of Irrigation: There are three resources of irrigation, 1. Rainfall 2. Surface Water 3. Ground Water

1. Rainfall: Rainfall can directly help irrigation by precipitation occurring over the crop area or indirectly by adding its runoff to the rivers. This runoff is then stored by weir, barrage or dam downstream or it may replenish as an underground reservoir. Direct rainfall is the most helpful for the plant and crop growth if it occurs in proper amount at proper time .But it is unreliable as a source of irrigation water. It varies from year to year and it may fall altogether. It is irregularly distributed throughout the year as well as within the same season.

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In Pakistan, It occurs particularly in the summer season in the form of high showers resulting in the heavy rainfall. As the temperature is high evaporation rate is also increased. It is a great booster for agriculture. For canal irrigated areas the rains compliment the irrigation water. In Pakistan the mean annual rainfall ranges from 4 to 30 inches in the lower Indus region to the northern foot hills. Only a small proportion of this annual rainfall makes any direct or useful contribution to irrigation water supplies. According to World Bank consultants report the figure ranges from 1 to 17 inches. The rest is either converted to Direct Runoff or becomes a part of the ground water. While a small proportion is lost by evaporation. According to estimation the present direct contribution to the crops is 6 MAF / Annual. 2. Surface Water: Surface water include water diverted from the stream and stored into dams and barrages and then applied to the land through canals or pumped from rivers, lakes and canals . In dry months melting snow adds a great amount of water to the river discharge. Snow remaining on ground provides storage greater than any man made reservoir for 1 foot snow holds 1-4 inches of water. Snowfall usually occurs over many square miles on the mountainous terrain providing a surface reservoir which is then released in the summer months. The most important thing for irrigation engineer is when and how fast this vast quantity is released. In Pakistan, the rivers carry the melting snow and rains from the northern hills to the areas where they can be used for irrigation purpose. River water available in PAKISTAN for irrigation is, Average flow of River Indus = 90 MAF Average flow of River Jhelum = 23 MAF Average flow of River Chenab = 27 MAF Average flow of River Ravi = 3 MAF Average flow of River Sutlej = 2 MAF Total

Surface

Flow

= 145 MAF TOTAL SURFACE FLOW (145 MAF)

CANAL DIVERSION (105 MAF)

WASTAGE TO SEA (40 MAF)

TOTAL WATER AVAILABLE FOR FIELD CROPS (72 MAF)

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3. Ground Water: Along with the rainfall and surface water the ground water is an important source of irrigation. In Pakistan we have enormous ground water reservoirs. In rainy season, due to rain, most of water seeps into the earth thus raising the water table in ground. This water is then taken out with the help of pumps and tube wells for irrigation purpose. The areas for which there is no access of canals, there we can get water for irrigation from underground sources of water. Ground water can cause water logging sometimes due to rise in water table and this can be avoided by pumping out water from the ground using several techniques i.e. pumps, tubewells etc. In the underground water, there are less chances of the presence of impurities but it does not contain silt which is helpful for crop production acting as a fertilizing agent. In Pakistan we normally use all three sources of irrigation. But based on quality, sometimes it may be desired to use single source of water or it may be necessary to mix the ground water with the surface water so that the combined salts of both sources mixed in any quantity may not cause any damage to the crops. FLOW CHART OF RAINFALL FROM SOURCE TO FIELD: RAIN FALL

AVERAGE FLOW AT FORM-GATES (NAKKAS) (13 MAF)

WATER AVAILABLE FOR CROPS 72% (9 MAF)

FIELD APPLICATION LOSSES 28% (4MAF)

FLOW CHART OF SURFACE WATER FROM SOURCE TO FIELD: 7



GROUND WATER

GOVERNMENT TUBE WELLS (9MAF)

CONVEYANCE LOSSES 15% (1.35 MAF)

PRIVATE TUBE WELLS (35MAF)

AVERAGE FLOW AT NAKKAS 85% (7.65 MAF)


AVERAGE FLOW AT NAKKAS 95% (33.25 MAF)

FIELD APPLICATION LOSSES 28% (12 MAF) WATER AVAILABLE FOR CROPS 72% (29 MAF)

FLOW CHART OF GROUNDWATER FROM SOURCE TO FIELD: AVERAGE ANNUAL FLOW AVAILABLE IN RIVERS (146 MAF)

DIVERSION TO CANAL IRRIGATION SYSTEM 75% (109.5 MAF)

HEAD AT WATER COURSES OUTLETS 75% (82.125 MAF)

FLOW TO FORM-GATES 55% (45.16875 MAF)

WATER AVAILABLE FOR CROPS 72% (32.5215 MAF)

FLOW TOWARDS SEA 25% (36.25 MAF)



FIELD APPLICATION LOSSES 28% (12.64725 MAF)

Types Of irrigation System: There are three types of irrigation, 8



(i)Gravity Flow or Surface Water Flow Irrigation (ii)Tank or Reservoir Irrigation (iii) Lift Irrigation (i) Gravity Flow Irrigation: Gravity flow irrigation is providing water where water flow is due to gravity, not under any mechanical means. Due to gravity water flows from higher areas to the lower areas. After which it is distributed in the fields. Silt in the canal water has a manurel value (fertilizing agent).The whole canal irrigation in our country is gravity irrigation. The gravity flow is cheaper and the quality of water is very good because of the presence of silt content. (ii) Tank irrigation: If the runoff is more than the required amount then headwork and barrages are constructed to store the water. A head work consists of a weir, canal head regulator, gate structure (barrage). So, a headwork is a complete system of structures. Whereas barrage is a part of a headwork, it is constructed in the path of the river to obstruct water. The flow of a river is seasonal flow. Sometimes more water is required like in December but source is scanty and sometimes less water is required like in March but the source is high. So in order to regulate the flow the reservoirs are constructed in order to 1. Fulfill the irrigation requirements 2. Generate the hydraulic power 3. Regulate the river flow so as to avoid flood In some areas small dams are constructed for the irrigation purposes. As the topography of these areas, does not allow the possibility of constructing a canal. (iii) Lift Irrigation: When the main source is at the lower level than the supply level then we try to supply water by using some mechanical means, such type of irrigation is known as Lift Irrigation. This can be done by the following methods, (a) Lift from canals (b)Open Wells (c) Tube wells (a)Lift From canals (Rivers): Pumps are used to lift the water from canals or rivers at lower level to the area at higher level for irrigation purpose. (b)Open Wells: In villages there are some open holes whose depth intercepts the water table. So the water is taken out from lower level to the surface for irrigation purpose by adopting different manual and mechanical methods. 9



(c)Tube Wells: It is the lifting of water by pumping from underground reservoir. Extensive surface irrigation results in an increase in the ground water level due to percolation and seepage which causes water logging in large areas. Irrigation by this method will reduce the yield. Tube well irrigation offers a remedial measure by providing sub-surface drainage. Tube well irrigation can be obtained more quickly than from surface water project. Large capital costs involve in canal irrigation for the construction of dam, canal and headwork system but tube well construction cost is very less.

