A range of emitter types is available to suit particular situations.
Drippers.Drippers are designed to give a slow application at frequent intervals, targeting the crop root zone. The drippers are located either on the outside of the lateral (on-line) or installed inside the lateral at manufacture (in-line). Laterals are installed permanently in die case of vineyards and orchards, either laid out on die ground or suspended from a wire (where they are more visible and clear of vegetation). In orchards and plantations of larger trees at wider spacings, multiple drippers are used around each tree, attached to an off take from the lateral. In die case of row crops, the laterals are laid out for the duration of the irrigation season, then wound up on a large reel for later re-use. For plants grown in containers, drippers can be attached to the lateral by a short length of microtube. It is also possible to install in-line dripper laterals below the ground, which may have application in landscape irrigation, lucerne and other crops, but the risks of root penetration and die lack of visibility of die dripper may deter some users.
Figure 7.1 Drip irrigation of winegrapes. The dripline is carried on a wire above the ground. The wetted surface area is visible.
For standard drippers, there is a particular relationship between water pressure in die lateral and dripper output. To achieve a uniform output for each dripper in die field (a flow variation of 10 per cent or less is usually acceptable), it is necessary to provide a reasonably uniform water pressure along and between each lateral. Consequently, a drip irrigation system requires careful hydraulic analysis of die pipe network to ensure uniformity. This is difficult to achieve for long laterals and steeper sites, but pressure compensated drippers are available. These are manufactured with a small diaphragm mat automatically adjusts the dripper orifice size to provide a constant output over a range of water pressures. A high degree of manufacturing accuracy is necessary to ensure this occurs.
Figure 7.2 In this nursery, each container receives water from the lateral through a small piece of microtube.
Figure 7.3 Drip irrigation of corn. The driplines, connected to hand-shift aluminium sub-mains, are wound up each season and re-used.
Dripper spacing needs to be matched to the soil's hydraulic conductivity and dripper output, and die peak irrigation requirement of die crop must be met width die required irrigation interval. In-line drippers can be selected to suit a wide range of standard spacings. On-line drippers can be installed at any position in die lateral, provided die water pressure along die lateral is kept reasonably uniform.
Blockage of drippers is a major risk, because of die small orifice size used. Manufacturers go to great lengths to minimise die risk of blockage. Rather man a single pin-hole type of orifice, the flow path through die dripper should be a tortuous one, because this enables a short but larger diameter flow path. Also, the flow through it is more likely to be turbulent, (rather man laminar) which will help keep suspended material moving through die dripper. The inlet to die flow path should have its own screen to prevent particles entering. Some online drippers can be disassembled and cleaned manually.
Other factors to consider in dripper selection include:
• The ability of me dripper (and diaphragm) materials to resist chemicals in the irrigation water.
• The uniformity of output and range of operating pressures for pressure compensated drippers.
• Temperature changes on dripper performance (as well as expansion and contraction of laterals).
Perforated or permeable tube.There are various versions of perforated tube available where water is delivered through a pin hole through the wall of the tube, with perforations made at fixed intervals along the length of the tube. The holes are relatively small and easily blocked, so some types utilise secondary chambers built into the tube to allow a larger hole to be used.
Permeable tubes are also available, where water oozes continuously along the full surface of the tube. One type is manufactured from a plasticised paper material, and another from a special re-cycled rubber which gives to release particles which might cause a blockage.
These may have application in vegetable, flower or greenhouse crops and can be buried below the soil or growing medium.
Microtubes.These are pieces of small diameter plastic spaghetti tubing inserted into the wall of a lateral. Discharge rate is governed by the length of microtube and occasionally a spreading device is inserted on the end. Their relatively narrow diameter can result in easy blockage, and although coiled types are available, the microtube can get in the way and be damaged or pulled out. They have been mostly superseded by other emitters.
Microjets.This term describes a family of emitters which spread water over a much larger surface area than a single dripper. This may be necessary in larger tree crops or ornamental applications, or where a higher discharge rate is required. Microjets are in common use in domestic garden watering.
Various types of fan type spray patterns are available (full circle, part circle, directed jets, mist). Most use small perforations in the top of the jet to produce die wetting pattern, or alternatively, the water stream strikes a stationary anvil to cause spreading.
Microjets have no moving parts (which distinguishes them from minisprinklers). Operating pressure is nominated by the manufacturer and discharge rate is selected by choice of orifice size in the base of the jet. Discharge rates up to 200 L/hr are typical, with wetted diameters to around 2 m. The wetting pattern at the surface is often uneven, but this may not be a problem with low application rates, considering lateral infiltration in the root zone. Microjets need to be clear of vegetation often encountered at ground level, and need to be oriented vertically. Although they can be inserted directly into the wall of a lateral, they are best installed on a short riser or stake, with a connecting tube from the lateral to the jet.
Figure 7.4 Cross section through one type of in-line dripper, indicating the tortuous flow path designed to help prevent blockage.
Blockage of microjets is still a potential problem, since water must pass through a single, small-diameter orifice.
Minisprinklers.This describes another family of emitters, which use a rotating component to spread water over a larger area (2-5 m). This type of emitter also delivers a higher rate of discharge. Orifice size is often larger, so blockages may be less likely.
Many different types are available. Some use a spinning disc or cap, others a rotating arm or arms, and others resemble small versions of conventional sprinklers. Selection is based on crop requirements for frequency and amount of watering, operating pressure and discharge rate, wetting pattern, and emitter spacing.
Figure 7.5 Microjet irrigation
Figure 7.6 Minisprinklers have moving parts to spread water over a larger wetted area whilst maintaining reasonably low operating pressure