Irrigation Networks.pdf

Irrigation structures Chapter -1

Originally prepared by Zeleke Agide and modified by Ermias Alemu for DDSA course

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1. Irrigation Networks 1.1 Introduction The aim of irrigation facilities is to divert water from a source, convey it to appropriate locations in the system and distribute it within the field so that water reaches the roots of the crops in an optimal manner to ensure improvement of agricultural production. Irrigation networks are man made facilities put in place to achieve the above mentioned objective. Irrigation networks also include drainage networks which are responsible for the removal of excess water from the field and road networks which are required for access to the various parts of the irrigation system. Drainage is usually a complementary component of irrigation and irrigation schemes without a drainage facility is seldom successful particularly in poorly drained heavy soils. The network of irrigation consists of the entire physical infrastructure for water acquisition, conveyance, protection, regulation, division, measurement, distribution and its final application to the crop. At the head of the irrigation network there will be some kind of headwork for diversion of water. At the headwork facility which can be a diversion weir, dam, pumping station etc, water enters the conveyance system that transports it to the required delivery point. This delivery point can be a secondary or tertiary off-take or turnout. At such a structure the water will be divided and distributed to the various parts of the network via a distribution system. The water then will be applied to the fields (crops) using field irrigation methods. Three management levels in irrigation networks:

Main level: Consists of the facilities for water acquisition and conveyance being managed by irrigation agencies.

Off-farm level: Consists of facilities for water distribution at off-takes being managed by a group of farmers or water users:

Field level: Consists of the facilities for application of water to the individual plots and is managed by the individual farmer or irrigator.

1.2 Irrigation Areas For the sake of ease of management, operation and design, irrigation areas are divided into units of various sizes at various levels as discussed hereunder. Field units: These are the smallest areas within the irrigation system being irrigated. It is an individual farm plot which receives water from the irrigation network via a field inlet or turnout. Quaternary unit: is an area to which water is supplied to a group of water users of farmers via a common inlet point. Each farmer then will have his own water control farm inlet to control his water intake. Tertiary unit: is an irrigation unit which consists of a couple of quaternary units or fields. A tertiary unit receives water from a tertiary off-take. Based on the size of the irrigation scheme, the size of the tertiary unit can also vary greatly. In some small schemes, it may consist of only one filed or farm plot; while in large schemes it can it can be a combination of several small fields or large farms. The size of a tertiary unit can vary from 3 or 4 ha to over 70ha.



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Secondary unit: is an area consisting of two or more tertiary units and receive water from a secondary off-take point. A branch or secondary canal usually conveys the water from the secondary off-take to a group of tertiary units. Irrigation area (Command area): is the total area of the irrigation scheme. It consists of two or more secondary units and the total area receives water from the headwork.

Fig. Irrigation areas and off-takes

Based the size of the irrigation scheme, not all the above mentioned levels present in all irrigation systems.

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●

●

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● ● Secondary unit

Tertiary unit

Diversion/Headwork structure

Secondary off-take Structure

Tertiary off-takes

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Fields/Farm plots



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Figure An irrigation network with irrigation units and flow control structures

1.3 Structures in Irrigation Networks In gravity irrigation systems, water is conveyed in open channels while in pressurized system water is conveyed and applied via a closed conduit under pressure. For a gravity irrigation system, the infrastructure consists of

Open canals and



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Control structures The flow control structures in gravity irrigation systems include the following: Head works: Diversion weir, barrage, pumping station, free intake (dam) Conveyance structures: Drops, aqueduct, chute, flume, inverted siphon Regulating structures: Discharge regulator, water level regulator, division work, check Measuring structure: Broad, sharp, short-crested weirs, V-notch weir, Parshall and RBC

flumes, Romijn weir Protective structures: Over chute, spillway, waste way, side drainage, stilling basin Off-take structures: Tertiary off-take, secondary of-take, turnout Miscellaneous structures: Culvert, sand traps, roads, drainage structures, trash racks

Fig. Typical tertiary off-take structure

1.4 Conveyance and distribution systems Irrigation water can be conveyed, distributed and applied to the fields by either or a combination of the two methods:

Gravity system Pressurized system

Gravity conveyance and distribution systems comprise of open canal systems and canal structures for controlling and regulating the flow. Water flows by gravity from the headwork to the required off-take point and be distributed again by gravity. Even the field application of water to the crops is by gravity in which the water flows on the surface of the soil where it infiltrates. Irrigation canals can be lined or unlined. For gravity irrigation systems, sedimentation



