In this article we will discuss about:- 1. Scope of Micro-Irrigation Systems 2. Principles and Components of Micro-Irrigation Systems 3. Benefits.
Scope of Micro-Irrigation Systems:
The Water Use Efficiency (WUE) in the Indian agriculture is only about 30-40%, which is one of the lowest in the world. Thus, judicious use of irrigation water needs more attention to enhance total agricultural production and area under irrigated agriculture.
It can be achieved by introducing the advance method of irrigation like micro-irrigation (MI) coupled with other improved management practices. The MI systems consist of drip, sprinkler, micro-sprinkler and mini-sprinkler.
It is proved by studies that drip and sprinkler methods of irrigation help to save water and improve WP. Though both drip and sprinkler methods of irrigation are treated as MI, there are distinct characteristic differences between the two in terms of flow rate, pressure requirement, wetted area and mobility.
The Drip Irrigation (DI) system supplies water directly to the root zone of the crop through a network of pipes with the help of emitters, whereas the sprinkler irrigation (SI) system sprinkles water similar to rainfall into the air through nozzles which subsequently break into small water drops and fall on the field surface. Unlike flood irrigation method (FIM), DI system supplies water directly to the root zone of the crop, instead of land, and therefore, the water losses occurring through evaporation and distribution are completely absent.
In SI system, water-saving is relatively low (up to 70%) as compared to DI system since SI system supplies water over the entire field of the crop. According to past studies reported in literature, productivity gain due to use of MI is estimated to be in the range of 20-90% for different crops. Reduction in water consumption due to DI system over the surface method of irrigation varies from 30-70% for different crops.
Principles and Components of Micro-Irrigation Systems:
a. Principles of Drip Irrigation Systems:
Drip irrigation involves supplying water to the soil very close to the plants at very low flow rates (0.5-10 litre hr-1) from a plastic pipe fitted with outlets (drip emitters). The basic concept underlying the drip irrigation method is to maintain a wet bulb of soil in which plant roots suck water. Only the part of the soil immediately surrounding the plant is wetted. The volume and shape of the wet bulb irrigated by each drip emitter are a function of the characteristics of the soil (texture and hydraulic conductivity) and the discharge rate of the drip emitter. Applications are usually frequent (every 1-3 days) to maintain soil water content in the bulb close to field capacity.
b. Components of Drip Irrigation Systems:
Drip irrigation system, also known as trickle irrigation, involves dripping water on the soil at very low rates, and thus, provides a very favourable high moisture level in the soil in which plants can flourish. Drip irrigation is most suitable for row crops (vegetables, soft fruit), tree and vine crops where one or more emitters can be provided for each plant. Because of the high capital costs of installation, only high value crops are grown in drip system. A drip irrigation system is comprised of several components.
The basic components of any drip irrigation system are broadly divided into the following components:
I. Pump unit
II. Control head
III. Mainlines, Submains and Laterals
IV. Emitters or drippers.
I. Pump Unit:
It consists of prime mover (motor/engine)/water pump, which takes water from the source and provides the right pressure for delivery into the pipe system consisting of G.I. pipes.
II. Control Head:
It consists of valves (bye pass valve and air release valve) to control the discharge and pressure in the entire system. It consists of filters to clear the water. Common types of filter include screen filters and graded sand filters, which remove fine material suspended in the water. Screen filters or disk filters may be used with groundwater. A 200-mesh screen or equivalent is considered adequate for drip irrigation.
When the water contains sand, a sand separator should be used. Rapid clogging may occur when no filter or the incorrect type of filter is used. A filter needs to be cleaned when the difference in pressure across the filter (measured before and after the filter) is greater than 5-8 psi. A drip-irrigation system should never be operated without a filter even if the filter requires frequent cleaning. Failure to use a filter will result in clogging of the emitters, often resulting in poor uniformity and sometimes in crop loss.
The filter should be cleaned as often as needed. Efforts should be made to understand the cause of the rapid clogging, and remediation for the problem should be developed. The presence of the filter after the point of fertilizer injection means totally soluble fertilizers must be used. Otherwise, fertilizer particles may contribute to filter clogging. Some control head units contain a fertilizer or nutrient tank.