Flow Irrigation: The irrigation system in which the water flows under gravity from the source to the agricultural land/field. It is also called canal irrigation. Canal 10



It is an artificial channel constructed on the ground to carry water to the field either from a reservoir tank or river. Classification of Canals: The canals are classified on the bases of, 1. Based on the nature of source of supply (a) Perennial Canals (b) Non-Perennial Canals (c) Inundation Canals CANAL CLASSIFICATION BASED ON THE NATURE OF SOURCE

Non-Perennial Canals

Perennial Canal

Inundation Canals

(a) Perennial Canals These are the canals which get continuous supplies by permanent source of supply like a river or reservoir are called as permanent canals or perennial canals. These irrigate the field throughout all the year with equitable rate of flow. (b) Non-Perennial Canals These are the canals which irrigate the field for only one part of the year usually during summer season or at the beginning and end of winter season, called as non-perennial canals. These canals take-off from rivers which do not have assured supply throughout the year. (c) Inundation Canals These are the canals in which the supply depends upon the periodical rise in the river from where these take off. When the water level rises above the bed level of the canal the water starts flowing through the canal. As the water level fall below the bed level of the canal. The flow of water through the canal stops. No regulator is provided at the head of such canal. This draws lot of quantity of silt which is really beneficial for the crops.

2. Based on the function of canals (a) Navigation Canals (b) Irrigation Canals (c) Power Canals CLASSIFICATION BASED ON THE FUNCTION OF CANAL (d) Link Canals (e) Feeder Canals

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Navigation Canals

Irrigation Canals

Power Canals

Feeder Canals

Link Canals

(a) Navigation Canals The canal which is constructed to carry water from the source to the agriculture land for the purpose of irrigation is known as irrigation canal. In this canal the velocity of flow is kept high so that the water may carry silt in suspension for good command areas. (b) Irrigation Canals These are the canals which are used for providing transportation and voyage facilities nationwide and internationally. Sometimes these are also used for irrigation purposes. (c) Power Canals The canal which is constructed to supply water with very high force to the hydroelectric power station for the purpose of moving turbine to generate electric power is known as power canal. (d) Link Canals These are the canals which are constructed to transfer water to the other conveyance structure which contain in-sufficient quantity of water. These transfer water from river to canal system. e.g. Sidhnai Mailsi Link Canal (e) Feeder Canals These are constructed to provide water to other conveyance structures. These are not used for irrigation. These canals feed two or more canals. e.g. Lower Chenab canal Feeder 3. Based on the discharge: (a) Main Canals (b) Branch Canals (c) Distributory Canals (d) Field Canals CLASSIFICATION BASED ON THE DISCHARGE OF CANAL

Main Canals

Branch Canals

Distributory Canals

Field Canals

(a) Main Canals 12



The main canal carries discharge directly from river therefore it carries large supply of water and cannot be used for direct irrigation. In main canals the discharge is greater than 10 cumecs. The water is taken to the field through the branch canal, distributory channel and field channel. (b) Branch Canals The branch canals are taken from either side of the main canal at suitable points so that whole command area can be covered by the network. The discharge varies from 5 to 10 cumecs. (c) Distributory Canals These take-off from branch canals. The discharge capacity of these channels varies from 0.25 to 3 cumecs. These are divided as, (i) Major Distributory (ii) Minor Distributory (i) Major Distributory These take-off from branch canals. Sometimes they may also take-off from main canals but their discharge is always less than the branch canals. These are real irrigation channels because they supply water to the field directly through outlets. The capacity varied from 0.25 to 3 cumecs. (ii) Minor Distributory These distributaries take-off from major distributaries or sometimes from branch canals. They also provide water to the water courses through outlets provided along with them. The discharge capacity in this type of canals is 0.25 to 3 cumecs. (d) Field Canals These channels are taken from the outlets of the Distributory channels by the cultivators to supply water to their own lands. These channels are maintained by the cultivators.

Object of Canal Lining: The following are the main objects of canal lining, (a)To Control Seepage The seepage loss is the maximum loss in unlined canals. Due to seepage the duty of canal water is much reduced which involves enhancement of storage capacity of a reservoir by constructing high dam. So, to control seepage loss through the bed and sides of the canal, the lining of the canal is necessary. (b) To Prevent Water-logging 13



Along the course of the canal, there may be low lying areas on one side or both sides of the canal. Due to the seepage of water through the sides of the canal, these areas may get converted into marshy lands. This water logging makes the land alkaline which is unsuitable for agriculture. This water logging area may become the breeding place of mosquitoes which are responsible for many infectious diseases. (c) To Increase the Capacity Of Canal In unlined canal, the velocity of flow should be fixed such that the silting and scouring is avoided. In practice, the velocity should always be kept below 1 m/s. Due to this low velocity, the discharge capacity of the canal becomes low. In unlined canal, if the capacity of the canal is to be increased the cross sectional area has to be increased which involves more land width. So, the lining of the canal should be such that the velocity and the discharge of the canal are more with minimum cross-sectional area. (d) To Increase the Command Area If the lining is provided in the canals the various losses can be controlled and ultimately the command area of the project may be enhanced. (e) To Protect the Canal From The Damage By Flood The unlined canals may be severely damaged by scouring and erosion caused due to high velocity of flood water at the time of heavy rainfall. So, to protect the canals from the damage, the lining should be provided. (f) To Control the Growth of Weeds The growth of various types of weeds along the sides of the canals is a common problem. Again, some types of weeds are found to grow along the bed of the canals. These weeds reduce the velocity of flow and the capacity of the canals. So, the unlined canals require excessive maintenance works for clearing the weeds. If lining is provided in the canal, the growth of weeds can be stopped and velocity and the capacity of the canal may be increased. Types of Lining: There are varieties of linings that are available today but we will be discussing the following three, 1. Plain Cement Concrete Lining 2. Reinforced Cement Concrete Lining 3. Brick Lining 1. Plain Cement Concrete Lining This lining is recommended for the canal in full banking. The cement concrete lining is widely accepted. It can resist the effect of scouring and erosion very efficiently. The velocity of flow may be kept above 2.5 m/s. It can eliminate completely growth of weeds. The lining is done by the following steps; (a) Preparation of sub-grade

