Installation
It is nearly always recommended that an experienced installer be engaged. This is certainly the case regarding installation below ground level.
System monitoring
You can do your own pressure and discharge testing by taking measures at the extremities of the system, as well as any high points and weak patches.
- Pressure testing
Tapered attachments are available for use in conjunction with standard pressure gauges. Place the tapered attachment on the emitter until it is sealed over the emitter. The highest reading obtained is the emitter operating pressure. Most drip systems should be operating around 100kPa. Check this against your system design.
- Discharge
The most popular and time efficient method is to capture the discharge of an emitter for 36 seconds into a measuring cylinder. Multiply the ml captured by 100 = l/hr. Most drippers should emit 2 – 4 l/hr. Once a patch has been tested, calculations can be made to determine that the variability in pressure and discharge is acceptable within a valve unit (< ± 5% for discharge, < ±10% for pressure in non pressure compensating drip systems).
System maintenance
The maintenance required on drip systems is significantly lower than all other irrigation systems, however, the maintenance that is required is extremely important and must be regularly carried out.
- System flushing
Drip irrigation systems are typically not flushed often enough. In regions with poor quality water, systems must be flushed every couple of weeks, in others only a few times per year. Frequency can be determined by observing the colour of the discharge flushing. The system should be completely flushed in the following order - main, submain, laterals. When flushing the laterals, the taps should be left open long enough to observe two ‘slugs’ of coloured material come through.
- Oxidation
Oxidation is the term used to describe the injection of a chemical to control organic matter within an irrigation system. Organic matter can include algae, mussels, shells etc. Historically this has been carried out by chlorine, however, other chemicals are becoming popular such as hydrogen peroxide, and halosan. Calculation sheets are available to allow managers to determine the amount of chlorine required for injection. The amount will vary depending on the flow rate of the system and the degree of oxidation required.
Intermittent chlorination is the most common method (1-3 times per year), with10-15 mg/L typically injected, resulting in 0.5-2mg/L detectable at the end of the system. Test strips are the best method of determining if chlorine has adequately reached the end of the system. Do not rely on smell. Chlorine should then be left to sit in the system for at least 2 hours.
The system should be flushed following chlorination to remove the material which has been killed. Fertigation tanks must be thoroughly cleaned as chlorine is explosive if mixed with nitrate based fertilisers.
- Acid injection
Acid is used to lower water pH to a level required to dissolve mineral deposits. It is important to determine whether the problem is organic or mineral based prior to injection. Cut open and inspect emitters to determine the nature of the problem (and if a problem actually exists).
Mineral deposits can be an issue depending on the water supply, but are most commonly due to an error in fertiliser mixing with fertigation, where incompatible inputs may have been mixed. An acid titration can be carried out by a service provider to determine the amount of acid required to drop the pH of the water to a desired level. A series of calculations are used, accounting for the irrigation system flow rate, to determine the volume of acid needed. Many managers prefer to employ contractors to carryout both oxidation and acid injection. OH&S issues exist and must be followed.
Drip agronomy
- Mulching and soil management
Soil improvements should be based around where the roots are concentrated. All manures, gypsum, fertilisers and any other soil additives should now be applied along the dripline. Mid-row cover crops can be cut and thrown onto the vine bank. This practice results in the dripline gradually being covered. This may lead to root intrusion, especially during drought situations. Managers have overcome this by mechanically lifting the dripline periodically, or permanently suspending the dripline on a wire.
- Scheduling tools
Drip irrigation in hot arid regions requires frequent application. Three to five irrigations per week is not uncommon in the peak of summer. For this reason continuously logged scheduling tools are preferred where readings may take place frequently and automatically.
- Salinity
Salt tends to accumulate on the outer edge of the wetted zone of drip irrigation. Furrow and overhead sprinkler irrigation have the opposite effect and tend to encourage greater salinity levels below the vine row due to a lower level of leaching in this area. Therefore when drip irrigation is installed in these situations a large concentration of salt is leached out from the vine row. This is often visible on the soil surface to the side of the vine bank, near the wheel track. You should not be alarmed at this but must ensure that this salt is not allowed to move back into the active part of the rootzone. This can occur following rainfall, or during RDI when smaller irrigation volumes are applied.
Drip irrigation and heatwaves
As mentioned previously irrigation scheduling is critical for successful drip irrigation, particularly in the first year following conversion. Many irrigators are concerned that drip irrigation cannot be successful in hot arid regions because of heatwave conditions experienced. Drip irrigators have overcome this concern by adequately planning ahead using weather forecasting services. If heatwave conditions are predicted ample irrigation should be applied in preparation for the heatwave.
Drip irrigation and rainfall
Managers need to read their soil moisture monitoring equipment following rainfall to determine how effective this rainfall is. You may find that an irrigation is required much sooner than initially thought.
During winter when no canopy exists, rainfall can leach the salt that accumulated around the wetted zone of drip irrigation, back into this active part of the rootzone. This is not of great concern as little water use occurs at this time of year and the next irrigation will generally remove this salt.
During the season when a canopy exists, managers may ignore the need for irrigation following heavy rainfall. However, with drip irrigation most of the rootzone is concentrated under the canopy, along the vine row, leaving this rootzone drier than expected.
However the area outside the normal drip wetted area will have become moist following rainfall, and this soil may be highly saline. If the active part of the rootzone is dry, vines will begin accessing this saline moisture, and salt burn may result.
Other
- Dripline location
Some managers prefer to run their dripline on a wire suspended 30-40cm above the soil surface. Most of this is done for aesthetic reasons. It does avoid the danger in having root penetration as manures etc are added to the vine bank. It also adds to the cost, particularly as wire prices continue to rise. Also some mangers have observed some compaction occurring below the emitter, resulting in surface runoff.
- Retain existing irrigation system
Regardless of the previous irrigation system (even furrow irrigation), it is highly desirable to retain this existing system. This allows full cover irrigation to be applied when desired, such as to grow a green manure crop, frost control, vineyard cooling or irrigate alternative crops.
Filtration
Media or disc filtration are the only real options for primary filtration for drip irrigation. Screen filters are used in some regions but are less successful if algae is a problem. The choice between disc or media is a personal choice, quite often the personal choice of the irrigation designer! Four aspects to consider are; backflush volumes, floor space, level of filtration and cost.
Author
Jeremy Giddings & Allison Deegenaars - NSW Department of Primary Industries
