2.2. Types of spray systems
A wide range of different types of spray irrigation are in use.
Permanent installation. Permanent installation is expensive, since the equipment cost is dedicated to a fixed area and cannot be moved, but gives best control over irrigation management.
Permanent sub-mains and spraylines. It is common to permanently install underground mains and sub-mains to form the reticulation network of a spray irrigation system. It is less common to permanently install the sprayline except on high value perennial crops, playing fields, ornamental applications and greenhouses. Such systems are easily automated.
Movable sprinklers on fixed laterals. For turf and ornamental irrigation, it is possible to permanently install laterals, but use special bayonet type fittings to enable individual sprinklers to be moved from one station to another. This makes management simpler, and allows pipes to be permanently pressurised, but enables removal of sprinklers (for example, from playing fields) and better utilisation of sprinklers. (New installations would use "pop-up" sprinklers, which automatically retract into a hurried housing when water pressure is closed off.)
Solid set system. This is a type of above-ground installation used in some annual crops where the spraylines and sub-mains are installed, without shifting, for the life of the crop. The equipment is installed after die last operation of ground preparation and sowing, then removed after die last irrigation before harvest. Special couplings and fittings are used, which are more easily assembled and dismantled in die field.
Portable systems. With portable systems, die spraylines or machines are moved between runs or stations. This enables die equipment to be used over a larger area, thereby reducing capital cost per hectare, but possibly increasing labour costs. The equipment is selected so that it completes its cycle of runs or stations before irrigation is required again at die start of die cycle.
Hand shift aluminium spraylines. Couplings at me end of each length of pipe enable quick attachment or removal so that die sprayline can be shifted manually to each sprayline position. Each coupling has a gasket which prevents leakage when die water is under pressure, and provision for attachment of die sprinkler or riser. Skids or stands are attached to die coupling to ensure die sprinkler is positioned vertically and/or to provide some height for die sprinkler to clear die crop.
Connection to die sub-main is accomplished by a short length of flexible pipe into a hydrant. The hydrant is just a stand-pipe and valve connected to the buried sub-main, with a coupling to suit. The flexible pipe can be long enough to enable two or three sprayline positions to be serviced from die one hydrant.
Hand shift systems incur a significant labour cost in shifting pipes, often under adverse paddock conditions. The systems described in the following sections are designed by various means to reduce labour requirements.
Figure 6.1 Hand shift spray irrigation of pasture.
End tow. The sprayline is connected end-to-end by a steel cable, or by a special fixing method at each coupling, and each coupling is mounted on a skid. This enables the whole sprayline to be towed by die tractor. This is suitable only where die shifting can be accomplished in the same direction die sprayline runs, but can be useful in intensive cropping applications (such as vegetables).
Angle tow. This type of equipment is also sometimes referred to as "end tow". Each section of sprayline is supported by a pair of wheels which are able to pivot. The tractor pulls die sprayline from one end, in a 45° direction to die sprayline, to move it "crablike" halfway toward its next position. It is then pulled from me opposite end to complete die shift. This enables long spraylines to be quickly moved. This method is common in small to medium areas of pasture and lucerne irrigation. The sprayline will buckle if moved when full of water, so couplings contain a valve which opens to release water when the water pressure is turned off.
Figure 6.3 Angle tow sprayline on pasture.
Figure 6.4 Typical side roll spray irrigation application.
Side roll. The sprayline is mounted on relatively large wheels, such that die whole sprayline can be moved in a parallel direction to its next position. This movement can be done manually or the equipment can be fitted with an engine or water motor to provide slow continuous movement. Variable speed allows for alteration to the depth of water applied. Powered systems simplify moving the sprayline back to its start position at die completion of an irrigation cycle. Side roll machines can also be towed from the end to move die spray line to a different field, by die temporary use of transport wheels under each large rolling wheel. The diameter of die rolling wheel permits irrigation of taller crops, but traction over wet soil needs to be considered. With powered systems, each sprinkler is connected to a weighted connecting pipe, so that it always remains vertical as die sprayline rolls.
Figure 6.5 One type of travelling rigator.