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The sub grade is prepared by ramming the surface properly with a layer of sand (about 15 cm). Then slurry of cement and sand (1:3) is spread uniformly over the prepared bed. (b) Laying of concrete The cement concrete of grade M15 is spread uniformly according to the desired thickness, (generally the thickness varies from 100mm to 150 mm). After laying, the concrete is tapped gently until the slurry comes on the top. The curing is done for two weeks. As the concrete is liable to get damaged by the change of temperature, the expansion joints are provided at appropriate places. 2. Brick Lining This lining is prepared by the double layer brick flat soling laid with cement mortar (1:6) over the compacted sub-grade. The first class bricks should be recommended for the work. The surface of the lining is finished with cement plaster (1:3). The curing should be done perfectly. This lining is always preferred for the following reasons, (a)This lining is economical. (b)Work can be done very quickly. (c) Expansion joints are not required. (d) Repair works can be done easily. (e)Bricks can be manufactured from the excavated earth near the site. However this lining has certain disadvantages, (a) It is not completely impervious. (b)It has low resistance against erosion. (c)It is not so much durable. 3. Reinforced Cement Concrete Lining Sometimes reinforcement is required to increase the resistance against cracks and shrinkage cracks. The reduction in the cracks results in less seepage losses. However this reinforcement does not increase the structural strength of the lining. This reinforcement adds 10 to 15 percent to the cost and for this reason steel reinforcement is usually omitted except for very particular situations. Advantages of Canal Lining 1. It reduces the loss of water due to seepage and hence the duty is enhanced. 2. It controls the water logging and hence the bad effects of water-logging are eliminated. 3. It provides smooth surface and hence the velocity of flow can be increased. 4. Due to the increased velocity the discharge capacity of a canal is also increased. 5. Due to the increased velocity, the evaporation loss also can be reduced. 6. It eliminates the effect of scouring in the canal bed 7. The increased velocity eliminates the possibility of silting in the canal bed. 8. It controls the growth of weeds along the canal sides and bed. 9. It provides the stable section of the canal. 10. It reduces the requirements of land width for the canal, because smaller section of the canal can be used to produce greater discharge. 15



11. It prevents the sub-soil salt to come in contact with the canal water. 12. It reduces the maintenance cost for the canals. Disadvantages of Canal Lining 1. The initial cost of the canal lining is very high. So, it makes the project very expensive with respect to the output. 2. It involves many difficulties for repairing the damaged section of lining. 3. It takes too much time to complete the project work. 4. It becomes difficult, if the outlets are required to be shifted or new outlets are required to be provided, because the dismantling of the lined section is difficult.

FACTORS AFFECTING TYPE OF LINING Question: What are the factors which affect the type of canal lining? Answer: The selection of type of lining depends on the following factors, (1) Imperviousness When the canal passes through the sandy soil the seepage loss is at maximum and the canal is unstable. So, to make the canal perfectly impervious and reasonably stable, the most impervious types of linings should be recommended such as cement concrete etc. (2) Smoothness The smoothness of the canal bed and sides increases the velocity of flow which further increases the discharge of the canal. Due to the increased discharge, the duty of water will be more. So, to increase the duty, the canal surface should be made smooth. The lining like cement concrete, pre-cast cement concrete etc gives smooth surface to the canal. (3) Durability The ultimate benefit of any project depends on the durability of the hydraulic structures, canals, etc. So, to make the canal section more durable against all adverse effects like scouring, erosion, weather action, etc. the most strong and impervious types of lining should be recommended. (4) Economy The lining should be economically viable with the benefits that may be accrued from the expected revenue, yield of crop, etc. So, by studying the overall benefits the type of lining should be recommended. (5) Site Condition The canal may pass through the marshy land, loose sandy soil, alluvial soil, black clayey soil, hard soil, etc. So, according to the soil and site condition the type of lining should be recommended. 16



(6) Life of Project Every project should be designed to serve the future three or four decades successfully. The type of lining should be recommended keeping in mind the life of the project. (7) Availability of Construction Materials The expenditure of lining depends on the availability of construction materials, carriage charges, etc. To reduce the expenditure of lining, the materials which are available in the vicinity of the project should be utilized. MANNING’S ROUGHNESS COEFFICIENTS Question: What are the Manning’s roughness coefficients for brick, earth, R.C.C and P.C.C? Answer: It is denoted by “n”. Manning’s Roughness coefficients are: Brick: 0.014-0.017 (usually taken as 0.015) Earth: 0.02 P.C.C: 0.014 R.C.C: 0.012

Cross Drainage Works: A “cross drainage work” is a hydraulic structure which needs to be constructed at the crossing of a natural stream and an irrigation canal flowing normally at right angles underneath or over the natural stream. It is generally a very costly item and should be avoided by, i. Diverting one stream into another. ii. Changing the alignment of the canal so that it crosses below the junction of two streams. Necessity of Cross- Drainage Works: The following factors justify the necessity of cross drainage works, 1. The water shed canals do not cross natural drainages. But in actual orientation of the canal network, this ideal condition may not be available and the obstacles like natural drainages may be present across the canal. So, the cross drainage works must be provided for running the irrigation system. 17



2. At the crossing point, the water of the canal and the drainage get intermixed. So, for the smooth running of the canal with its design discharge the cross drainage works are required. 3. The site condition of the crossing point may be such that without any suitable structure, the water of the canal and drainage cannot be diverted to their natural directions. So, the cross drainage works must be provided to maintain their natural direction of flow. Types Of Cross Drainage Works: Depending upon the relative bed levels, maximum water levels and relative discharges of canals and drainages the cross drainage works may be of following types, 1.

Type1-Irrigation Canal passes over the drainage: In this type of C.D work, an irrigation canal is taken over the drainage

This condition involves construction of following, a) Aqueduct: The hydraulic structure in which irrigation canal is passing over the drainage is known as aqueduct. This structure is suitable when bed of canal is above the highest flood level of drainage. In this case, the drainage water passes clearly below the canal. b) Siphon Aqueduct: The hydraulic structure in which irrigation canal is passing over the drainage, but the drainage water cannot pass clearly below the canal is known as siphon aqueduct. It flows under siphoned action. This structure is suitable when the bed level of canal is below the highest flood level of the drainage. Advantages of Type1: • •

The canal running perennially is above ground and is open to inspection. Damage done by floods is rare.

Disadvantages of Type1: •

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During high floods, the foundation can be scoured or the water way of the drain may be chocked with trees. Compiled Muhammad Farooq Zia By: (2005-CE-44) Muhammad Sajid Nazir


1.

Type2-Drainage passes over the irrigation canal: In this type of cross drainage work, drainage is taken over the canal.

This condition involves the construction of the following, a) Super Passage: The hydraulic structure in which the drainage is passing over the irrigation canal is known as super passage. This structure is suitable when the bed level of drainage is above the flood surface level of the canal. The water of the canal passes clearly below the drainage. b) Siphon Super passage: The hydraulic structure in which the drainage is taken over the irrigation canal, but the canal water passes below the drainage under siphonic action is known as siphon super passage. This structure is suitable when the bed level of drainage is below the full supply level of the canal. c) Canal Siphon: If two canals cross each other and one of the canals is siphoned under the other, then the hydraulic structure at crossing is called “canal siphon”. For example, lower Jhelum canal is siphoned under the Rasul-Qadirabad link canal and the crossing structure is called “L.J.C siphon” Advantages of Type2: •

C.D works are less liable to damage then the earthwork of canal. Disadvantages of Type2: • •

Perennial canal is not open to inspection. It is difficult to clear the silt deposited in the barrels of the C.D.work.

