Get subsurface drip irrigation (SDI) for sugar cane right
By Matt Clift
If you have chosen to irrigate your cane with subsurface drip (SDI), congratulations – this is a decision that will yield economical and environmental benefits for years to come.
With the decision to use SDI, there are a number of options that you are presented with — lateral configuration, dripper type and flow rates to mention just a few.
For over 30 years, Rivulis has been designing and installing subsurface drip irrigation (SDI) systems for sugar cane. Based upon this experience, we have developed a number of general recommendations for growers using SDI in sugar cane.
Choosing the right layout
In countries with mechanical harvesting, an increasingly common crop configuration is 1.5 – 2.0 meters centers with dual rows of cane. The placement of the drip line laterals will depend upon the water application rate required during peak growth days and the exact planting configuration. However, it is not uncommon for water requirement to reach 12 mm a day in some regions. As such, for dual row applications specifically, we recommend one line of drip tape under the rows of cane (drip laterals every 0.75 – 1.00 m). Not only does this configuration provide the water required for peak growth, the reduced distance between the tape and the active root zone increases efficiency of the system.
For single row plantings, and other planting configurations, please contact your local Rivulis representative for a recommendation.
Remember – you need to design for peak water requirement, not average requirement. If the average water requirement is 8 mm a day, but peak is 12 mm a day, you must be able to deliver 12 mm because cane growth loss to water deficit cannot be recovered at a later date.
Distance between drip emitters
Correct spacing between each drip emitter can make a big difference. Early sugar cane drip irrigation systems used drippers spaced at 50 – 60 cm intervals. However, this practice has generally been superseded by closer dripper spacing of 25 – 30 cm. The benefits of closer dripper spacing are numerous, but are all related to water movement. When irrigating, you want water to move laterally, not down through the soil profile where it is either lost (including any fertilizers added) or is harder for plants to uptake. By keeping drippers spaced at 25 – 30 cm intervals, water flows laterally quicker, ensuring a continued wet strip along the row of cane. Increased distance between drippers can lead to dry patches between them, and more water lost through the soil profile while trying to achieve lateral movement.
The thickness of the tape itself is commonly measured in mil (thousandths of an inch), with drip tapes and thin wall drip lines ranging from 6 to 25 mil wall thickness. Generally, we have found 12 – 15 mil wall thickness to be suitable for subsurface sugar cane, although there are cases where heavier wall thickness may be considered. We have 15 mil tape that is still in operation 20 years after being installed.
A common question we receive is: “Should a system use pressure compensating (PC) drippers?” A PC dripper has a membrane that keeps a constant flow over a wider range of pressures.
Here, there is no single answer. However, Rivulis is unique in having both drip tapes such as Rivulis T-Tape, and PC drip lines such as Rivulis D5000 PC. Therefore, we have the flexibility to offer what you need, not just to limit you to solutions that may not suit you.
The advantages of PC drip lines is that you can have very long-run lengths without concern of reduced flows towards the end of the block or variations on sloping ground. Design is also easier and it enables you to operate with smaller diameter sub-mains which reduce upfront costs.
However, PC drip lines do come at extra cost compared to non-PC tube, and overall system design and subsequent running costs should be considered. Generally you should evaluate whether you need PC. As large areas of sugar cane are on flat ground, and as Rivulis T-Tape is available in up to 35mm diameter (which can give you up to 700m of run length on flat ground while maintaining 90% emission uniformity), Rivulis T-Tape is an economical solution that is ideal for most applications. However, at the same time, you have the option of Rivulis D5000 PC available for when you need the performance of pressure compensating drip lines.
Main lines and submains
When designing a system, it can be tempting to reduce main-line and submain costs by using smaller diameter piping. Although you will save money upfront, our experience has shown this can lead to significantly increased energy bills. Pressure is lost as it passes through piping due to friction loss. Once you go over 1.5 meters per second, significant pressure loss is incurred, resulting in increased pumping costs to still achieve the required pressure. We would generally recommend keeping to a maximum of 1.5 meters per second for the main line and 2 meters per second across the submains dependent on the situation. The difference in cost is not to be underestimated. A poorly designed system with incorrect mainlines and submains compared to a correctly designed system we have modelled to equate to 46% increase in pumping costs.
Every situation is unique, and we recommend you engage professionals such as Rivulis hydraulic design team to develop a system specifically for your requirements.
Always ensure your designs are completed by an experienced drip irrigation designer, and only work with drip irrigation companies with a proven track record of drip irrigation in sugar cane. When we talk to growers who use drip irrigation, we often hear comments such as: “It is nowhere near as complicated as I thought it would be.” Drip irrigation is easy, and can yield great results if you work with partners who can provide the expertise to deliver a system that best meets your needs.