Travelling urigator (soft hose). This term refers to a family of irrigation machines, characterised by a single high pressure giant sprinkler (sometimes called a "rain gun") mounted on a carriage which is towed from one end of a run to the other by means of an anchored cable. A winch is located on the machine, which is turned slowly, by a turbine, piston or bellows type of water motor. As the winch turns, it winds up the cable, but because the cable is anchored at the far end, it has the effect of towing the machine along. Winch speed is adjustable to vary the rate of water application. Water is supplied to the sprinkler through a long flexible hose towed behind, but connected to a hydrant at the centre of the run.
The width of the run is governed by the trajectory of the water from the sprinkler, but can be up to 100 m. Adjacent runs are overlapped by around 30 per cent to give a somewhat smaller lane spacing. Length of run is determined by die length of hose that can be towed by the machine, but up to 400 m is possible.
Figure 6.6 Typical spray pattern from a travelling irrigator, showing the substantial wetted width, and a good breakup of the spray accross the width.
At the completion of the run, the sprinkler shuts down, with die cable fully wound up. To move to an adjacent run, die hose is wound onto a reel on die machine, power assisted by die tractor PTO on larger machines. The cable is re-anchored, die machine towed back to die hydrant, die hose connected, and die machine towed further back to die start position of die run.
These machines have the disadvantage of a high operating pressure, die possibility of structural damage to die surface of bare soil caused by impact of large droplets, die need for a special laneway to eliminate damage to tall crops by the dragging hose, and wind losses resulting from the long and high trajectory of die spray.
Hose reel type traveling irrigators (hard hose). In this type, die machine is kept stationary near die hydrant, and die rain gun is towed back toward die machine. This is accomplished by winding up the pipe running to the sprinkler, rather man towing the machine by a winch and cable. Such machines are characterised by a large reel mounted on a carriage, which is rotated slowly by a water motor to wind in the sprinkler. Polythene pipe is used rather man lay-flat hose, and because of this, the machine is sometimes referred to as a hard-hose travelling irrigator.
Figure 6.7 Top, the reel of one hose reel machine and bottom, the sprinkler. Mote the effect of insufficient water pressure on the distribution of water. A large proportion hits the ground at the end of the trajectory, without droplet breakup into a "curtain" effect.
From a crop point of view there is no difference compared to a conventional travelling irrigator, because the same type of rain gun is used. There are some possible management advantages, however:
A dedicated laneway for the sprinkler carriage may not be required.
The machine is easier and quicker to set up as it does not need an anchor cable.
It is parked near the hydrant. It only needs to be moved from one hydrant to another at die end of each run and die sprinkler pulled to the start position away from the machine.
Irrigation is accomplished on each side of the machine by simply rotating die reel 180° on its carriage.
Boom-type travelling irrigators. Some of the disadvantages of a conventional travelling irrigator can be reduced by replacing die giant sprinkler with a wide boom fitted with multiple sprinklers of lower operating pressure. This maintains a convenient with of run but reduces operating costs. Finer droplets and a more desirable wetting pattern are created. The boom is mounted on a similar type of carriage, propelled in a similar way. Moving the machine between fields becomes more complex, requiring the boom to be partly rotated to be parallel to the run, and a person holding each end of die boom steady with a rope, during the transport operation.
Some machines are fitted with a rotating boom.
Figure 6.8 Low pressure boom type travelling irrigator
Centre-pivot irrigators. These self-propelled machines are designed for spray irrigation of large areas. They consist of a large diameter spray line, supported on substantial towers, radiating from a central pivot point. Water is delivered to the pivot point in a buried main line, or die pivot point can be mounted directly over a bore. Electricity is delivered to die pivot point in underground cables, or is occasionally generated on-site.
Each tower is normally driven by its own electric motor, such that the machine as a whole travels in a circular fashion about die pivot, irrigating as it goes. The irrigated area is therefore circular, which will require adjustment to crop management practices. Travel speed is set at die outermost tower and is adjustable to allow for various application rates. The remaining towers are fitted with position sensors (of which there are various types) mat instruct die towers to keep in a straight line. Should they not keep in line, because of bogging or malfunction, die machine is designed to shut down. This with happen also as a result of water supply failure.
The size and construction of die machine results in a sprayline high off die ground. Newer machines are fitted with drop tubes and low pressure rosette type sprayheads, that deliver water quite close to crop height.