1. Type3-Drainage and Canal intersection at the same level: In this type of work, the canal water and drainage water 19



are permitted to intermingle.

a) Level Crossing: When the beds of the drainage and canal are practically at the same level, then a hydraulic structure is constructed which is known as level crossing. This is suitable for the crossing of large drainage with main canal. The level crossing consists of the following components. 1. Crest Wall: It is provided across the drainage just at the upstream side of the crossing point. The top level of the crest wall is kept at the full supply level of the canal. 2. Drainage Regulator: it is provided across the drainage just at downstream side of the crossing point. The regulator consists of adjustable shutters at different tiers. 3. Canal Regulator: it is provided across the canal just at downstream side of the crossing point. The regulator consists of adjustable shutters at different tiers. a) Inlet and Outlet: In the crossing of small drainage with small channel no hydraulic structure is constructed. Simple openings are provided for the flow of water in their respective directions. It is not necessary for the number of inlets and outlets to be same. There may be one outlet for two or three inlets. A canal inlet is constructed when the cross drainage flow is small and its water may be absorbed into the canal without causing appreciable rise. Advantages of Type3: •

Low initial cost

Disadvantages of Type3: • • • •

Regulation of such work is difficult & requires additional staff The canal has to be designed to carry the increased flood discharge of drain. The faulty regulation of the gates may damage the canal. There is additional expenditure of silt clearance.

Suitability of Cross-Drainage Works: 20



The factors which affect the selection of the suitable type of cross drainage works are: 1. Relative bed levels and water levels of the canal and drainage 2. Size of the canal and the drainage The following considerations are important, 1. When the bed level of the canal is much above the highest flood level (H.F.L) of the drainage, so that sufficient headway is available for floating rubbish etc and also for the structural elements of the work. An ‘aqueduct’ is the obvious choice. Similarly, if the bed level of the drain is well above the Flood surface level (F.S.L) of the canal, ‘Super-passage’ is provided. 2. The necessary headway between the canal bed level and the drain H.F.L can be increased by shifting the crossing to the downstream of the drainage. If, however, it is not possible to change the canal alignment or if such a shifting does not give sufficient headway between the two levels, a ‘siphon aqueduct’ may be provided. Thus in case of siphon aqueduct, the H.F.L of the drain is above channel (canal) bed. 3. When the canal bed level is much lower but the F.S.L of the canal is higher than the bed level of drainage, a ‘canal siphon’ is preferred. 4. When the drainage and the canal cross each other practically at the same level a ‘level crossing’ may be preferred. This type of work is avoided as far as possible. The considerations governing the choice between aqueduct and siphon aqueduct (or a super passage and siphon-super passage) are, (i)Suitable canal alignment (ii)Suitable soil available for bank connections and (iii) Nature of available foundation As discussed earlier, the relative difference between the bed level of the canal and the H.F.L of the drainage can be suitable altered by changing the canal alignment so that the point of crossing is shifted upstream or Downstream of the drainage. For example, if the canal alignment is such that headway is not available between the H.F.L of the drain and the bed of the canal, a siphon aqueduct is to be constructed at the crossing. But if the other conditions are not favorable for the construction of the siphon aqueduct, the canal alignment may be changed so that the crossing is shifted to the downstream and sufficient headway required for the construction of an aqueduct is available. PROPER SITE FOR DRAINGE CROSSING: The site selected for the cross drainage works should have the following main characteristics, 1. It should be such that it requires minimum disturbance regarding the approach and tail reaches of the drainage channel. 2. Suitable foundation soil should be available at reasonable depth. 3. Sufficient headway is available for the super structure of the aqueduct over the H.F.L of the natural stream. 21



4. Suitable existing topography, geological and hydraulic conditions for the cross drainage works at reasonable costs.

WATER REQUIREMENTS FOR CROPS It is defined as, “The quantity of water required by a crop in a given period of time for normal growth under field conditions.” It includes evaporation and other unavoidable wastes. Usually water requirement for crop is expressed in water depth per unit area. Mathematically, Total water requirement = A + B + C A = Consumptive use of water (Evapotranspiration) B= Application and conveyance losses of water C = Special needs of water (Water required for preparation of land etc)

Water losses produced after passing from Nakka is called application losses. FACTORS AFFECTING THE WATER REQUIREMENT The following are the factors which affect on the water requirements of the crops, 1. Water table: If the water table is nearer to the ground surface, the water requirement will be less & vice versa. 2. Climate: In hot climate the evaporation loss is more and hence the water requirement will be more and vice versa. 3. Ground Slope: If the slope of the ground is steep the water requirement will be more due to less absorption time for the soil. 22



4. Intensity of Irrigation: It is directly related to water requirement, the more the intensity greater will be the water required for a particular crop. 5. Type of Soil: In sandy soil water percolates easily so water required is more. While in clayey soils water requirement is less. 6. Method of Application of water: In sprinkler method less water is required as it just moist the soil like rainwater whereas in flood more water is required. 7. Method of Ploughing: In deep ploughing less water is required and vice versa. CONSUMPTIVE USE OF WATER It is the quantity of water used by the vegetation growth of a given area. Mathematically, Consumptive Use = Evapotranspiration = Evaporation + transpiration

It is expressed in terms of depth of water. FACTORS AFFECTIING THE CONSUMPTIVE USE OF WATER Consumptive use varies with, 1. Evaporation which depends on humidity. 2. Mean Monthly temperature. 3. Growing season of crops and cropping pattern. 4. Monthly precipitation in area. 5. Wind velocity in locality. 6. Soil and topography. 7. Irrigation practices and method of irrigation. 8. Sunlight hours. TYPES OF CONSUMPTIVE USE Following are the types of consumptive use, 1. Optimum Consumptive Use 2. Potential Consumptive Use 3. Seasonal Consumptive Use 1. Optimum Consumptive Use: It is the consumptive use which produces a maximum crop yield. 23



2. Potential Consumptive Use: If sufficient moisture is always available to completely meet the needs of vegetation fully covering the entire area then resulting evapotranspiration is known as Potential Consumptive Use. 3. Seasonal Consumptive Use: The total amount of water used in the evapotranspiration by a cropped area during the entire growing season. METHODS OF ESTIMATION OF CONSUMPTIVE USE: For the estimation of consumptive use there are two main methods, 1. Direct Methods/Field Methods 2. Empirical Methods 1. Direct Methods: In this method field observations are made and physical model is used for this purpose. This includes, (i) Vapour Transfer Method/Soil Moisture Studies (ii) Field Plot Method (iii) Tanks and Lysimeter (iv) Integration Method/Summation Method (v) Irrigation Method (vi) Inflow Outflow Method (i) Vapour Transfer Method: In this method, soil moisture measurements are taken before and after each irrigation. The quantity of water extracted per day from soil is computed for each period. A curve is drawn by plotting the rate of use against time and from this curve, the seasonal use can be estimated. This method is suitable in those areas where soil is fairly uniform and ground water is deep enough so that it does not affect the fluctuations in the soil moisture within the root zone of the soil. It is expressed in terms of volume i.e. Acre-feet or Hectare-meter (ii) Field Plot Method: We select a representative plot of area and the accuracy depends upon the representativeness of plot (cropping intensity, exposure etc).It replicates the conditions of an actual sample field (field plot). Less seepage should be there. Inflow + Rain + Outflow = Evapotranspiration

The drawback in this method is that lateral movement of water takes place although more representative to field condition. Also some correction has to be applied for deep percolation as it cannot be ascertained in the field. (iii) Tanks and Lysimeter: 24