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can be a serious problem particularly in unlined canals situated in unstable soils such as alluvial deposits. In pressurized irrigation systems, water is conveyed and distributed either on the whole or part of the system by closed conduit (pipelines) under pressure. In several irrigation systems around the world, a combination of gravity and pressurized conveyance and distribution system can be used. The water might be conveyed in some convenient part of the system by gravity and when it is required to lift it to a higher canal, a pump can be used to pump it to the higher elevation. Similarly, the field application of the water to the crops can also be done under pressure with pressurized system such as sprinkler and drip systems or a combination of surface and pressurized systems. The selection depends on a number of factors such as topography, soil type, water quality, water availability, affordability etc.

Fig. A combination of gravity and pressurized conveyance system

1.5 Management of conveyance and distribution Effective and efficient irrigation water conveyance and distribution to the various parts and level of the irrigation system requires proper and adequate management system. There are generally two management (operation) levels in most irrigations systems: The main system: Consists of the system for water conveyance and delivery. It is under the

management of the irrigation authority or agency. The irrigation agency in most cases is a government body responsible for the development of the physical irrigation infrastructure and prepares the strategies of water delivery. It is responsible for all the maintenance and operation of the system above a tertiary off-take.

The tertiary system: Is the system within the tertiary unit which is responsible for the distribution of water within the tertiary units to the quaternary canals and individual farm plots. It is under the management of a group of water users or water users’ association. The



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water users manage their water distribution and responsible also for the maintenance and the water distribution infrastructure within a tertiary unit.

1.6 Water delivery aspects The two most important components of water supply system are:

amount of water delivery (q) duration of water delivery (t)

The delivery schedules highly depend on the field irrigation methods and field irrigation requirement. As the objective of a water delivery and distribution system is to deliver water adequately, efficiently and reliably to the users there by improving production. The field water requirement varies based on the stage of the crops within a crop season. Therefore, a water delivery system is supposed to deliver a certain volume of water sufficient to meet the crop water demand over the irrigation interval. This volume is: The water supply (delivery) can be:

With constant flow rate Constant duration Variable flow rate and duration

In all the cases the system should deliver the required water that can sustain the field crops within the irrigation interval (T).

Fig: Irrigation scheduling (water supply) parameters Rotational flow (supply): In rotational flow, water is supplied to the required off-take point or users by turn. The rotation can be in the main system (secondary off-takes and tertiary off-takes) or within the tertiary unit to individual farmers.

Volume (V) = flow rate (q) * duration of delivery (t)

Interval

Durationq

Time



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If rotation is at the secondary off-take, all the water in the main canal will first be diverted to the first secondary off-take on the main canal and once all the low level off-takes on this canal have been irrigated, it will proceed with the second secondary off-take. If the rotation is at tertiary off-take the same principle will apply. If there is rotational flow within the tertiary unit (flow to each farm plots), the first farmer will irrigate first for some specified duration of time; once he has used his time his outlet will be closed the water again flows to the second farmer until all the farmers within the tertiary unit are irrigated. An important aspect of rotational supplies is that the duration of rotation should be less than or at least equal to the irrigation interval (T). Otherwise the crop will suffer water shortage. Thus, design of rotational irrigation systems should take into consideration all the outlets to be irrigated with the available flow and make appropriate irrigation duration to complete with in the irrigation turn. The interval between two consecutive irrigations is called irrigation interval or one irrigation turn. Continuous flow (supply): In this case flow in the given system or to a certain off-take occurs continuously during the irrigation season. The flow can be adjustable or constant continuous flow. Continuous flow is difficult to manage in small farms as the required flow is too small which poses problems to control and maintain it. The water supply to the water users can be either demand-oriented or supply-oriented. In demand oriented systems the supply is based on the actual demand of the tertiary units while in supply oriented systems the supply is limited by either water availability or the capacity of the physical infrastructure. Water delivery methods There are three basic parameters in establishing the water delivery methods and operational objectives of the main irrigation system. 1. The decision making procedure on water allocation to the tertiary units 2. The method of water allocation to the tertiary unit: how the water is going to be allocated to

the tertiary unit 3. The method of water distribution through the main system: how the water is distributed

through the main system.