These slowly add a measured dose of fertilizer into the water during irrigation through injectors. This is one of the major advantages of drip irrigation over other methods. The most common injectors used with small drip-irrigation systems are the Venturi injector and the Dosatron. Because Venturi injectors involve no moving parts and are less expensive, they are commonly used on small farms and located as close as possible to the irrigation zone, but before the filter.
III. Mainlines, Submains and Laterals:
These supply water from the control head into the fields. They are usually made from PVC or polyethylene hose and should be buried below ground because they easily degrade when exposed to direct solar radiation. Lateral pipes are usually of 13-32 mm diameter and are provided with end plug and pressure gage.
IV. Emitters:
These are also called drippers. These are the devices used to control the discharge of water from the lateral to the plants. These are usually spaced more than 1 metre apart with one or more emitters used for a single plant such as a tree. For row crops, more closely-spaced emitters may be used to wet a strip of soil. Many different emitter designs have been produced in recent years. The basis of design is to produce an emitter which will provide a specified constant discharge, which does not vary much with pressure changes, and does not block easily.
c. Principles of Sprinkler Irrigation Systems:
It is a method of applying irrigation water which is similar to natural rainfall. Sprinkler irrigation water is applied through a pressurized system. The pressure causes the water to flow out through the sprinkler nozzle and fly through the air so that it breaks up the water into small water drops which falls on the soil surface.
The main objective of a sprinkler system is to apply water as uniformly as possible to fill the root zone of the crop with water. This system is suited for most row, field and tree crops. However, large sprinklers are not recommended for irrigation of delicate crops such as lettuce because the large water drops produced by the sprinklers may damage the crop. Sprinklers are best suited to sandy soils with high infiltration rates although they are adaptable to most soils.
If sprinkler irrigation is the only method available, then light fine sprays should be used. The larger sprinklers producing larger water droplets are to be avoided. A good clean supply of water, free of suspended sediments, is required to avoid problems of sprinkler nozzle blockage and spoiling the crop by coating it with sediment.
d. Components of Sprinkler Irrigation Systems:
Atypical sprinkler irrigation system consists of the following components:
i. Pump unit
ii. Mainline and sometimes sub-main lines
iii. Laterals
iv. Couplers
v. Sprinkler Head
vi. Fittings and accessories such as valves, bends, plugs and risers.
i. Pump Unit:
It is usually a centrifugal pump, which takes water from the source and provides adequate pressure for delivery into the pipe system.
ii. The Mainline- and Sub-Mainlines:
These are the pipes, which deliver water from the pump to the laterals. In some cases, these pipelines are permanent and are laid on the soil surface or buried below ground. In other cases, these are temporary, and can be moved from field to field. The main pipe materials used include plastic or aluminium alloy.
iii. The Laterals:
These deliver water from the mainlines or sub-mainlines to the sprinklers. These can be permanent, but more often, they are portable and made of aluminium alloy or plastic so that they can be moved easily.
iv. Couplers:
These are used for connecting two pipes and uncoupling quickly and easily Essentially a coupler should provide (a) a reuse and flexible connection (b) not leak at the joint (c) be simple and easy to couple and uncouple (d) light, non-corrosive, and durable.
v. Sprinkler Head:
Sprinkler head distribute water uniformly over the field without runoff or excessive loss due to deep percolation. Different types of sprinklers are available. These are either rotating or fixed type. The rotating type can be adapted for a wide range of application rates and spacing.
They are effective with pressure of about 10 to 70 m head at the sprinkler. Pressures ranging from 16 to 40 m head are considered the most practical for most farmers. Fixed head sprinklers are commonly used to irrigate small lawns and gardens. Perforated lateral lines are sometimes used as sprinklers. These require less pressure than rotating sprinklers. These release more water per unit area than rotating sprinklers. Hence, fixed head sprinklers are adaptable for soils with high intake rate.
vi. Fittings and Accessories:
The following are some of the important fittings and accessories used in sprinkler system:
(a) Water Meter:
It is used to measure the volume of water delivered. This is necessary to operate the system to give the required quantity of water.
