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What do tsunamis and spinner cuts have in common?

Picture: Mike Morris, DEDJTR irrrigation researcher speaking about spinner cuts with the Humphris PIT team.

They can be both modelled using new research that the Department of Economic Development, Jobs, Transport and Resources (DEDJTR) are using to improve the precision of border check irrigation.

The Humphris PIT heard about this and the importance of good drainage at the second meeting for the Accelerating Change program. At the farm of Tim and Lyndal Humphris, of Tongala, discussion focused on the use of spinner cuts and how they can reduce the duration of transient waterlogging.

Now spinner cuts may not be the first thing that comes to mind in a project focusing on new technology! They have been around for a long time and have been tried and discarded by many farmers in the region, including Tim and Lyndal. However upgrading of infrastructure meaning faster flows on farm and new science which validates the benefits of them are bringing them back into focus. Paul Price, a member of the Humphris PIT who has been involved with drainage research with Mike Morris and his team at DEDJTR persuaded Tim to have another look at them.

Mike told the group that conventional irrigation bays are inherently imprecise because surface drainage from them is so much slower than the rate of wetting. “Its usual for ‘water off’ to take about ten times longer than ‘water on’ takes”. This means that areas within a bay will be under water for much longer than other areas.

With funding from Dairy Australia, Mike and his team have adapted a computer model originally developed to simulate tsunamis and have used it to simulate the flow of surface water on irrigation bays. What sets this model apart from other irrigation models is its ability to simulate water flows on and off irregular bay surfaces. Early simulations of irrigation water flow compared identical irrigations on three different bay surfaces – firstly a bay with a hypothetical, perfectly smooth surface, then the same bay with its surveyed elevation surface and finally, the surveyed bay surface with shallow spinner cuts installed down the length of the bay.

The results showed that the spinner cut surface reduced the duration of ponding and improved the uniformity of the irrigation when compared to the surveyed surface and, surprisingly, also when compared to the perfectly smooth surface. Spinner cuts not only reduced the total maximum time of ponding but also reduced variation in the duration of ponding on different parts of the bay. This happened because the spinner cuts drained the bay more quickly, so drainage happened at a rate closer to the rate that water was applied. Measurements taken at Paul’s farm this year on an irrigation bay before and after the installation of shallow spinner cuts have confirmed this in the field.

The degree of performance improvement depends on aspects that include the current condition of the bay and pasture, and the bay slope and length. Bays with spinner cuts installed will have increased volumes and rates of runoff, so surface drainage and reuse systems have to be in good condition. With shallow spinner cuts and well timed cut-off, runoff volumes need not be excessive, but will include water previously lost to evaporation and deep drainage.

Reducing the duration of surface water ponding should reduce stress on pastures and discourage shallower roots. Reducing the variation of ponding times within bays could allow more precise scheduling of irrigations, with the potential for greater production per megalitre. While shallow spinner cuts have been shown to improve bay surface drainage and irrigation performance, Mike and his team are now using the model to search for bay surface designs that can deliver these benefits without the cost and inconvenience of installing and maintaining long spinner cuts within bays. Over the next 2 years, Mike’s project team will install two new bay designs developed with the model and test them on dairy farms in the region.

On the Humphris farm, Paul knew that Tim’s old spinner cuts were not executed in a way that Tim would receive full benefit, and low flow rates on the particular part of the farm they were on would mean they would not be as effective. Paul assisted Tim to install shallow spinner cuts according to his practice which looked very different to the old ones Tim had on his farm.

Cameron Smith spoke about the potential economics of spinner cuts. The literature suggests that waterlogging causes penalties of up to 17% in pasture growth. By improving drainage and reducing ponding from 12-14 hours down to 4 hours you can achieve an extra 2.9 t/ha/year which roughly translates into around $33,000 worth of pasture.

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