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Making the most from Surface Irrigation

By Adrian Smith, Senior Land Services Officer - Mixed Farming Systems

With the 2018-19 irrigation season just in front of us, it is timely to look at how the performance of irrigation layouts can be enhanced while avoiding some mistakes of the past. With some concerns over the volume of general security allocations, it is important to ensure your surface irrigation systems operate as efficiently as possible.

Well planned, constructed and maintained surface irrigation systems, constructed on the right soils, can result in high levels of efficiency and production.

With surface irrigation, there are many factors that influence how well a particular layout works. Your channels need to have enough capacity to handle the flows you have (or want). With higher flow rates, new farm outlets, and the addition of water from other sources (such as recycle systems), you may be dealing with flows of well above 20 megalitres (ML) per day.

Channels need to have adequate capacity to handle these flows; they should be built with a minimum of 3-metre bed widths. Driving around the district, it is frustrating to see undersized channels being built, with weeds restricting flows, water lapping the top of banks and flows of only 4 or 5 ML/day. This is simply not good practice – and what’s more, it’s costing you money!

Structures such as pipes and checks are necessary to allow access and to control water movement. They can also be a significant cost. If they are undersized or installed incorrectly, they can be a source of significant headloss, which can ultimately affect the flows you can achieve and the area (and speed) that can be irrigated.

It is essential that structures are correctly sized and installed. The ‘bargain-priced’ pipes you got at the clearing sale down the road, which you rolled off the truck into your channel, are only going to result in a headache! Designers should identify levels on your design to which structures should be installed – if they don’t, ask them to. If they still won’t, go elsewhere. Importantly, when you do install them, make sure they are put in at the correct level.

Drains need to be constructed with adequate capacity to remove irrigation and rainfall runoff, which can be large on steep, landformed paddocks.

When it comes to getting irrigation water on and off your paddocks, there are a couple of ‘rules of thumb’:
  1. The flow rate through your bay outlet drives how fast water gets onto the bay.
  2. The grade (or slope) of the bay surface, and the distance to a drain (and the ‘roughness’ of the paddock) drives how fast water gets off the bay surface.
The quicker the water gets on and off, the less waterlogging (and so more plant growth) and the less potential there is to lose water through evaporation or deep drainage.

The key messages are:

  • install high-capacity outlets
  • maximise your supply flow rates
  • don’t have excessively long (or big) bays
  • aim for steeper grades.

There are many options worth considering, such as spinner cuts on bordercheck fields, ‘V-bays’ and larger capacity outlets to improve watering and drainage times. The key is to consider your options carefully before you start buying the pipes or shifting the dirt!

Of course, there are always going to be compromises - budget, natural slope, the amount of topsoiling you are prepared to accept, the crops you want to grow, the capacity of your supply and drainage systems and, importantly, the amount of time you or your workers can (or want to) devote to actually irrigating. But remember, while a quality job may cost a bit more upfront, the benefits will be evident over the next 15 or 20 years. Quality will stand the test of time!

These basic principles are not new – good designers and irrigators have been doing these things for many years. We do, however, continue to refine our irrigation systems. Take a drive around and look closely at those layouts that work well, and talk to the irrigator who uses them. You will quickly get a feel for what works and what doesn’t, and the reasons why!

For more information, or to discuss your individual requirements or circumstances, talk with your irrigation designer, or contact Adrian Smith, Senior Land Services Officer - Mixed Farming Systems at Murray Local Land Services on 03 5880 1412 or 0447 778 515.

Adrian Smith (right) speaks with rice farmer Josh Small. Good planning is essential for effective irrigation.