Farm advice: Variable Rate Irrigation systems


Ignoring the nuances of VRI systems can lead to inaccurate evaluations, potential mismanagement of water resources, and unnecessary modifications

Variable Rate Irrigation (VRI) systems have added another tool to the toolbox in agricultural water management, offering precise control over water application to optimise crop yield and water use efficiency.

Irrigation system design companies can work with farmers on a VRI solution that can be used to achieve all sorts of valuable outcomes, such as avoiding sensitive areas while optimising water flow through the remainder of the machine. Any testing must, therefore, take into account what the system is designed to do with the VRI turned on.

Assessing the performance of pivot irrigators requires specialised knowledge and attention to detail to ensure accurate results. In this regard, IrrigationNZ highlights the critical need for specialist knowledge before undertaking a bucket performance test for a VRI pivot irrigator.

VRI systems demand a thorough understanding of their unique components and operating principles to conduct meaningful performance assessments. Ignoring the nuances of VRI systems can lead to inaccurate evaluations, potential mismanagement of water resources, and unnecessary modifications.

IrrigationNZ continues to assert that only qualified personnel, holding the NZ Certificate of Irrigation System Performance Assessment, should undertake performance assessments for VRI systems and work in alignment with the Code of Practice for Performance Assessment.

A fundamental aspect of testing VRI pivot irrigators is the necessity to assess them with the VRI function activated, utilising an appropriate application plan. In most cases, this plan should reflect a rate significantly higher than the 100% application rate to ensure adequate coverage and uniformity is possible. Failure to do so can result in improper system evaluation reporting.

Assessments should not proceed without the presence of the farmer or farm manager responsible for the system,
as their input and oversight are invaluable in ensuring the accuracy and relevance of the results.

No assessor should change any panel settings without the appropriate supervision, instruction, or knowledge of how that specific system is designed to operate. Having access to and understanding the system commissioning report and sprinkler package schedule will help that decision.


The performance assessor will also need to determine the optimal bucket placement based on the intention of the VRI control system.

If the test is being conducted with the system operating with a variable rate pattern, then assessors need to take that into account in their assessment and interpret bucket data based on bucket position and expected application depths. DU cannot be assessed where the system is not designed to operate uniformly, for instance, where identified highly variable soil patterns across the irrigated area have been accommodated in the design.

If the VRI system has an application plan where the application depth can be applied uniformly, then the system should be evaluated with that plan activated, and DU can be assessed.

If the VRI system is only switching sections off to keep laneways dry, then the system should be evaluated as such, ideally in a typical irrigated field section.

If any buckets have been placed in the exclusion (dry) zone, this should be noted to aid data interpretation later.
Adhering to recommended assessment frequencies outlined in commissioning reports or manufacturers’ manuals is crucial for maintaining system integrity and compliance.

To reiterate, simply switching VRI systems “off” before testing is not part of the IrrigationNZ Code of Practice for Irrigation System Performance Assessment.

Stephen McNally

Without going into too much detail, a feature of VRI system design is the sizing of nozzles. VRI systems often employ oversized nozzles to compensate for inherent distribution uniformity fluctuation from the rapid switching sequences, but this helps ensure consistent performance across the field. Assessors must be aware of these design considerations, emphasising that you should see this in a commissioning report, and account for them during testing to obtain reliable results. Like all systems, obtaining the design sprinkler chart is critical if an assessor wants to challenge or assert
after testing that maybe wrong components are present.

Issues also arise when assessors overlook essential checks and observations that are part of the Code
of Practice for Performance Assessment, such as the status of maintenance, flow rates, pressures, and system speeds. These observations and recordings should be compared to the designer’s commissioning KPIs. Overlooking or not understanding this detail can lead to incomplete or inaccurate assessments, posing problems and confusion for farmers, irrigation supply companies, and regulators looking for consistency in FEP audit outcomes.

In conclusion, the evaluation of VRI pivot irrigators demands specialised knowledge and meticulous attention to detail. By adhering to recommended procedures and considering the unique characteristics of VRI systems, assessors can provide accurate assessments that empower farmers to optimise water use and enhance crop productivity. Through rigorous and informed evaluation, the full potential of VRI technology be realised in sustainable agricultural practices.

Images supplied by Adobe Stock

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