In this method, a watertight tank of cylindrical shape having diameter 2m and depth about 3m is placed vertically on the ground. The tank is filled with sample of soil. The bottom of the tank consists of a sand layer and a pan for collecting the surplus water. The plants grown in the Lysimeter should be the same as in the surrounding field. The consumptive use of water is estimated by measuring the amount of water required for the satisfactory growth of the plants within the tanks. Consumptive use of water is given by, Cu=Wa-Wd

Where, Cu=Consuptive use of water Wa=Water Applied Wd=Water drained off

Lysimeter studies are time consuming and expensive. Methods 1 and 2 are the more reliable methods as compare to this method. (iv) Integration Method: In this method, it is necessary to know the division of total area, i.e. under irrigated crops, natural native vegetation area, water surface area and bare land area. In this method, annual consumptive use for the whole area is found in terms of volume. It is expressed in Acre feet or Hectare meter. Mathematically, Total Evapotranspiration=Total consumptive use×Total Area Annual Consumptive Use=Total Evapotranspiration=A+B+C+D

Where, A=Unit consumptive use for each crop×its area B=Unit consumptive use of native vegetation ×its area C=Water surface evaporation×its area D=Bare land evaporation×its area

(v) Irrigation Method: In this method, unit consumption is multiplied by some factor. The multiplication values depend upon the type of crops in certain area. This method requires an Engineer judgment as these factors are to be investigated by the Engineers of certain area. (vi) Inflow Outflow Method: In this method annual consumptive use is found for large areas. If U is the valley consumptive use its value is given by, U = (I+P) + (Gs – Ge) – R

Where, U = Valley consumptive use (in acre feet or hectare meter) I = Total inflow during a year P = Yearly precipitation on valley floor Gs = Ground Storage at the beginning of the year Ge = Ground Storage at the end of the year R = Yearly Outflow

All the above volumes are measured in acre-feet or hectare-meter. 25



2. Empirical Methods: Empirical equations are given for the estimation of water requirement. These are, (i) Lowry Johnson Method: The equation for this method is, U = 0.0015 H + 0.9 (Over specified) U= Consumptive Use H = Accumulated degree days during the growing season computed from maximum temperature above 32 °F

(ii) Penman Equation: According to this method, U = ET = ∆H+0.27 Ea∆- 0.27

ET = Evapotranspiration or consumptive use in mm Ea = Evaporation (mm/day) H = Daily head budget at surface (mm/day) H is a function of radiation, sunshine hours, wind speed, vapour pressure and other climatic factors. Δ = Slope of saturated vapour pressure curve of air at absolute temperature in °F (iii) Hargreave’s Method: It is a very simple method. According to this method, Cu=KEp

Where, Cu = Consumptive Use coefficient (varies from crop to crop) Ep = Evapotranspiration K = Coefficient

DEFINITONS Gross Command Area (G.C.A): The whole area enclosed between an imaginary boundary lines which can be included in an irrigation project for supplying water to agricultural land by the network of canals is known as Gross command Area (G.C.A).It includes both the culturable and unculturable areas. Mathematically, Gross command Area (G.C.A) = Culturable Command Area+ Unculturable Command Area

Unculturable Command Area (Un-C.C.A): The area where the agriculture cannot be done and crops cannot be grown is known as unculturable area. The marshy lands, lakes, ponds, forests, villages etc are considered as unculturable. Culturable Command Area (C.C.A):

26



The total area within an irrigation project where the cultivation can be done and crops can be grown. Mathematically,

C.C.A=G.C.A-UnC.C.A

In formation of projects and schemes, C.C.A is roughly taken as 80%-90% of G.C.A depending upon the configuration of land. Intensity of Irrigation: It is defined as the ratio of cultivated land for a particular crop to the total C.C.A. It is expressed as a %age of C.C.A. For example, if the total C.C.A is 1000 hectares where wheat is cultivated in 250 hectares Then, Intensity of irrigation for wheat=2501000×100=25%

Area to Be Irrigated: It is the product of C.C.A and the intensity of irrigation. Mathematically,

Area to be irrigated=C.C.A×Intensity of Irrigation

Crop Ratio: It is defined as the ratio of the areas of the two main crop seasons, e.g. Kharif and Rabi. For example, if the area under Kharif crop is 2500 hectares and the area under Rabi crop is 5000 hectares then, crop ratio of kharif to Rabi is 1:2 (i.e. C.R. =25005000=1:2) Crop Season: The period during which some particular types of crops can be grown every year on the same land is known as crop season. i. Kharif Season: This season ranges from June to October. The crops are sown in the very beginning of monsoon and harvested at the end of autumn. The major Kharif crops are---- Rice, Millet, Maize, Jute, and Groundnut. ii. Rabi Season: This season ranges from October to March. The crops are sown in the very beginning of winter and harvested at the end of spring. The major Rabi crops are-----Wheat, Gram, Mustard, Rapeseed, Linseed, Pulses, Onion etc. Consumptive Use of “Rabi Crops” CROP Wheat Gram Barley Potatoes Sugarcane

CONSUMPTIVE USE “Δ” 37 cm 30 cm 30 cm 60-90 cm 90 cm

Consumptive Use of “Kharif Crops” 27



CROP Cotton Rice Maize

CONSUMPTIVE USE 25-40 cm 125-150 cm 45 cm

Crop Rotation: The process of changing the type of crop for the cultivation on the same land is known as crop rotation. It is found that if same crop is cultivated on the same land every year, the fertility of the land gets reduced and the yield of crop also gradually reduces. This is due to the reason that necessary salts required for the growth of a particular crop get exhausted. Few crop rotations possible are, i. Rice----Gram ii. Wheat----Millet----Gram iii. Rice---Gram---Wheat Time Factor: The ratio of the number of days the canal has actually been kept open to the number of days the canal was designed to remain open during the base period is known as time factor. Mathematically, Time Factor=No.of days the canal practically kept openNo.of days the canal was designed to keep open

For example, a canal was designed to kept open for 15 days, but it was practically kept open for 10 days for supplying water to the culturable area, then the time factor is 1015 Capacity Factor: It is the ratio of the average discharge to the maximum discharge (design discharge). Mathematically, Capcity factor=Average DischargeDesign Discharge

For example, a canal was designed or the maximum discharge of 50 cumecs, but the average discharge is 40 cumecs, then the capacity factor is 4050=0.8 Number of Watering: The total depth of water required by a crop is not applied at one time but it is supplied over the base period by stages depending upon requirement, these numbers of stages are known as “Number of Watering” Paleo: The initial watering which is done on the land to provide moisture to the soil just before sowing any crop is known as paleo or paleva. Kor Watering: The first watering which is done when the crop has grown to about three centimeters is called Kor Watering. Kor Period: 28



The portion of the base period in which Kor watering is needed is called “Kor Period” Cumec Day: The quantity of water flowing continuously for one day at the rate of one cumec is known as cumec day. 1 cumec day=1m3sec×24×60×60 sec 1 cumec day=24×60×60 m3 1 cumec day=24×60×6010000×1m 1 hectare=10000 m2 1 cumec day=8.64 hectare-metre

Crop Period: It is defined as the total time period from the time of sowing of a crop to the time of harvesting it. It is the period in which crop remain in the field. It is expressed in number of days. Base Period: It is the period from the first to the last watering of the crop just before its maturity. It is denoted by “B” and expressed in number of days. Delta: It is the total depth of water required by a crop during entire base period. It is also called consumptive use. It lies in base period. It is expressed in terms of depth and denoted by “Δ’. Field Capacity: It is defined as the amount of water held in the soil after the excess gravitational water has been drained. Permanent Wilting Point: (Wilting Coefficient) It is the water content at which plants can no longer extract sufficient water from soil for its growth. Permanent Wilting point=Field capacity2 to 2.5

Water Allowance: It is the total cusecs required at the outlet to irrigate 1000 acres of C.C.A.