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Fig. Three parameters of Operational objectives (water delivery policy)

1. The decision making procedure for water allocation to tertiary units may fall in one of the following three categories:

Imposed Allocation: Also called dictated pr arranged delivery is an allocation in which the irrigation agency decides beforehand on the delivery schedules and amount of water. Each irrigator thus gets water based on a preset schedule may be for one irrigation season or a year. The schedule may be prepared based on the knowledge of water requirements of the crops or based on water availability in case of supply-based deliveries. On-request allocation: also called semi-demand allocation, in which the water users request for certain amount of delivery at a certain time for certain duration from the irrigation agency. The irrigation agency will process the requests of all the water users and decide on the requested deliveries based on the availability of water and suitability of schedules. In this method, processing of the requests may delay the actual water delivery. On-demand allocation: in this delivery system the water users have direct access to water. They can decide on the amount and duration of delivery and will have it immediately.



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Fig. Decision making procedure for water delivery

For the selection of the decision making procedure, refer the following figure.

Fig. Selection of the decision making procedure

2. The actual water delivery to tertiary units can be in one of the following ways: Proportional flow: the flow will be diverted at a fixed ratio based on the width of the diverter throughout the main system and to tertiary units. Un-gated diversion structures are required for this purpose. Intermittent flow: the flow into the tertiary init will be intermittent. This is also called on/off flow. A simple on/off gate in needed for intermittent flow.



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Adjustable flow: variable flow rates will be diverted to the tertiary units in this case. However, the supply can be either continuous or rotational. Adjustable flow regulator is needed for the purpose.

Fig. Actual water allocation to tertiary units

3. For the water distribution throughout the main system the same methods are used as

the water delivery to tertiary units; proportional, intermittent, adjustable. 1.7 Flow control systems Flow control in irrigation systems is a special subject of control engineering as has been developed for mechanical systems. Flow control system is the regulation system of the structures to maintain the system in the desired state. The purpose of flow control systems is to control the flows in the canal system at bifurcations to meet the required level of irrigation service. Flow control structures are usually used to control the flow. Flow control structures are structure that control and regulate the state of the canal such as the water level regulators and discharge regulators. A combination of water level control and discharge control are often used together. Usually flow rates are controlled indirectly by water level control. Variation in water levels either on upstream or down stream of control structures will be associated with a change in discharge. The most common flow control systems can be classified into three:

Proportional control Upstream control Downstream control

Proportional control: In this control system, water will be divided and distributed according to a fixed ratio. Irrigation systems in proportional control are simple in construction and operation. Such a control system is applied in systems where uniform cropping pattern is followed throughout the system and each irrigation unit have to receive an amount f water that is proportional to the irrigated area. There is no active regulation of structures for this control system. Only un-gated proportional flow division structures are required at each off-take.



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Fig. A canal reach under proportional control.

For a certain variation in discharge of the canal, there will be a corresponding proportional variation in discharge of the off-takes. All the off-takes will be affected equally under water shortage if there is any.

Fig. Off-take flows in proportional control

Upstream control: This is by far the most common type of flow control systems around the world. In this system, target water levels are set at upstream of the water level control structures; thus called upstream control. It is suitable for either imposed or on-request allocation. Although it is a water level control it is essentially a discharge control. Upstream controlled systems require active regulation of structures to maintain the water levels at the set points at the target water level. Manually operated structures, electro-mechanical regulation and hydro-mechanical regulation structures can be used as water level regulators. Typical problem with upstream controlled systems is that there is a ‘negative dynamic storage’ in the canal and there is significant lag-time before a new equilibrium condition can be attained. This means that the response time of the hydrodynamic system is large.

Fig. Canal reaches under upstream control



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Fig. Vertical gate for upstream water level control

Downstream control: In this control system, the set point is located on the downstream of the structure; and thus called downstream control. The regulators will keep a constant the water level at the downstream of the structure irrespective of the discharges. It is usually used for on-demand water allocation to tertiary off-takes. Downstream controlled systems are usually equipped with hydro-mechanical gates. The system will respond to any changes to water levels on the downstream side. It means that more water will flow through the system when water level drops due to withdrawal of irrigation water. Thus, the downstream demand will automatically be supplied at each regulator. The water level indicated by 1 corresponds to no discharge in the canal and that indicated by 3 corresponds to the maximum flow in the system. Water level at 2 indicates intermediate condition. In downstream controlled systems, since there is appositive storage in the canal reach between the two regulators, the system immediately responds to any

changes in the

downstream reach of the canal.

Positive storage wedge

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Irrigation Networks.pdf