International Irrigation Award

Background

Gary and Julie Davis are well known within the macadamia industry as growers of premium macadamias on their property at Donnybrook. I met Gary and Julie while doing irrigation design work on the macadamia farms of James and Aimee Thomas. Aimee is their daughter.

Late 2016, Gary rang me and said that, with his son Mitchell, they had bought a macadamia farm on Gates Road, Canina just out of Gympie. It was irrigated and had an old pump, and could I identify it. It was an ancient Regent RM15, probably 25 – 30 years old and certainly had had a long working life.

I visited the farm and after inspection and discussions, it was decided that the sub-mains, laterals and sprinklers needed to be re-designed and replaced. Further investigation brought to light the DN80 PVC main was in fact a DN65 PVC main. This too would need to be replaced, however the pump would remain.

Water supply for the farm is a 30 megalitre dam so the irrigation design would be for a supplementary system only. This was an adequate system for the 15.8 hectares of macadamias.

On James and Aimee’s irrigation project, I suggested to them to change the system from one sprinkler to two sprinklers in between the trees. This would improve the Co-efficient of Uniformity, the Distribution Uniformity and the Scheduling Co-efficient of this sprinkler placement in comparison to only one. Having seen the difference in the single sprinkler and dual sprinkler wetting patterns, Mitchell was in a good position to see the difference between the differing distribution patterns and chose the 2 x 28 per low throw sprinkler option. These lower rate sprinklers were better for his clay soil types as they required a low infiltration rate to prevent pooling and water running down the slopes. Also, I suggested to Mitchell to install a local zone fertigation system which allows each zone to have an injection point at each station.

The property is steeply undulating with contours ranging from 100 metres to 160 metres. The macadamia rows run down the slopes at 90 degrees to the contour lines. The challenge in this design was to accurately size sub-mains and lateral lines in the steep topography so that the operating pressures stayed within the pressure limits of both the Low-Density Polyethylene tube and the optimum operating range of the pressure regulating micro sprinklers. The topography prohibited the use of non-compensating sprinklers.

Sub-main sizing on these slopes telescoped down from DN80 PVC to DN25 PVC and lateral tubing from 25mm LDPE down to 10mm LDPE.

If one aspect of this design had to be named as critical to the successful operation of the design, it would have to be the decision to use of the 10mm LDPE. No other aspect of the design caused as much heartache and discussion as the inclusion of this pipe. In the course of discussing the project with supply companies, Mitchell enquired about using 10mm LDPE and was told:

With both of us sitting around the computer doing the hydraulic modelling it was obvious, we had to use it or completely re-design and re-route the mainlines. This would also mean sub-division of the existing irrigation zones into smaller zones to offset the slopes.

In the event the 10mm tube was available, we could get fittings and we did use it.

It has proven to be the decision that has made this system operate within the limitations of the pipe pressure rating and the sprinkler operating pressure range.

The owners chose to install the new system with staged site visits by JP WATER. The installation had to be carried out around normal farm operations and with at times, inclement weather. As is all installation work, it was long and tedious.

Then came the day, the button was to be pushed. Would the old pump do it? Would the use of the 10mm LDPE be justified?

All worked as designed, pressures were right, but the old pump struggled. Finally, it gave up the ghost and a new one was installed.

The Award

JP WATER P/L is a member of the American Society of Irrigation Consultants, a worldwide body based in East Lansing, Michigan, USA. Every year the Society present awards for outstanding and unique designs.

As a member of ASIC, Jim Phillips asked Mitchell, Gary and Julie if they would allow their design to be entered for consideration by ASIC’s panel of American irrigation designers. The panel would assess the design from a professional designer’s perspective and discuss with Gary, Julie and Mitchell over the phone, the design and the relationship between them as clients and JP WATER. It also meant that JP WATER had to send to them all aspects of the design for assessment by the panel.

The design was presented to the international body. We were advised that GJ&M Nuts design had been awarded their highest award, that of the Honour Award. We were invited to San Diego for the presentation but, unfortunately, none of us were available to go, this year.

The positive outcome of this design and its award highlights the importance of close collaboration between an irrigation designer and the grower. While the designer can apply his or her knowledge of hydraulics, pumps, soils and climate, it is of limited use if it is not applied with the intimate knowledge a grower has of his or her farm and crop. A design must be part of the overall management strategy of the farm and complement the day to day operation, not hinder it.

In this instance, Mitchell did the research for the sprinkler he wanted, and became involved in the decisions affecting the hydraulic design of his system. This practical involvement in the design process means that he has a working understanding of the potential and the limitations of his system and how to manage it.

From my perspective as the designer, it meant that I was not offering Mitchell a fait accompli based only on my decisions, but a system that drew on the knowledge and experience of both of us.

It was a rare pleasure for me to work with Mitchell, Gary and Julie on this design and then see it go on to win this Award.

Read the Submission

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