Duty: The duty of water is defined as number of hectares that can be irrigated by constant supply of water at the rate of one cumec throughout the base period. It is expressed in hectares/cumec and is denoted by “D”. For example if 3 cumecs of water is required for the crop sown in, an area 5100 hectares, the duty of the irrigation will be 51003=1700 hectares/cumecs and the discharge of 3 cumecs is required throughout the base period. 29



RELATION BETWEEN DUTY, DELTA AND BASE PERIOD: In M.K.S System: Let

Duty=D (hectares/cumecs) Delta=Δ meters Base period=B days

By definition,

One cumec of water flowing continuously for “B” days gives a depth of water “Δ” over an area of “D” hectares. Volume of water @ 1m3sec in one day=1×24×60×60=86400 m3 Volume of water @ 1m3sec in "B" days=1×24×60×60=86400B m3=86400 m2m---(i) As, 1 Hectare=10000 m2 1 m2=1104H Then, equation ibecomes, Volume of water @ 1m3sec in "B" days=86400B m3=86400B×1104 H-m Volume of water @ 1m3sec in "B" days=8.64×B H-m---(ii) Depth of water required by crop,Δ=VolumeArea Δ=8.64×B H-mD H Δ=8.64×B D m

In F.P.S System: Let

Duty=D (Acres/cusecs) Delta=Δ feet Base period=B days

By definition,

One cusec of water flowing continuously for “B” days gives a depth of water “Δ” over an area of “D” acres. Volume of water @ 1ft3sec in one day=1×24×60×60=86400 ft3 Volume of water @ 1ft3sec in "B" days=1×24×60×60=86400B ft3=86400 ft2ft---(i) As, 1 Acre=43560 ft2 1 ft2=143560Acre Then, equation ibecomes, Volume of water @ 1ft3sec in "B" days=86400B ft3=86400B×143560 Acre-ft Volume of water @ 1ft3sec in "B" days=1.983×B Acre-ft---(ii) Depth of water required by crop,Δ=VolumeArea Δ=1.983 B Acre-ftD Acre Δ=1.983×B D ft

FACTORS AFFECTING DUTY: The factors that affect the duty are described below, 1. Soil Characteristics: If the soil of the canal bed is porous and coarse grained, it leads to more seepage loss and consequently low duty. If it consists of alluvial soil, the percolation loss will be less and the soil retains the moisture for longer period and consequently the duty will be high. 2. Climatic Condition: When the temperature of the command area is high the evaporation loss is more and the duty becomes low and vice versa. 3. Rainfall: 30



If rainfall is sufficient during the crop period, the duty will be more and vice versa. 4. Base Period: When the base period is longer, the water requirement will be more and the duty will be low and vice versa. 5. Type of Crop: The water requirement for various crops is different. So the duty varies from crop to crop. 6. Topography of Agricultural Land: If the land is uneven the duty will be low. As the ground slope increases the duty decreases because there is wastage of water. 7. Method of Ploughing: Proper deep ploughing which is done by tractors requires overall less quantity of water and hence the duty is high. 8. Methods of Irrigation: The duty of water is high in case of perennial irrigation system as compared to that in inundation irrigation system. 9. Water Tax: If some tax is imposed the farmer will use the water economically thus increasing the duty.

METHODS OF IMPROVING DUTY: Various methods of improving duty are: (1) Proper Ploughing: Ploughing should be done properly and deeply so that the moisture retaining capacity of soil is increased. (2) Methods of supplying water: The method of supplying water to the agriculture land should be decided according to the field and soil conditions. For example, Furrow method…………For crops sown ion rows Contour method………..For hilly areas Basin……………………For orchards Flooding………………..For plain lands 31



(3) Canal Lining: It is provided to reduce percolation loss and evaporation loss due to high velocity. (4) Minimum idle length of irrigation Canals: The canal should be nearest to the command area so that idle length of the canal is minimum and hence reduced transmission losses. (5) Quality of water: Good quality of water should be used for irrigation. Pollution en route the canal should be avoided. (6) Crop rotation: The principle of crop rotation should be adopted to increase the moisture retaining capacity and fertility of the soil. (7) Method of Assessment of water: Particularly, the volumetric assessment would encourage the farmer to use the water carefully. (8) Implementation of Tax: The water tax should be imposed on the basis of volume of water consumption.

IRRIGATION EFFICIENCY: The ratio of the amount of water available (output) to the amount of water supplied (input) is known as Irrigation Efficiency. It is expressed in percentage. The following are the various types of irrigation efficiencies, (a) Water Conveyance Efficiency: (ηc) It is the ratio of the amount of water applied, to the land to the amount of water supplied from the reservoir. It is obtained by the expression, ηc=WlWr×100

Where, ηc= Water conveyance efficiency Wl = Amount of water applied to land Wr = Amount of water supplied from reservoir

(b) Water Application Efficiency: (ηa)

32



It is the ratio of the water stored in root zone of plants to the water applied to the land. It is obtained by the expression, ηa=WzWl×100

Where, ηa= Water application efficiency Wz = Amount of water stored in root zone Wl= Amount of water applied to land

(c) Water Use Efficiency: (ηu) It is the ratio of the amount of water used to the amount of water applied. It is obtained by the expression, ηu=WuWl×100

Where, ηu= Water use efficiency Wu = Amount of water used Wl = Amount of water applied to land

(d) Consumptive use Efficiency: (ηcu) It is the ration of the consumptive use of water to the amount of water depleted from the root zone. It is obtained by the expression, ηcu=CuWp×100

Where, ηcu =Consumptive use efficiency Cu = Consumptive use of water Wp = Amount of water depleted from root zone

ASSESSMENT OF IRRIGATION WATER The water which has been supplied to the farmer is at government expenses. Some nominal charge must, therefore be leaved on the farmer for using this water. This is called “Assessment of irrigation water”. Therefore the knowledge of the same is very essential to engineers. WHY ASSESSMENT OF WATER IS NEEDED: The charges must be leaved on the farmers for the following reasons, (a) To recover the cost of construction of the project by which it has been possible to supply water. (b) To recover the maintenance cost of the various works staff for certain improvement. (c) To check the cultivators against uneconomical and careless use of water. METHODS OF ASSESSMENT OF IRRIGATION WATER: There are five methods of the assessment, 1. Assessment on area basis or crop rate basis 33



