Dept. of Agril. Engg, BAU Lecture Note No. 7 FNRM-121 (Hydrology, Soil and Water Conservation) Instructor- Dr. M. Job First Year Semester II Dated: 15 th July &08 th Aug Drip Irrigation: It can be defined as the process of slow application of water in the form of discrete, continuous drops or miniature sprays through mechanical devices called emitters located at selected points along the water delivery lines. Water soluble and liquid fertilizers can also be effectively applied to individual plants using drip irrigation system. Drip System, also called as trickle system, has greatest potential where water is expensive or scarce, soils are sandy, rocky or difficult to level and high value crops are produced. Model Layout/Components of a Drip Irrigation System
Merits Increased beneficial use of available water Enhanced plant growth and yield Improved fertilizer and other chemical applications Limited weed growth Reduced operational labour Decreased energy requirement Improved cultural practices Drawbacks Persistent maintenance requirement Salt accumulation near plants if source of water is saline Economic limitation Components of Drip Irrigation System: A typical system consists of a source of water supply, pumping unit, main lines, laterals and emitters. Auxiliary components include filters, pressure regulators, valves and equipment for mixing fertilizers etc. When the water source is at higher elevation than the field to be irrigated, the system can be conveniently operated on gravity with a overhead storage tank. In most of the cases it is pressurized irrigation through pumps. Head control Unit: It consists of Pumping unit, Filtration unit, fertilizer applicator, Gate Valve, Bypass assembly, Pressure gauges, Air release valves etc. These are usually housed in concrete raised platforms. Pipes used in head control unit is Galvanized Iron.
In case of gravity system, water lifted for irrigation is stored in a head tank, at least with a capacity of 1000 litres resting on a raised platform to maintain pressure head of 3 to 5 m. 1. Filters: Filters are used to remove undesirable material from the water supply before it enters the distribution system and creates the potential for emitter clogging. Filtration is generally done in two stages i.e primary filtration and secondary filtration. a. Gravel Filter (Sand Filter): It is also called media filter. These filters are effective against both inorganic and organic suspended solids. This type of filter is essential for using water from open reservoir in drip irrigation. The dirt and algae is stopped and accumulated inside the media in the filter unit. Gravel filter consists of small basalt gravel or sand (1 to 2 mm dia) placed in a metallic cylindrical tank. Water with suspended impurities enters from the top, flows through the filters bed and clean water comes out at the bottom, leaving the dirt in the filter. The filter is cleaned by reversing the direction of flow from time to time. b. Screen filter: Screen filters are the most common type of filters used in drip irrigation system. A Screen filter is often placed downstream to the sand filter and this may be installed with or without gravel filter, depending upon quality of water. The screens are usually cylindrical in shape and are made up of non corrosive metal or plastic material. The filters are cleaned by flushing the screen with a stream of water. After cleaning, the screen is checked for tears and the gasket should be checked and replaced when necessary. With relatively clean water, only screen filters may be used. 2. Main lines: The main lines carries the water from the filtration system to the submain. They are normally made of rigid PVC pipes. Usually they are placed 60 cm below the ground, so as not to interfere with cultivation practices. Their diameter is based on the system flow capacity. 3. Submain: The submain distribute the water from main line to the laterals. They are also buried in ground below 45 cm to 60 cm and made of PVC material. The diameter of submain is usually smaller than main line. There may be a number of submain from one mainline depending upon the plot size and crop type.
4. Laterals: The lateral distribute water to the emitter which deliver water directly to the root zone. Laterals are small diameter flexible pipes or tubing made of low density polyethylene (LDPE) or linear low density poly ethylene (LLDPE) and having usually 12 mm and 16mm diameter. They can withstand a maximum pressure of 4 Kg.cm 2. They are connected to submain at predetermined distance. The pressure variation between two extreme points of lateral should not be more than 20 per cent and discharge variation should not be more than 10 per cent. 5. Emitters or drippers: The dripper or an emitter is a device used for discharging water from lateral to the plants root zone. There are various types and sizes of drippers, based on different operation conditions. They are made of plastic such as polyethylene or poly propylene. Their discharge ranges between 1 to 16 lit/hr. Drippers can be categorized as pressure compensating and non pressure compensating types. Emitters may be punched externally on the lateral or may be fitted in the laterals. Accordingly they are called on line or in line emitters, respectively. 6. Control valves: These are used to control the flow through the submain pipes. Generally, they are installed on filtration system, mainline and on all submains. They are made up of gun metal, PVC, cast iron and their sizes ranges from 20 mm and can go upwards depending upon the selected size of pipes. 7. Flush valve: It is provided at the end of each submain to flush out the water and dirt accumulated at the end of submain. 8. Air release valve: It is provided at the higher point in the mainline to release the entrapped air during the start of system and to break the vacuum during shut off. 9. Non- return valve: It is used to prevent the damage of pump from back flow of water in rising main line of drip irrigation system. 10. Pressure gauge: It is used to indicate the operating pressure of the drip system. The pressure gauges are installed at the inlet and outlet of the san and screen filters. 11. Grommet and take-off: These are used to connect the lateral to submain. A hole is pinched with hand drill of pre determined size in sunmain. Grommet is fixed into the hole on submain. Take-off is pressed into the grommet. Grommet acts as a seal. The sizes are different for 12 mm and 16 mm laterals.
12. End cap: They are used to close the lateral ends, submain ends or main ends. Submains and mains are preferably provided with flush valve. 13. Fertilizer system: It is used to add the chemicals ( nutrients, herbicides, pesticides etc.) to the irrigation water. Since these are toxic, care must be taken to prevent the back flow of irrigation water into the source. Fertilizer application in drip irrigation system commonly called Fertigation is accomplished by devices called ventury or fertilizer tank. Design Considerations of Drip Irrigation System: Design of drip system for a given situation needs consideration of soil, crop, climate and topography besides the equipment availability as indicated below: Inventory of resources Weather data for computing crop water needs Crop and management factors Soil characteristics Quality of irrigation water Capacity of the system Optimum water application rate Irrigation interval Selection of emitters Design of laterals Design of main line Selection of pump and power unit General consideration for layout Standard agricultural engineering books may be referred for further detail on drip irrigation design.