Over the past few years many growers in southern NSW have moved to overhead irrigation in an attempt to increase their water use efficiency.

Unfortunately many systems are costing up to twice as much to run as they should, simply because of poor design.

This has occurred due to several factors:

• Most machines being sold are American, and American design criteria do not suit Australian conditions and needs,
• A lack of engineering knowledge to reduce running costs among growers and some resellers,
• Irrigation requirements in southern winter-spring rainfall areas, particularly for summer crops and pastures, are quite high,
• The lean times experienced over the past few years combined with a lack of knowledge have led buyers to go for the cheapest capital cost option.

Before buying a system it is vital that you have it designed for minimum running costs.

A relatively small increase in capital cost can lead to savings in running costs which last for the life of the machine.

This can amount to many thousands of dollars.

The main component of running costs is friction (measured as pressure) which increases dramatically as the velocity (metres per second) of water in pipes increases.

The most common design problem is under sizing pipes to reduce capital cost.

Only slightly smaller diameter pipes mean large velocity increases requiring larger power units to pump water through the system and much higher energy costs.

NSW DPI’s Primefact 98, “Centre pivot irrigation in the Riverina” shows how the extra initial capital cost of increasing mainline pipe size to halve friction load can be paid for in savings made over a little more than two years.

The National Centre for Engineering in Agriculture and the Cotton Research and Development Corporation’s “Centre Pivot and Lateral Move – Planning and Design for informed purchase” shows that an increase in capital cost of 6.3 per cent incurred by increasing pipe sizes in the inner two-thirds of the machine spans can reduce friction load to half.

Every aspect of the design must be matched to requirements.

It is extremely costly and usually impossible to rectify mistakes or change individual parts after the machine has been assembled, paid for and commissioned.

A supplier-designer should be able to give a range of capital and running cost options for your proposed system – if not, you need to go elsewhere or seek independent expert advice.

For more information and help in assessing your design contact NSW DPI, Deniliquin.

Contact Graham Barron, 03 5881 9918.

- EDDIE PARR

This story appears in Agriculture Today.