2. Volumetric assessment 3. Assessment on seasonal basis 4. Composite rate basis 5. Permanent assessment or Betterment levy basis 1. Assessment on Area Basis or Crop Rate Basis: There are fixed charges for different types of crops on area basis. It is a very old system. The rates for different crops have been fixed on the basis of, (a) Cash value of the crops and hence the paying capacity (b) Water requirements of the crop (c) Time of demand of water 2. Volumetric assessment: In this method water rates are charged on the basis of actual volume of water supplied at outlet head. It is very difficult to maintain as measurement of water is difficult. 3. Assessment on seasonal basis: In this method rate of assessment is based on the type of crop grown in a particular area during certain crop season. 4. Composite rate basis: It is a combination of water charges and land revenue i.e Malia and Abiana . 5. Permanent assessment or Betterment levy basis: In this system farmers have their own sources of supply but they use the irrigation water only in the instance of a drought. In such a case the farmers are levied at a fixed rate every year this levy is known as betterment levy. And the farmers are authorized to use the water in a drought. METHODS OF APPLICATION OR DISTRIBUTION OF WATER There are various methods of application of water some of which are listed as under, 1. Surface method (a) Furrow method (b) Flooding method 2. Sub- Surface method 3. Sprinkler method 1. Surface method: It includes the furrow method and the flooding method. In this method water is distributed through the small channels which flood the area. (a) Furrow method: The irrigation water is supplied to the land by digging narrow channels known as furrows at regular intervals. This method is best suited for potatoes, tobacco, sugarcane etc. 34



(b) Flooding method: In this method the field is flooded with water with the help of field channels. It may be controlled and uncontrolled. 2. Sub- Surface method: In this method water is applied to the root zone of the crops by underground networks of pipes. This method is also called as drip method or trickle method of irrigation. 3. Sprinkler method: In this method the water is applied to land in the form of spray like rain. It is done by the network of main pipes, sub main pipes and lateral pipes. PROBLEMS FROM IRRIGATION ENGINEERING BY NN BASAK Problem No. 01 A channel is to be designed for irrigating 5000 hectares in kharif crop and 4000 hectares in Rabi crop. The water requirement for kharif and Rabi are 60 cm and 25cm respectively. The Kor period for kharif is 3 weeks and for Rabi is 4 weeks. Determine the discharge of the channel for which it is to be designed. Solution: Crop

Area

Base Period

Consumptive Use cm M

Duty D=8.64×BΔ

Discharge=AreaDuty

Hectares/cumecs

cumecs

Hectares

week

days

Kharif

5000

3

21

60

0.6

8.64×210.6=302.4

16.53

Rabi

4000

4

28

25

025

8.64×280.25=967.6 8

4.13

Problem No. 02 The gross command area of an irrigation project is 1.5 lakh hectares, where 7,500 hectares are unculturable. The area of kharif crop is 60,000 hectares and that of Rabi crop is 40,000 hectares. The duty of Kharif is 3000 hectares/cumec and the duty of Rabi is 4000 hectares/cumec. Find (a) The design discharge of channel assuming 10% transmission loss. (b) Intensity of irrigation for Kharif and Rabi. Solution: Given that, G.C.A=150000 hectares Un-C.C.A=7500 hectares C.C.A for Kharif crop=60000 C.C.A for Rabi Crop=40000 Duty of Kharif crop=3000 hectares/cumecs Duty of Rabi crop=4000 hectares/cumecs We have to calculate 35


WATER RESOURCES AND IRRIGATION ENGINEERING CED-401 Design Discharge=? At 10% transmission losses Intensity of irrigation for a) kharif=? b) Rabi=?

Now, Culturable command area = G.C.A-Un-C.C.A=150000-7500=142500 hectares

Discharge for Kharif crops, Area of harif Crop = 60000 hectares Duty of Kharif Crop =3000hectarescumecs Required discharge of channel =AreaDuty=600003000=20 cumecs Considering 10 % losses, Discharge=20×110100=22 cumecs

Discharge for Rabi crops, Area of Rabi crops=40000 hectares Duty of Rabi crops=4000hectarescumecs Required discharge of channel=AreaDuty=400004000=10 cumecs Considering 10 % losses, Discharge=10×110100=11 cumecs a) So, the design discharge of the channel should be 22 cumecs, as it is maximum. b) Intensity of irrigation for kharif=60000142500×100=42.11% Intensity of irrigation for Rabi=40000142500×100=28.07%

Problem No. 03 The gross command area of an irrigation project is 1 lakh hectares. The culturable command area is 75% of G.C.A. The intensities of irrigation for Kharif and Rabi are 50% and 55% respectively. If the duties for kharif and Rabi are 1200 hectare/cumec and 1400 hectares/cumecs respectively, determine the discharge at the head of the canal considering 20% provisions for the transmission loss, overlap allowance, evaporation loss etc. Solution: Given that, G.C.A=10000 hectares Culturable command areaC.C.A=100000×75100=75000 hectares

For kharif crops, Area under Kharif=75000×50100=37500 hectares Duty for kharif=1200hectarescumec Required discharge for kharif=375001200=31.25 cumecs

For Rabi crops, Area under Kharif=75000×55100=41250 hectares Duty for kharif=1400hectarescumec Required discharge for kharif=412501400=29.46 cumecs

So, to meet up the actual water requirement of the crops, the discharge of the canal at the head of the field should be 31.25 cumecs (as it maximum). Now considering 20% provision for losses, Required discharge at the head of canal=31.25×120100=37.5 cumecs

Problem No. 04

36



Determine the head discharge of a canal from the following data. The value of time factor may be assumed as 0.75. Duty In Crop Base Period In Days Area In Hectare Hectares/Cumecs Rice 120 4000 1500 Wheat 120 3500 2000 Sugarcane 310 3000 1200 Solution: Crop Rice Wheat Sugarcane

Base Period In Days 120 120 310

Area In Hectare 4000 3500 3000

Duty In Hectares/Cumecs 1500 2000 1200

Discharge=ARE ADUTY

2.667 1.750 2.500

As, the base period of sugarcane is 310 days, it will require water both in Kharif and Rabi seasons. Actual discharge required in Rabi season =2.667+2.500=5.167 cumecs Actual discharge required in Rabi season =1.750+2.500=4.250 cumecs So, the maximum discharge in Kharif season i.e.5.167 cumecsshould be taken into consideration as it will be able to serve both the seasons. Time factor=Actual dischargeDesign discharge 0.75=5.17Design discharge Design discharge=5.1670.75=6.889 cumecs

So, the required head discharge of the canal is 6.889 cumecs

Problem No. 05 Find out the capacity of a reservoir from the following data. The culturable command area is 80,000 hectares. Intensity Of Crop Base Period In Days Area In Hectare Irrigation In % Rice 120 1800 25 Wheat 120 2000 30 Sugarcane 320 2500 20 Assume that canal and reservoir losses are 5% and 10% respectively. Solution: Calculating Delta for each crop, Δ=8.64×BD Delta for rice=8.64×1201800=0.576 m Delta for Wheat=8.64×1202000=0.518 m Delta for Sugarcane=8.64×3202500=1.106 m

