DRIP IRRIGATION:

DRIP IRRIGATION

          Drip irrigation is defined as the precise, slow application of water in the form of discrete or continuous or tiny streams of miniature sprays through mechanical devices called emitters or applicators located at selected points along water delivery lines.  The terms trickle or drip irrigation are used synonymously.

          In the year 1940, Symcha Blass an Israeli engineer, observed that a large tree near a leaking faucet exhibited vigorous growth than other trees in the area.  In 1964 he developed the first patented drip irrigation system.  By the late sixties, this concept spread to Australia and USA and eventually to several countries in the world.

COMPONENTS OF DRIP IRRIGATION

          Drip irrigation system consists of the following components:

1. pump to lift the water from the source of supply,
2. a head unit consisting of a tank to maintain the required pressure for circulation of water,
3. a central distribution system, connected to the main water supply, which regulates water pressure and quantity,
4. a fertilizer tank, connected to the central distribution system, to supply soluble plant nutrients along with irrigation water,
5. a filter, connected to the central distribution system, to remove materials suspended in water,
6. PVC main supply pipe of suitable diameter and length to deliver the desired discharge,
7. sub-mains/or laterals of suitable diameter and length connected in a parallel way to the main, and
8. plastic drippers inserted in the laterals at the desired spacing (equal to the intra-raw spacing of the plant) which control the release of the desired quantity of water.

 Pump

     The duty of the pump in terms of flow and pressure is determined after the diameter, length and discharge of all the mains, laterals and emitters are decided and the friction losses are estimated.

Head

     The water lifted from the source of supply is stored in a head tank, usually 3x3x3 m size, resting on a raised platform to maintain pressure head of about 4 to 5m.  The head is connected to the central distribution system.  It regulates the pressure and amount of water supplied.

Central Distribution System

     It consists of check valves, water meters, gauges, fertilizer and filtration units fixed on the mains.  A check valve just down stream of the pump is open when flow is from the pump and closed when the flow is from the opposite direction towards the pump when pump is shut down.  This prevents water containing suspended materials, fertilizer nutrients or pesticides from flowing back to the pump.  A water meter is installed in the main line close to the tank for measurement of the discharge that passes through.  Each main line and sub-main should have a valve at its upstream end to provide ‘on’ and ‘off’ service and for isolation purposes.

Fertiliser Unit

     The drip system may include equipment for metering into the irrigation system, fertilizers, pesticides and anticlogging chemicals.  This unit consists of a tank, flow meter, regulator valves, liquid fertilizer pump or venture control valves and sometimes injector pumps and pressure reducing valves.  This unit is installed in the upstream of the filter.  The fertilizer is injected into the system at a predetermined rate.  Fertilizer injection requires that the pump operates at a higher pressure than the drip system.  The capacity of the tank and injection system depends on the concentration rate and frequency of application.  The fertilizer tank capacity is important.  The entire quantity of fertilizer must be dissolved initially demanding large size of tank and the use of highly soluble chemicals.  Less fertilizer solution and more frequent application requires smaller and less costly units.  If two injection points are provided, one before and another after the filter, the filter can be bypassed if filtering is not required.  The discharge line from the fertilizer tank is sometimes provided with a filter and also the suction side of the cylinder injector.  Injection points are provided so that injected fertilizers are properly mixed before the flow divides in several directions. 

Filtration System

     Filtration of irrigation water is essential to prevent clogging of emitters which can be a major problem in the drip system.  The clogging may be due to presence of salts in water, microorganisms, suspended organic and inorganic matter, clays, silt etc.,  Filter is connected to the central distribution system.  There are two common types of filters:

Screen (mesh) Filter

     This is useful primarily for removing suspended inorganic particles in water containing sufficient amounts of organic matter.  The screen filter does not remove large amounts of suspended particles and organic particles without reducing the flow of water through the filter.  It is, therefore, necessary to frequently flush the screen mesh filter to remove accumulated particles.

Sand Filter

     Sand filter is most effective in the removal of inorganic and organic particles from water.  It can extract and retain large quantity of suspended solids without reduction in delivery of the rated flow of filtrated water.  The sand filter is normally provided with a back flushing arrangement. 

Main Line

     The main line design is based on topography, the operating pressure, the field layout of laterals and submains and the required discharge from each outlet along the main line.  The main line system has changing flow capacity with respect to length.  It has higher discharge in the upstream sections than in the downstream sections.  The main line design is to select the proper pipe diameter for each section to deliver water at the required rate to all submains and distribution lines in the system.  There are a number of pipe sizes to meet the hydraulic requirement of a given layout.

     The Darcy-Weisbach equation is the most aacurate in predicting friction loss in pipe lines.  But it is cumbersome to use.  Hazen and Williams formula is the most widely used.  Tables and nomographs using this equation are available for use.  The tables and nomographs provide head losses, pipe sizes, velocities etc.

     The mains are PVC pipes usually of 25 mm to 75 mm in diameter.  In the nomenclature of PVC and HDPE pipes only the Outer diameter is quoted.  Pressure rating of PVC pipes is determined by the PVC material used and dimension ratio.  The dimension ratio is a function of pipe diameter and wall thickness and together form pipe pressure rating.  PVC pipes are generally available in 2.5,4.0, 6.0 and 10.0 kg/cm² pressure rating.

Sub-main

     The sub-main distributes the same discharge to all the lateral fitted to it.  The area covered by the sub-main is dependent on field lay out, slope of the land, water supply and irrigation demand and uniformity requirement within the subunit.  Usually pipes of 25-50 mm diameter and suitable length are used.

Laterals

     The lateral are provided in the main line or sub-main for each row of the crop.  Correct evaluation of friction head loss in the laterals is essential to achieve optimum uniformity for emitter characteristics and variability of manufacturing in addition to pressure variation.  In the case of lateral design as water flows through laterals, it is gradually discharged through the drippers.  A close value for head loss of pipe which loses water along its entire length can be obtained from Christiansen’s equation.  Considerations for design are: elevation, multiple outlet factor, frictional loss in the lateral and emitter discharge versus pressure performance.

     The uniformity of discharge through the extreme dripper on a lateral should be based on 20 per cent of the difference in flow or under turbulent conditions within the lateral, the pressure difference may be 44 per cent.  The significance of 44 per cent total head pressure difference is that it allows for greater span in head design and therefore, longer and more economic drip laterals.  The laterals should be designed to carry uniform discharge for any supply of water through the drippers with acceptable uniformity.  The lateral size should be selected to carry the maximum water required for one row per unit time.  The slope of the lateral line affects the discharge through the emitters as it causes pressure changes in the line.  In the drip laterals the pressure drop between the lateral lines must not exceed 20 per cent of the emitter operating pressure.  Where number of emitters are fixed, the length of the laterals is determined by the pressure drop between the lateral lines or the uniformity of emitters discharge.  In the drip system the dripper uniformity must not be less than 90 per cent.  Usually 9,12 and 15 mm pipes are used as laterals and fitted on both sides of sub-main or mains.  They are usually placed above the ground. 

Drippers/Emitters

     A perfect dripper should meet the following objectives:

1. compact, serviceable and inexpensive,

2. relatively of low discharge,

3. not vary significantly with pressure, and

4. a relatively large cross-section area to avoid clogging.

     Water application rate is adjusted by using drippers which discharge at the desired application rates.  The drippers discharge about 2-4 litres/hour.