Most centre pivot machines are permanently located in die field. However, mobile types are available that enable the same machine to be used on multiple circles. Corner watering attachments are available (but not common because of the cost) to allow watering of the corners of a square field to maximise production per field.
Figure 6.9 Pivot end of a centre-pivot irrigator. Electricity and water are delivered to the pivot point, then along the machine. This older style machine has medium pressure sprinklers mounted on the spray line.
Figure 6.10 A tower on a centre pivot irrigator. An electric motor, located in the can between the wheels, drives the tower forward when the position sensors, located at the top of the tower where sections of spray line join, instruct it to move
Figure 6.11 Drop tubes on this machine bring water closer to crop height, minimising wind disturbance, and enabling selection of low pressure nozdes.
Figure 6.12 A corner watering extension is filed to this centre-pivot machine. The last section of the sprayline can swing in and out to follow a non-circular boundary to the field.
Lateral move irrigators. The limitations of circular irrigated areas are overcome by the use of lateral move machines. These resemble centre pivot machines in construction and method of water application, but they move continuously in a straight line to water large rectangular areas. This type of machine can cover the largest area of all spray irrigation methods, with individual spraylines up to 1500 m long, travelling up to 4000 m, although smaller machines have been introduced to provide for more intensive production and/or overcome site difficulties.
Figure 6.13 Top, the centre of one lateral move irrigator. An engine driven pump draws water from a supply channel (its fuel tank is mounted on the other tower). The trailing tank contains fertiliser concentrate and an injection pump. Bottom, his view of the (empty) supply channel gives some impression of the area this machine can cover.
With larger machines, the centre span of the spray line carries an engine to drive a pump and an alternator. The alternator supplies electric power to the drive motors for each tower. The pump draws water from a channel, which runs die length of die centre line of die area to be irrigated, and delivers it to die spray line which straddles die channel. Similar methods to centre pivot machines are used to keep die sprayline straight but additional, quite sophisticated, guidance systems are used to keep the whole machine tracking parallel to die channel.
The channel supplying water to the machine must be straight and would normally be constructed with only a slight gradient. This partly conflicts with die ability of die machine to negotiate gently undulations, although some systems have been installed with stepped channels to accommodate steeper slopes.
Lateral move irrigators are available where water can be supplied to die sprayline from a large diameter hose, connected to hydrants along die edge of die irrigated area. Machines as small as one span, with overhanging extensions, are available for more intensive production. These smaller machines can be towed from the end to additional sites, thereby increasing their application to fields of unusual shape and maintaining a low cost per hectare.
As with other spray irrigation machines, lateral move systems can be easily automated and give good control over water application.
Figure 6.14 Top, the drive carriage of a hose-pull lateral move irrigator and bottom, the trailing hoses.
Sprinkler design and performance
Sprinkler characteristics greatly influence sprinkler performance in achieving a satisfactory application of water.
Types of sprinklers. Sprinklers vary according to their method of generating droplets, and die water pressure at which they normally operate.
Medium pressure rotary knocker. Most agricultural sprinklers are of this type, designed with an operating pressure in the range 200-400 kPa (30-60 psi). Water is forced out of single or dual nozzles, breaking up into droplets as it leaves the nozzle. The water hits a rotating striker arm, which performs two functions:
As the arm strikes the water stream, driven by a spring, it deflects some of the water to a smaller radius, improving the distribution of water near the sprinkler.
The force with which the arm hits the sprinkler causes the sprinkler body to rotate part of a turn, thereby accomplishing rotation. Spring tension can be adjusted to vary the speed of rotation.
Various sized nozzles are available. Nozzle size(s) and water pressure at the nozzle can be selected to provide the required discharge rate and wetted diameter. This in turn determines the sprinkler spacing for optimum uniformity and application rate over the field. A sample sprinkler performance table is shown in Figure 6.15
The angle of trajectory of the sprinkler is normally from 20-25° from the horizontal. Special low trajectory types are available for watering under trees or in high wind sites.
Nozzle blockage can be a problem; for example insects and spiders can occupy the nozzle between waterings. Frequent checking for blocked nozzles is recommended, as it is for sprinklers failing to rotate or not standing in the vertical position. Sand and silt in die water may cause nozzle wear, but most high quality sprinklers are designed to resist damage to bearings and seals caused by die presence of sand. Plastic sprinkler parts may become brittle with rime and therefore more susceptible to damage.
