Subsurface Drip Irrigation (SDI) Maintenance

However, as I have met growers around the world, I have seen two opposite scenarios. On the one hand, I meet growers with SDI systems who have realized incredible yield increase and cost reduction. Their systems have given them many years of return, with some growers still using the same drip tape that was installed 20+ years ago. In fact, if I look at our older SDI installations, a 10-15 plus year life span for 15 mil (0.38mm) thickness drip tape is quite common.

On the other hand, unfortunately I have also seen SDI systems which have failed in just a few seasons. These growers are unhappy with their system and experience problems such as clogging and the inability to deliver enough water to their crops during peak water demand. 

Why is there such a significant difference in grower experience? The answer in most cases is maintenance. No one would expect to buy a car, never changing the oil or air-filter, and expect it to continue performing. The same holds true with SDI systems. A good maintenance regime will help ensure that you are in the ‘happy grower’ category, which has the corresponding economic benefits that SDI provides.

As we have been providing SDI systems for many decades now, the purpose of this article is to give you a selection of recommended important maintenance tips. Of course, this is provided as a general guide only, and an irrigation professional should be involved to consider your specific needs. Additionally, this article references uses of acids, herbicides and other chemicals. These products can be extremely dangerous and are not suitable for all systems. Proper handling and adherence to local legislative/regulatory requirements is a must.

 

It all starts with the design

Some people may think that a maintenance article will focus on what to do after your system is installed; however, what we have seen time and time again is that maintenance starts with proper system design. If your system is not designed correctly, you will never be able to perform the necessary maintenance.

Flushing the system is the key preventative maintenance regime required. The system must be able to achieve a flushing velocity of 0.3-0.5 m/ second at the end of the laterals in order to expel contaminants. However, if you work backwards, you will quickly calculate that to achieve this velocity, you will need to be able to move a much higher volume of water for flushing than for irrigation. This consideration impacts everything from the laterals, to the sub-mains, valves, pump, etc.

Most systems are unable to flush an entire block. Therefore, the ability to isolate smaller sections (e.g. 25% of a block) at any time for flushing will need to be considered in the initial design depending on block geometry, topography and hydraulic factors. 

Aside from designing a system which can be flushed, we strongly recommend the use of a flushing manifold. As shown in the diagram, a flushing collector manifold connects the ends of the laterals and opens for flushing only.

There are numerous other design considerations for SDI; here’s a list of a few key factors to consider:

• Vacuum breakers are key. At minimum, you need one in every field zone and collector. They should be placed at the highest elevation point.

• As you are designing a system to last many years, your filtration needs to be better than what you use for single season use where the drip tape / line needs to last just one season. In most cases, we would recommend media filtration as the primary filtration. For backup/secondary filtration, we recommend in-field semi-automatic filters.

• Soil ingestion / suck-back is a risk with buried drip irrigation. Choosing the right drip tape/line is important. To help reduce risk, choose either a drip line/tape with a slit outlet that closes upon shutdown, or choose an anti-siphon (AS) drip line which is designed to seal when there is a vacuum in the line.

• In order to carry out your maintenance program properly, the system will also need to be set up with chemical injection capabilities.

 

Installation & Commissioning

Once you have your design, the next step is correct installation and commissioning.

The ground should be suitably prepared to the depth that the tube will be embedded, especially if you are installing thinner wall thickness drip line/tape.

All equipment should be inspected and monitored. E.g. The laying shoots for the drip irrigation should be inspected to ensure that they do not have any sharp edges / burrs. One burr in a laying shoot can cause significant damage.

Although not absolutely critical, we would highly recommend the use of GPS guided installation and cropping for use in row crops where the centers will be wider than 1.0 m. This will ensure that future crops are planted close to the drip line/tape.

Upon commissioning, flush every component in the correct order (main lines, sub-mains and laterals), and ensure that every lateral is expelling water before it is connected to the flushing lateral.  Check that the filter backflush program is functioning and that the field valves are set correctly. Take pressure and flow measurements to make sure that they correspond to the hydraulic design. 

 

Measurement

Your two key factors from a measurement perspective are flow and pressure. Any changes to these measurements are warning signs. In general, an increase of flow is a sign of leakage, and an increase of pressure is a sign of blockage. When you see these warning signs, you need to undertake a field investigation to discover the cause and to address the problem.

Frequent crop observation is also paramount. However, as farms become increasingly large and geographically spread over greater distances, it is impossible to be everywhere the same time. Practically speaking, for some crops in particular, it is difficult to see what is happening in the middle of the field. To discover problems before they are too late, we recommend the use of satellite-based crop monitoring technology. Below you will see two screen shots from our Manna Irrigation Intelligence service. The Plant Wetness Variability map detected an irrigation problem that the grower was then able to resolve. The use of monitoring technology is invaluable for SDI.

 

Flushing 

Flushing is one of the most important preventative maintenance activities for your SDI system. Sub-mains should have a minimum velocity of 0.5 m/s, and drip laterals should have a velocity of at least 0.3 m/s at the end of the laterals.

Flushing should occur at the beginning of every season, after any damage or repair, and during the season. Flushing during the season is dependent on water quality. In general, it is monthly for clean water, every two weeks for poor quality water, and weekly for very poor quality water

 

Chemical treatments

There are effectively two forms of treatments; acid treatment for dissolving mineral sediment, and chlorine/peroxide treatment for oxidizing organic material, iron and magnesium, prevention of algae formation and elimination of sulfur bacteria. 

Due to the complexity of treatments, a high-level overview of treatments is provided only. An irrigation professional should be engaged to help determine the treatment plan suitable for your system. Be very careful of possible interactions of system components and fertilizers.

Acid treatment. Protection against calcium carbonate precipitation can be achieved through continuous injection of an acid in low dosage that achieves a water pH level of 6.0 – 6.5.

For treatment of mineral sediment, one-off chemical treatments are used. Generally, in the middle and at the end of the season. The objective is to bring the pH of the water to 2.0 for half an hour, and then to flush afterwards. 

Hydrochloric acid is the most common form of acid. Other acids are more potent, but are also more dangerous. The calculation of acid injection should be made with the consultation of an irrigation professional. It is imperative that you check that all the components of your system can handle acid treatment.

Chlorine/peroxide treatment. Continuous injection regimes of low concentration chlorine are generally defined as one hour every three days to create a surplus chlorine concentration of 0.2 ppm at the end of the last treated lateral.

Periodic injection regimes of high concentration chlorine involve injection for 1 hour several times during the season. The goal is to achieve 3.0 – 5.0 ppm concentration at the end of the last treated lateral.

Keep in mind that temperature and pH of the water impact chlorine effectiveness.

Lower temperatures and lower pH increase the effectiveness of chlorine, while conversely, higher temperatures and higher pH decrease the effectiveness. 

 

Root Intrusion

Maintaining an irrigation regime that prevents water stress to the plants will prevent plant roots from ‘chasing’ water into the dripper. However, in practice, a number of crops require water stress prior to harvest. In this case, herbicide treatment is the most effective preventative maintenance against root intrusion.

 

Rodents

Rodents can damage the drip line and therefore rodent management is important. Starting with good farm hygiene, such as slashing/mowing to prevent buildup of protein is an easy and effective starting point. In addition, rodent extermination programs are best conducted in the colder months.

 

Conclusion

At the start of the article, I spoke about growers whom I have met who have yielded the full range of benefits of SDI, ranging from economic (yield increase and cost saving) to environmental and operational. It is our goal that, with the introduction of proper maintenance regimes, more and more growers will experience the benefits of SDI.     

 

Author
by Matt Clift, director of Global Product Marketing at Rivulis