Drip irrigation (also referred to as trickle irrigation or micro irrigation) utilizes the concept of applying small amounts of water through perforations in distribution lines to specific areas of application. The system essentially consists of a perforated plastic pipe laid on the ground or buried at a shallow depth. Water is supplied through these pipes.
Perforations and any fixtures fitted on them to release water are known as emitters. They are designed to emit a trickle rather than a jet of water. The emitters are placed so as to produce a wet strip along the crop row or a wetted bulb of soil at every plant. All the field pipes are left in place for the duration of the growing season of the crop. Fertilizers are usually applied in solution along with the water.
Israel, a country with acute shortage of fresh water, pioneered extensive use of the system. This system has been used with a number of horticultural crops like citrus, apples etc., and also on several vegetable crops like tomatoes, peppers, etc. Use of this method of irrigation with other crops is under way in several parts of the world.
The trickle irrigation system has the following advantages:
1. Water distribution in close proximity to plant roots along plant row and therefore water distribution is uniform and controlled.
2. Elimination of land levelling and irrigation on steeper slopes.
3. No surface flow and as such no tail water loss or soil erosion.
4. Permits use of poor quality water. Frequent irrigation is easily accomplished and has the effect of keeping soil moisture tensions low, i.e., between field capacity and saturation.
Therefore, the crop is able to withstand the higher osmotic tensions inherent in waters of higher salinity.
5. Concurrent water and fertilizer application.
6. Concurrent cultural operations during irrigation on such crops as trees and vines.
7. Restriction of weed growth to wetted areas.
8. Water savings and generally increased yields.
The disadvantages of the trickle system lies in the higher initial cost of installation; the problem of blockage of the outlets and the durability of the components.
Components of a 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. When the water source is at higher elevation than the field to be irrigated the drip system can conveniently operate on gravity.
The pipe system is usually made of flexible PVC pipes. The emitters are also made of PVC material so that they are not damaged when using with saline water or water mixed with fertilizers. Appropriate connections are to be used between the pipelines and other equipment. Fig. 15.21 shows a general layout of a drip irrigation system.
Emitters:
The emitters which are also known as drip nozzles or drippers allow the water to be applied at a very slow rate. Emitters are the most important component in the drip system. They should supply the water at the desired rate and should not get clogged either due to sediment or due to chemical reaction with the water.
Emitters may be classified according to –
(i) Principle of operation,
(ii) Flow regime, and
(iii) Lateral connection.
Depending on the principle of operation, emitters could be classified as orifice type, long path, double-wall pipe or perforated pipe. Orifice emitters are connected to the lateral and consist of an orifice producing a jet of water.
The jet strikes a cap which acts as a pressure dissipater. These are simple but have the problem of clogging due to silt or dissolved salts. The orifice emitters range from 0.5 to 1.0 m and operate at pressures of 2 to 5 m.
Long path emitters are long narrow tubes (0.8 to 2.5 mm dia) operating at relatively high pressures (10 to 15 m). Pressure dissipation is by friction during flow. Several types like the microtubes and the screw type emitter are available.
Clogging is not a major problem in these emitters. Double- wall pipes are two flexible polyethylene pipes, one within the other. Water flows inside the inner tube at high discharge rate and pressures (3 to 15 m), passes through small perforations in the inner pipe to the space between the two, from where it flows out at low pressure through large perforations in the outer wall.
Porous tubes or perforated pipes are pipes with walls having small pores, through which water is drawn out by the soil suction. These are buried in the soil and deliver water all through their length. These are easily clogged unless some filter material is provided around them.
Based on operating pressures, emitters are referred to as low pressure (2 or 5 m) or high pressure (8-15 m) emitters. Discharges below 4 lph are termed low, 4-10 lph medium and 15 lph or more high. Emitters are also referred to as point source or line source emitters depending on whether the outlet is at one point or all along the line.
Pressure or flow compensating emitters are designed to discharge water at a constant rate over a selected range of pressure variation. In these emitters, variation of pressure distorts a flexible membrane which causes the passageway cross-section to decrease as the pressure increases.
Emitters are also classified depending on the type of connection to the lateral. The inline emitter is placed between two lateral sections whereas the on-line emitter is attached to the outer wall of the lateral with an insert into the tube. If the lateral is buried on-line riser is used for connecting the emitter. This is done when the emitters are placed far apart and it is advantageous to bury the lateral from cultivation requirements.
The value of x characterizes of flow regime and discharge versus pressure relationship of the emitter. The lower value of x, the less discharge will be affected by pressure variations. In fully turbulent flow, x = 0.5 and in laminar flow, x = 1.0. Non-compensating orifice and nozzle emitters are always fully turbulent. The exponent of long-path emitters may range anywhere between 0.5 and 1.0. For pressure compensating emitters, the value of x = 0.
The flow regime is characterized by the Reynolds number, Re. It is determined by the diameter of the flow cross-section and the discharge passing through it.
In a nozzle or orifice emitter water flows through a pore or opening of small diameter, where most of the head loss takes place. The flow regime is fully turbulent and the discharge of the emitter is given as – 
From Eq. (15.59) it is possible to calculate the orifice cross-sectional area necessary to provide the desired head loss and discharge.
Flow in Long-Path Emitters:
Head loss in a long-path emitter occurs in the long-flow path. Length of the path needed for required loss of head and a known discharge is given by the Darcy-Weisbach equation.
Clogging of Emitters:
Clogging of emitters is one of the main causes of failure of drip systems. Clogging could occur due to suspended particles carried in water. The suspended particles may be organic moss, aquatic plants or soil particles.
Chemical precipitation could occur at the emitter outlets due to salts present or when fertilizers are mixed with the irrigation water. Biological deposition like algae could also clog the emitters. Apart from using filters, injection of acids, oxidants, algaecides and bacteriacides are treatments used to control chemically and biologically caused clogging.
Emission Uniformity along Laterals:
As with sprinkler irrigation laterals, head loss due to pipe friction and changes in ground surface elevation cause variation in emitter discharges. In case of drip systems, water temperature could also cause variation in emitter discharges. Uniformity of application may be measured, as in sprinkler irrigation by means of Christiansen’s coefficient of uniformity, Cu is defined by –
A uniformity coefficient of 90% and above is considered to be acceptable. The acceptable values are higher than in case of sprinkler systems.
In order to ensure the manufacturing quality, the manufacturer’s coefficient of variation is determined from flow rate measurements for several identical emitters and is computed with following equation. 