High pressure rain guns. A single high pressure (400-700 kPa) giant sprinkler is used on travelling irrigators and hose reel machines. It operates on a similar principle to a medium pressure type, but nozzle size and water pressure are much higher, giving a much larger wetted diameter and sprinkler discharge.
Low pressure sprinklers. Rotary knocker type sprinklers require a certain water pressure to drive me striker arm. Low pressure (100-200 kPa) sprinklers use other means to accomplish droplet generation. In many centre pivot and lateral move irrigators, water is directed onto a stationary "anvil", shaped to break die water stream into a fan type spray. Omer devices include spinning discs and spinning arms, often used with microirrigation systems.
Others. Many horticultural and ornamental applications use a reaction type sprinkler. In diis type, two nozzles pointing in opposite directions are carried on die ends of a relatively wide arm. Water leaving die nozzles causes die arm to rotate with die angle of trajectory of die nozzles variable to adjust die rate of rotation. Often, this same mechanism is used to provide power to a cable winch to propel die machine along.
Various types of pop-up and quick coupling sprinklers are available, mainly for turf and grounds watering.
Figure 6.16 Sprinkler irrigation of roses. Many horticultural crops prefer not to have wetted foliage.
Sprinkler performance criteria
These relate to the ability of the sprinklers to provide a uniform pattern of "rainfall" on the ground surface.
Droplet size distribution. A range of droplet sizes is produced as the water stream breaks up leaving the nozzle. Larger droplets will have a greater momentum and therefore travel further from the nozzle, but will increase the risk of soil and even plant damage (for example, to flowering parts). Small droplets may be preferred, but will be much more susceptible to wind drift and evaporation in the air, thereby reducing application efficiency.
Smaller droplets are achieved with higher pressures and/ or smaller nozzle sizes.
Wetted diameter and sprinkler spacing. The diameter of the wetted area increases as nozzle size and pressure increase. The relationship is not linear, that is, a doubling of pressure does not result in a doubling of wetted diameter, since the effects of droplet size come in to play.
The wetting pattern from a single sprinkler usually results in a large variation in depth of application, from maximum at die sprinkler position to zero at the perimeter of me wetted area. The pattern is distorted further by the action of wind. Consequently adjacent sprinklers must be positioned so their wetting patterns overlap. For rectangular spacing of sprinklers, me distance between sprinklers should be 0.55-0.75 per cent of the wetted diameter. The same applies to adjacent runs of self-propelled machines. Measurements of die actual wetting pattern in the field gives more accurate recommendations.
Sprinkler discharge. The flow rate from the nozzle also increases as nozzle size and water pressure increase, with a marked variation possible. The sprinkler discharge for a given spacing determines the application rate to die field as follows:
where spacing is die distance between sprinklers along die sprayline, and shift is the distance between sprayline positions. In the case of self-propelled machines, die travel speed influences die application rate: it is easiest to refer to die manufacturer's specifications.
Relationship between the above
Sprinkler manufacturers publish performance charts mat relate sprinkler discharge, nozzle size, water pressure and wetted diameter. Some sample data is included in Figure 6.15. As a starring point, it is necessary to decide the required application rate first (in mm/h). Sprinkler spacing is often governed by standard pipe lengths or standard models of self-propelled machines. This information can then be used to select die optimum nozzle size and water pressure to suit any given operational requirement.
Need for risers. Although the sprinkler can be attached directly to me sprayline coupling (see Figure 6.1), there are two reasons why the sprinkler should be mounted on a riser:
to ensure the sprinkler is above crop height,
to provide non-turbulent water flow into the sprinkler.
High sprinklers may become unstable and topple over. In permanent installations the riser may need to be staked. In portable systems the sprayline coupling could be fitted with a stabilising foot, or it could be fitted with legs.
Figure 6.17 Ideal spray pattern – single sprinkler
Application rates. The maximum application rate should not exceed the infiltration rate of the soil surface. If it does, the water depth at the surface will increase and runoff will occur.
The application rate may be determined by the need to irrigate a given area in a particular time. For permanently installed equipment, mere is usually no limitations. For mobile equipment, which must water a certain number of shifts or stations within the irrigation interval, die application rate must be sufficient to get the required amount of water on within the prescribed time. Given mat a slow application rate has some advantages over excessive application rates, mere must therefore be enough equipment installed, and of sufficient capacity to suit.