(b) Flange, couplings and nipple used for proper connection to the pump, suction and delivery.
(c) Pressure Gauge:
It is necessary to know whether the sprinkler system is working with desired pressure to ensure application uniformity.
(d) Bend, tees, reducers, elbows, hydrants, butterfly valve and plugs.
(e) Fertilizer Applicator:
Soluble chemical fertilizers can be injected into the sprinkler system and applied to the crop. The equipment for fertilizer application is relatively cheap and simple and can be fabricated locally. The fertilizer applicator consists of a sealed fertilizer tank with necessary tubings and connections. A Venturi injector can be arranged in the mainline, which creates the differential pressure suction and allows the fertilizer solution to flow in the main water line.
The most common type of sprinkler system consists of a system of light-weight aluminium or plastic pipes, which are moved by hand. The rotary sprinklers are usually spaced 9-24 m apart along the lateral, which is normally 5-12.5 cm in diameter. This is so it can be carried easily. This is the simplest of all systems. Some use more than one lateral to irrigate larger areas. A common problem with sprinkler irrigation is the large labour force needed to move the pipes and sprinklers around the field.
In some places, such labour may not be available and may also be costly. To overcome this problem, many mobile systems have been developed such as the hose reel rain gun and the centre pivot. Another system, which does not need a large labour force is the drag-hose sprinkler system. The uniformity of sprinkler applications can be affected by wind and water pressure. Spray from sprinklers is easily blown about by even a gentle breeze, and this can seriously reduce uniformity. To reduce the effects of wind, the sprinklers can be positioned more closely together.
Benefits of Micro-Irrigation System:
1. The MI increases yield of different crops upto 27-88% and water-saving is achieved up to 36-68% over the conventional flow irrigation systems.
2. It enables farmers to grow crops which would not be possible under conventional systems since it can irrigate adequately with lower water quantities.
3. It improves the quality of crops.
4. It saves costs of hired labour and other inputs like fertilizer.
5. It reduces the energy needs for pumping, thus reducing energy per ha of irrigated area because of its reduced water needs. However, overall energy needs of the agriculture sector may not get reduced because most farmers use the increased water efficiency to bring more area under irrigation.
6. It is adaptable to any farmable slope. In drip systems, the crops are normally planted along contour lines and the water supply pipes (laterals) would be laid along the contour also. This is done to minimize changes in emitter discharge as a result of land elevation changes.
7. Drip irrigation is suitable for most soils. On clay soils, water must be applied slowly to avoid surface water pounding and runoff. On sandy soils, higher emitter discharge rates will be needed to ensure adequate lateral wetting of the soil.
Literature studies based on several experiments suggest that the crops being cultivated under MI require relatively less amount of water to produce one unit of crop output. The WP of different crops under flood and MI presented in Table 12.3 clearly confirm that the WP is much higher under both SI and DI systems as compared to FIM.
Among SI and DI systems, DI system appears to be more efficient in terms of producing output per unit of water. Since the application efficiency of water is much higher in DI system, the WP is also substantially higher.
According to the reports of INCID (1994, 1998), about 80 crops (both narrow and widely spaced crops) can be grown with MI system. Although DI system is considered to be highly suitable for wide-spaced and high value commercial crops, it is also being used for cultivating oilseeds, pulses, cotton and wheat crops. Closely-grown crops such as millets, pulses, wheat, sugarcane, groundnut, cotton, vegetables, fruits, flowers, spices and condiments are found to be suitable to cultivate under SI system.
The water-saving capacity of DI system is expected to be different for various crops as the consumption and the requirement of water varies from crop to crop. The water-saving in DI system for vegetable crops varies from 12- 84% over the conventional method of irrigation (CMI). Similarly, water-saving in DI system varies from 45-81% in fruit crops. In crops like cotton, coconut and groundnut, water-saving varies from 40-60%. Importantly, water-saving with DI system in sugarcane, which is one of the water-intensive crops, is more than 65% in comparison to that with CMI. The DI system can also be adopted over different type of lands, viz. undulating terrain, rolling topography, hilly areas, barren land and areas which have shallow soils.