Sub-surface Drip Irrigation Systems
This Information Note has been developed for irrigators who are considering the installation of sub-surface drip (SSD) irrigation systems.
We encourage irrigators to adopt and use systems that reduce water losses and help achieve optimum water use and maximise crop yields and quality.
It is strongly recommend that before you make the decision to purchase a sub-surface drip irrigation system, you identify your individual requirements. This will help guide your decision in relation to the type of irrigation system you purchase.
Installation of this type of equipment is a significant business decision and irrigators are encouraged to complete a financial assessment that appropriately considers the investment. This should consider the cash flow impacts, capital expenditure versus operation and maintenance, the payback period, tax implications, and the benefits (ie productivity and labour) of the investment.
Too many decisions to purchase all types of irrigation equipment are made on the capital price alone and after a period of use the grower realises the equipment does not meet their expectations, a poor investment.
This Information Note can help you choose a system that will ultimately meet your short and long term expectations and deliver the benefits you require for your particular situation.
Do not commit to a system until you have considered and understand the following (each of which is expanded on below):
- What are the key components of the system?
- What am I trying to achieve with a sub surface drip irrigation system?
- What are my soil properties in relation to lateral movement of water?
- What crop type and rotation am I going to adopt?
- What are the design considerations?
- Do I have a reliable source of water?
- Do I know how to take full advantage of the system benefits?
- Do I understand the importance of installation?
- Have I adequately considered the other general considerations?
What are the key components of the system?
The design of the system is the most critical component - have the system designed by a qualified and experienced sub-surface drip irrigation designer. Proper consideration of all aspects at this point will save time and money later and ensure the system works efficiently and effectively.
The design should describe the specific pump requirements in terms of the flow rate required and the total pressure head requirement of the system. You should approach a number of reputable pump suppliers with this information and ask for quotes. High efficiency pumps will provide lower running costs for the life of the project and should be considered. Ensure the pump can deliver an appropriate velocity throughout the system for optimum flushing.
Filter systems are a critical component which will increase the life expectancy of the system if selected appropriately. There are a range of filter options with mesh, disk and media type filters the most common. These provide variable levels of filtration and your choice will be based on the quality of irrigation supply you have. Settling ponds and cyclones may be needed where heavy silt or sand loads exist (ie some groundwater). Other things such as algae will need to be considered along with dissolved ions. Better quality filters (such as disc and sand) will minimise maintenance required in the field.
Mains and Sub-mains
These are usually PVC and deliver water to the lateral drip tapes. Whilst choosing the location, diameter and length considered the system pressure, flow rate, water hammer and cost.
Lateral Dripper Tape
These are made of polyethylene and have built in emitters. They come in a range of diameters, emitter spacings and flow rates which should be determined by the crop water demand and soil water holding capacity. Wall thickness is important in minimising damage, the thicker the wall the more resistant to damage the dripper tape will be.
These can be fairly short and welded on the inside of the tape or quite long and part of the tape seam. There are several consideration you will need to understand regarding emitters and these are expanded on later.
Other components that you should ask your supplier and designer about and be included as integral components of the system are:
Pressure regulator valves - these ensure the required pressure is maintained within the system so that the required flow rate is achieved at all the emitters and the overall flow rate variation (uniformity) remains at an acceptable level.
Zone valves - as the name suggests these control the flow to various zones or blocks within the paddock serviced by sub-surface drip irrigation.
Air and vacuum release valves - prevent soil being sucked back through the emitters during shutdown or when tapes are being drained. They also allow air to be removed from the system minimising water hammer.
Flushing manifold - critical component at the tail end of the system where lateral drip tapes are connected, used for system flushing.
Pressure gauges - are essential for managing and maintaining the system.
Flow meter - measures the volume of water applied through the system and is a critical monitoring tool that also helps determine system performance.
Fertiliser injector - this provides the irrigator with the ability to fertigate through the irrigation system.
Backflow preventer, check or one way valve - these prevent the back flow of fertilisers and/or particulates into the water supply system. Used where the supply is potable water.
What am I trying to achieve with a sub-surface drip irrigation system?
Many of the considerations discussed in this document will help you answer this question. It is important that you seriously consider the alternatives such as border check irrigation, lateral move or centre pivot. Given the appropriate soil conditions, design, construction, operation and maintenance (O&M) these systems can be cost effective and efficient. Talk to an Irrigation Surveyor and Designer and/or other irrigators with SSD irrigation and do a whole of life cost/benefit analysis of the investment to ensure a sub-surface drip irrigation system is right for your particular situation.
Once you are certain a sub-surface drip irrigation system is what you want or need consider the following.
What are my soil properties in relation to lateral movement of water?