The advantages of slow application rate include:
reduced runoff,
maintenance of good soil surface structure,
reduced labour per day (although total labour requirement for me irrigation is unaffected),
more uniform infiltration,
better match to the yield from bores.
Figure 6.18 Ideal spray pattern -Multiple Sprinklers, 50 per cent overlap
Figure 6.19 Actual spray pattern - single sprinkler
Uniformity of distribution
The desired level of uniformity at the soil surface is difficult to determine, because of the effects of lateral water movement through the soil profile. Generally speaking, for a given sprinkler, nozzle size, pressure and wind pattern, there will be an optimum sprinkler spacing where the distribution of water between the sprinklers is most uniform. This spacing may then need to be modified to suit the equipment in use, standard pipe lengths, layout of the field and the pipe network.
Areas of die field which do not receive sufficient water will not reach their yield potential, and may ripen prematurely. Areas of die field which receive too much water will suffer yield loss due to temporary waterlogging. Consequently, there is an optimum depth of water to apply and it is desirable to minimise the variation from this.
The uniformity of distribution can be analysed by looking at sprinkler spray patterns, where a graph can be used to represent depth of water applied according to distance from die sprinkler.
Figure 6.20 Actual spray pattern -multiple sprinklers
If an ideal spray pattern was created, die spacing between sprinklers should be half of die wetted diameter; that is die next sprinkler should be located at the wetted perimeter of die first. This would result in wetting patterns overlapping, but the combined wetting pattern would be uniform.
However, in die field, irregularities and distortions occur, resulting in significant departure from an ideal wetting pattern.
It is also convenient to observe how depth of water application varies in body directions about die sprinkler, and depth charts can be constructed.
The position of a single sprinkler, and lines represent die contours of water depth. Simple measuring containers can be located in die field on a grid or radial pattern under the sprinkler to measure such a distribution in the field.
Adjacent spray patterns can then be combined to represent the distribution between adjacent sprinklers, as an aid to determine optimum sprinkler spacing.
Sprinkler manufacturers sometime publish coefficients of uniformity for the wetting pattern of their sprinklers. Check that the tests are conducted by an independent organization.
Summary of design procedures
The peak irrigation requirement and interval are determined, which in turn determines how much water must be applied and how quickly. This determines flow rate through the pipe network and pump output. Consideration should be given to crop and soil characteristics, and site topography.
Determine the discharge rate per sprinkler, based on irrigation amount and expected sprinkler spacing. In conjunction with this decision decide the nozzle size(s) and water pressure to provide the necessary discharge. Consult sprinkler performance charts to determine the optimum combination. For self-propelled machines, the run spacing and travel speed of the machine need to be ascertained, based on manufacturer's data.
Determine the diameter of the sprayline lateral, given the flow rate per sprinkler, the number of sprinklers, and the length of the sprayline. For self-propelled machines, this decision is pre-determined by the manufacturer, to suit the performance specifications of the machine.
Design the network of sub-mains and mains. Consider layout or positioning of spraylines or runs, distance from proposed pump site, fittings, and so on. Decisions need to be made on pipe material and diameter, method of installation, and optimum balance between cost and layout. Consider the effect that layout will have on pumping duty; for instance will the pump be able to handle the pressure and flow rate when the sprayline is furthest away from the pump, compared to its nearest position? This is particularly important to sites of irregular shape or layout.
Select pump and associated equipment. Consult manufacturers' performance data to help this decision.
Select power supply and associated equipment.
Maintenance. Apart from maintenance of the pumping station, spray irrigation systems require only routine maintenance. Blocked nozzles should be cleared, and damaged sprinkler parts replaced. Spare parts should be kept on hand.
Damage could be caused by:
Careless handling of me equipment; sprinklers, hydrants, above ground pipes.
Wear caused by excessive abrasive material in the water.
Normal ageing of seals and gaskets.
Movement of buried pipes, and subsequent failure of points of connection in particular, usually caused by inadequate initial installation.
Self-propelled machines will require more substantial routine maintenance, particularly the lubrication requirements of gearboxes and bearings, but also attention to tyre pressures and electrical components. The owner's manual for the machine will have specific instructions.