Downfalls of a Long Irrigator

Up until the early 2000’s when dairy conversions were a plenty, the centre pivot irrigators that were being installed seemed to be getting longer and longer. It seemed that the longer the pivot you could put on, the cheaper the per ha cost of the system became. However, several studies more recently carried out have brought to light the negative effects and inefficiencies of some poorly designed long pivot irrigators.

A standard pivot in the USA is a quarter mile long (approx. 400m) which enables it to irrigate a half mile by half mile section, the development of centre pivot technology has primarily been based on these standard specifications. Akin to the common Kiwi approach of creating innovative solutions to suit our farming conditions Kiwi’s went about lengthening these standard pivot designs. It didn’t take long though for farmers and irrigation installers to realise that there was a major trade-off between system length and the instantaneous application rate of water going on out towards the end of the irrigator. The problem being that the further out you get the faster the machine is travelling, therefore the higher the instantaneous application rate needs to be. This introduces major issues including crop and soil damage and surface ponding. With surface ponding comes runoff, bypass flow and leaching – all things that should be avoided!

Alarm bells should start ringing when you start talking about pivots longer than 660m. Now, a pivot longer than 660m isn’t necessarily bad, but you need to consider the design carefully to ensure that you are irrigating within tolerances of the soil and farming system. The Irrigation New Zealand Design Code Of Practice has some great information about what soil features should be considered during the design process.

In a nutshell, when looking at the length of an irrigator, you need to consider:

1. Soil type
2. The infiltration rate of that soil
3. And the maximum instantaneous application rate of the pivot

The latter should not exceed the infiltration rate of the soil as otherwise the negative outcomes previously mentioned are likely to occur. There are also other properties such as the water holding capacity and the effective crop root depth that should be factored into the design to ensure that you are getting enough water to the plant, but not over-watering and causing problems.

The other issue is engineering. A longer, higher discharge rate machine must have bigger pipes which means more weight and stress on motors and gearboxes (and soil). And also all the water has to be pressurised to meet the requirements of only a little of the water right at the end of the machine - which is not energy efficient. A short machine can cover the extra capital cost per hectare very quickly by reducing operating costs, obviously each situation is different, but it may be worth getting your irrigation dealer to help you do some figures.

If you do happen to have a very long pivot, you may be experiencing some of the problems mentioned inherent with the design. If you are seeing some of these issues, there are a few steps you can take to help improve the performance of your system.

1. Increase the wetted footprint of your sprinklers, effectively lowering the instantaneous application rate over any particular point. You can do this by alternately attaching sprinkler drops to the forward and rear truss rods, changing the sprinkler heads on your machine to sprinklers with a larger throw, or putting tees in and increasing the number of sprinklers while still slinging them over truss rods. This will only go so far towards solving the problem but it may help.
2. Using Variable Rate Irrigation could help depending on the variation in your soils and your topography. By knowing more about the physical constraints beneath your irrigator, you can then use Precision VRI to better manage to those constraints. Agri Optics can help with their EM Mapping and Data Management services.
3. And the most expensive capital-wise, but potentially the most effective and highest potential for reducing operating costs is to decrease the length of your pivot. This may mean reconfiguring pivots on your farm to still irrigate the greatest area. There is a significant capital cost, but if you do the numbers as some research studies have shown, it could well be worth it to increase your irrigation efficiency.

The one thing in common with all of these approaches is that you need to know the characteristics of your soils (including the variability in soils) and then design your system to work within the constraints.

But there is no substitute for a well-designed system. For well-designed irrigation solutions or to discuss how you could improve the efficiency of your centre pivot contact your local Zimmatic dealer.

We hope you find the information in this post useful. The information was supplied by Stu Bradbury who has many years of experience creating irrigation solutions from the ground up so to speak. The ideas began to spark while spinning spanners and installing irrigators in the early 2000's. He has gained a wealth of knowledge through numerous research studies and much trial and error over the years. Stu's current role is VRI Solutions Advisor for Lindsay NZ.

Stu Bradbury working on a Precision VRI system on a centre pivot