This is the key determinate for lateral tape spacings. Sandier soils require closer spacing which increases the capital cost. Wider spacings are possible with heavier soils as lateral movement of water is greater in these soils. If laterals need to be close together (less than 0.8 metres) you should ask whether sub-surface drip irrigation is the right system for your soil type.
What crop type and rotation am I going to adopt?
Germination of crops, particularly in sandier soils, needs to be considered along with the economics of growing that crop/rotation with a high capital cost technology. Getting good establishment of crops relying solely on sub-surface drip irrigation can be high risk. This risk can be reduced by ensuring sowing can be done at times when rainfall is more likely.
Lucerne reduces this risk because it can be sown anytime of the year (ie take advantage of wetter times to achieve germination) versus something like maize which will need to receive adequate moisture (rain or irrigation) to achieve germination at a drier time of the year.
Sub-surface drip irrigation is a high capital cost technology. To ensure you get the most out of the system, give appropriate consideration to long term rotation of crops you hope to grow. For example, high value crops such as tomatoes can be grown followed by double cropping on a long term rotation. If lucerne is to be grown it is important that the yield and water savings possible with this type of system are achieved. This will require you to run the system at its optimum to help ensure the highest possible returns.
What are the design considerations?
What is the shape of the block?
This determines the location and size of sub main/s and how flexible the management of the system will be, ultimately allowing the cost to be reduced if considered properly. Simple square or rectangle shaped blocks allow similar sized sections to be designed therefore reducing cost and improving flexibility of management. Odd shapes suit this type of technology also but the options for design will be greater and can impact capital cost along with longer term O&M. Flexibility of management could also be compromised if all options are not explored properly.
What is the run length of lateral dripper tape?
Best results will be achieved if runs are less than 300 to 400 meters. It may be possible to go longer but you need to understand the consequences of this and be confident the system will operate efficiently and effectively. Pressure compensating emitters may help if lateral dripper tape distances are excessive (longer than 300 to 400 metres) (see separate section).
What is the topography of the block?
Sub-surface drip irrigation is well suited to undulating country but pressure compensating emitters may be required.
Where is the existing infrastructure?
It seems obvious but it's important that you think about access to power and the irrigation supply. Diesel can be an appropriate option if your crop rotation requires you to move sites regularly (ie tomato growers have "pumping sets" they pick up and move). For more permanent systems electricity should be seriously considered.
What about expansion?
If you are going to add to the system in the future then this needs to be considered at the design stage to ensure it can be expanded without significant changes to the system and additional cost.
Do I have a reliable source of water?
What is the source of and quality of the water to be used? This impacts the filtration design (discussed earlier). Water supplies that have heavy silt loads or particles (ie sand) will require greater filtration. Other things such as algae will need to be considered along with dissolved ions. If algae is in your supply for a considerable time throughout the year then it will be an issue that will need to be addressed regularly. Emitters may become blocked and regular chemical treatment will be required.
You should plan to use the system every year. This is not the type of irrigation system that can be moth balled without serious consequences. It needs to be used to reduce maintenance requirements and ensure it is maintained in an operational state. Insect problems and root intrusion can be a significant problem if the system is not consistently used. In addition there are potential water use and productivity benefits that should be taken advantage of. It is a capital intensive system so you need to gain a return from it. You should have a good understanding of what your water budget is and the minimum requirement to run the system each year should be achievable.
Do I know how to take full advantage of the system benefits?
If you plan to run the system like a border check irrigation system then sub-surface drip irrigation is probably not the system for you. There are other advantages that will help maximise the investment and your returns such as fertigation, more frequent irrigation to maintain the plant in the optimum moisture state, full automation of the system using sensor and alarm technology and flow meters.
Do I understand the importance of installation?
Proper installation of the system is critical to maximizing the system benefits and ensuring it performs at its optimum long term. There are a number of considerations (following) that you need to discuss with the installation company/contractor to ensure they understand what is required.
Field preparation is critical. Ripping the paddock adequately to at least 50mm below the tape depth is a minimum requirement. This will help avoid bones (hard areas under the surface that may effect the tape). Cross ripping is important as well to avoid this. After ripping smudge and harrow to reduce clods.
Tape should be laid with GPS, this is highly recommended. This allows accurate identification of where tape is for crop management later. This is particularly important if you are going to be rotating crops from beds to broadacre. It also ensures the tapes are evenly spaced. Depth of laterals must be consistent across the block as well.
Think about tracks, head lands and access at the design phase. Avoid situations where the ends of lateral lines need to be manually installed. Often these areas are close to fences and the risk is the system will be damaged as machinery and implements use these areas for a range of activities that could damage the system. These areas should be designed as access tracks.
The start and end of all laterals must be as deep or deeper than the rest to avoid pick up damage by implements when working fields post installation.