Calculation of Area for each crop, Area for Rice=C.C.A×Intensity of irrigation in % Area for Rice=80000×25100=20000 hectares Area for Rice=80000×30100=24000 hectares Area for Rice=80000×20100=16000 hectares

Volume of water Required for each crop, 37


WATER RESOURCES AND IRRIGATION ENGINEERING CED-401 We know that, Volume=Area×Delta Volume for Rice=20000×0.576=11520 ha-m Volume for Wheat=24000×0.518=12432 ha-m Volume for Sugarcane=16000×1.106=17696 ha-m Total volume of water=41648 ha-m

Considering canal loss of 5%, Water required at the head of the canal=41648×105100=43730.40 ha-m

Considering reservoir loss of 10%, Capacity of the reservoir=43730.40×110100=48103.44 ha-m

Problem No. 06: The command area of a channel is 4000 hectares. The intensity of irrigation of a crop is 70%. The crop requires 60cm of water in 15 days, when the effective rainfall is recorded as 15cm during that period. (a) The duty at the head of field (b) The duty at the head of channel (c) The head discharge at the head of channel Assume total losses as 15%. Solution: Given that, G.C.A=4000 hectares Depth of water required=60mm Effective rainfall=15 cm Depth of irrigation water,Δ=60-15=45 cm Delta=45 cm=0.45 m , B=15 days From relation, Δ=8.64×BD Duty, D=8.64×BΔ=8.64×150.45=288hectatescumecs a) So, duty at head of field=288 hacumec Due to the losses of water the duty at the head of the channel will be reduced. b) So, the duty at the head of channel=288×85100=244.80hacumec Duty will be reduced due to loss Total area under crop=4000×70100=2800 hectares c) The discharge at the head of the channel=2800244.8=11.438 cumecs

PROBLEMS FROM IRRIGATION & WATER POWER ENGINEERING BY B.C. PUNMIA Example no. 3.2: A crop requires a total depth of 92 cm of water for a base period of 120 days. Find the duty of water. Solution: Given that, Base period, B=120 days Depth of water, Δ=92 cm=0.92 m

As, D=8.64×BΔ=8.64×1200.92=1126.95 hectares/cumecs

Example 3.3: An irrigation canal has gross commanded area of 80,000 hectares out of which 85% is 38



culturable irrigable. The intensity of irrigation for Kharif season is 30% and for Rabi season is 60%. Find the design discharge at the head of the canal if the duty at its head is 800 hectares/cumecs for Kharif season and 1700 hectares/cumecs for Rabi season. Solution: Given that,

G.C.A=80000 hectares Duty at head of canal for kharif season=1700hectarescumecs Duty at head of canal for Rabi season=800hectarescumecs Culturable command area=80000×85100=68000 hectares Area under Kharif season=68000×30100=20400 hectares Area under Rabi season=68000×60100=40800 hectares

Now,

Water required at the head of the canal to irrigate the land under Kharif season=Area under Kharif seasonDuty for Kharif season=20400800=25.5 cumecs Water required at the head of the canal to irrigate the land under Kharif season=Area under Rabi seasonDuty for Rabi season=2408001700=24 cumecs Canal must be designed in such a way that it must carry the discharge of 25.5 cumecs

Example 3.4: A water course has a culturable commanded area of 2600 hectares, out of which the intensities of irrigation of perennial sugar-cane and rice crops are 20% and 40% respectively. The duty for these crops at the head of water course is 750 hectares/cumecs and 1800 hectares/cumecs respectively. Find the discharge required at the head of water course if the peak demand is 20% of the average requirement. Solution: Given that,

C.C.A=2600 hectares Duty for Sugarcane=750hectares/cumecs Duty for Rice=1800 hectares/cumecs Area under Sugarcane=2600×20100=520 hectares Area under Rice=2600×40100=1040 hectares Now, Water Required for Sugarcane=AreaDuty=520750=0.694 cumecs Water Required for Rice =AreaDuty=10401800=0.577 cumecs Since sugarcane is perennial crop, it will require water through out the year. Hence the watercourse must carry a total discharge of 0.694+0.577=1.271 cumecs Design discharge to meet the peak demand=120100×1.271=1.52 cumecs

Example 3.5: The left branch canal carrying a discharge of 20 cumecs has culturable commanded area of 20,000 hectares. The intensity of Rabi crop is 80% and the base period is 120 days. The right branch canal carrying a discharge of 8 cumecs has culturable commanded area of 12,000 hectares, intensity of irrigation of Rabi crop is 50%, and the base period is 120 days. Compare the efficiencies of the two canal systems. Solution: a) For Left canal

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WATER RESOURCES AND IRRIGATION ENGINEERING CED-401 Area under Rabi crop=2000×80100=16000 hectares Discharge=20 cumecs Duty=AreaDischarge=1600020=800 hectares/cumecs

b) For Right canal Area under Rabi crop=12000×50100=6000 hectares Discharge=20 cumecs Duty=AreaDischarge=60008=750 hectares/cumecs

Example 3.6 A water course has a culturable commanded area of 1200 hectares. The intensity of irrigation for crop A is 40% and for B is 35%, both the crops being Rabi crops. Crop A has a kor period of 20 days and crop B has kor period of 15 days. Calculate the discharge of the water course if the depth for A crop is 10 cm and for B it is 16 cm. Solution: a) For crop A Culturable command area =1200×40100=480 hectares Kor Period, b=20 days Kor depth,δ=10 cm=0.1 m Duty=8.64×bδ=8.64×200.1=1728 hectares/cumecs Design discharge=Area under irrigationOutlet factor=4801728=0.278 cumecs

b) For crop B Culturable command area =1200×35100=420 hectares Kor Period, b=15 days Kor depth,δ=16 cm=0.16 m Duty=8.64×bδ=8.64×150.16=810 hectares/cumecs Design discharge=Area under irrigationOutlet factor=420810=0.518 cumecs Thus, Design discharge of water course=0.278+0.518=0.796≅0.8 cumecs

Example 3.7: A water course commands an irrigated area of 600 hectares. The intensity of irrigation of rice in this area is 60%. The rice transplantation of Rice crop takes 12 days and total water depth of water required by the crop is 50 cm on the field during the transplantation period. During the transplantation period, the useful rain falling on the field is 10 cm. the duty of the irrigation water for the crop on the field during transplantation, at the head of the field and also at the head of the distributory, assuming losses of water to be 20% in the water course. Also, calculate the discharge required in the water course. Solution: Transplantation period=12 days Depth of irrigation water applied in the field,Δ=50-10=40 cm=0.4 m As we know that, D=8.64×BΔ D=8.64×120.40=259.5 hectares/cumecs on the field Since the losses in the canal are 20%, 1 cumec of water discharge at the head of the water course will be reduced to 0.80 cumec at the head of the field. Hence, it will irrigate 259.5×0.8=207.6 hectares only. Hence, the duty of water at the head of the water course will be 207.6 hectares/cumes Now, Total Area under Rice Plantation=600×0.6=360 hectares Discharge at the head of water corse=360207.6=1.735 cumecs

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irrigation and water resources created by sajid and farooq

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