Consider the quality of fittings when connecting to sub mains. It is important to use quality fittings especially with modern crop rotations and having the irrigation system in the ground and operational for as long as possible.
Have I adequately considered the other general considerations?
There are a range of other general considerations that do not specifically fit within a particular heading but require appropriate consideration. These include:
Consider the clogging resistance, coefficient of variation and emitter exponent. The larger the passage and higher the flow rate the less clogging. The coefficient of variation relates to the variation in flow rate between the best and worst emitter and should be as small as possible. This is the critical component of irrigation distribution uniformity and effects how consistently your crop is irrigated across your system. The emitter exponent is a measure of the sensitivity of emitters to pressure variation. A lower emitter exponent is more important where there are large variations in pressure due to undulating ground or long lateral tape runs.
Pressure compensating emitters
These ensure that a consistent flow rate is deployed from each dripper with differences in pressure across or along a dripper tape. This is especially relevant on sloping land where the pressure will be greater at emitters lower in the profile. They add significantly to the capital cost of a system (and to a lesser extent the operation costs) therefore you need to ensure they are required on your property. They may also be useful in situations where dripper tape lengths are high (greater than 300 to 400 metres). This would need to be considered in conjunction with the design to ensure savings are made elsewhere (ie reduced sub mains) to warrant the additional cost of pressure compensating emitters.
Emitter flow rate
This will vary depending on crop water demand and the soil water holding capacity. Lower flow rates are more conducive to lateral movement of water. 1 litre per hour is considered the most appropriate flow rate for most situations. As dripper flow rate decreases problems can arise with emitters blocking. O&M may also increase because longer run times will be needed to deliver the required volume of water.
This is a trade off between longevity and robustness of product versus capital cost. The thicker the wall the greater the cost. Longer term installations on cropping rotations generally better match thicker walled tape. Shorter term crops such a tomatoes can justify thinner walled tape.
Irrigators need to be able to calculate run times for the various crops they may grow. One way to do this is by using evapo-transpiration (ET) and a crop factor to calculate water use in millimeters. For example:
Flow rate (litres/hr)/lateral spacing x emitter spacing = application rate
|1 (ltr/hr)/1(m) x 0.5(m) = 2mm/hr|
Using the above example if ET was 30mm you would need 15 hours of run time (30/2=15) to deliver the required moisture. How that 30mm is replaced will be dependant on agronomic aspects (ie root zone depth) and soil type. For example in the Shepparton area (heavier soils) you might replace the 30mm with one irrigation versus Mildura (very light sands) where you might do 3 separate 10mm pulses across the day. You may want to take advantage of off peak power and this may influence how it is applied.
Flushing of the system is the main and most important consideration. This involves opening the ends of the system and using an appropriate velocity to flush sediment and algae build up from the sub-mains and tapes. This is a critical component of the system design, if it is not done properly then the system may never flush properly.
Chemical treatment (chlorine or acid) will probably be required at some point. If inorganic (ie sediment or colloidal) material is present then acid should be used. If organic (ie algae) material is present then chlorine would be used. You can easily identify between inorganic and organic material by the color and appearance of the material coming out of the system.
General pump maintenance
Periodic maintenance of the pumping equipment will be required from time to time to ensure the system is in good order therefore reducing the incidence of breakdowns.
Leaks and holes
From time to time leaks or holes may occur. These can usually be repaired easily. The design of the system and ensuring it is consistently used and appropriate maintenance is performed will help reduce the incidence of leaks and holes. It also ensures your crop receives the moisture required.
Depending on the quality of the water being used automatic filters will need to be manually cleaned. This will ensure the quality of water being delivered through the system is high and will reduce running costs by ensuring pumps do not have to work harder than necessary.
Diesel or electric motor
Often there is no choice because it is too costly to connect power but if you have an option there are some subjective and economic considerations. To help make this decision ask your designer to provide an estimate of the pumping costs for both options and what the capital costs would be. Depending on the volume of water you plan to pump each year and the availability of electricity it may be attractive to install a diesel motor. Also consider such things as refueling diesels, bunding around fuel tanks, automation, safety, greenhouse impacts, operating life and general maintenance requirements.
What ongoing operation cost per ML am I comfortable with?
System flushing manifold valves
A relationship exists between pipe size, pump efficiency, head loss and energy costs that will impact the cost to pump a ML of water for the life of the system. If you get this wrong the overall cost (capital expenditure plus operation and maintenance) will be significantly higher than it needs to be. You should always aim to design a system that delivers the required flow whilst minimising energy requirements. This ensures the system remains as economically efficient as possible for the life of the equipment.
Irrigation Survey and Designer Group
Dennis Moon private irrigator with significant experience in Subsurface Drip irrigation.