Irrigation system assessments

The Department of Primary Industries and Regional Development (DPIRD) recently engaged Irrigation Australia to manage a project to conduct free system assessments of horticultural irrigation systems in the Carabooda, Nowergup and Wanneroo districts north of Perth, WA. A total of 22 system assessments were conducted by Willmott Irrigation on behalf of Irrigation Australia. Of the systems tested, 10 used drip irrigation, while 11 used overhead sprinklers and one system used under-tree mini sprinklers.

Each assessment consisted of gathering key information and performing tests on a grower’s irrigation system. The assessed system components could be broadly divided into one of two categories; the pumping system, and the delivery system.

Pumping system The pump is a major component of any irrigation system. A pump that runs efficiently reduces costs to the grower by using less energy and requiring less maintenance. Any pumping system operating at less than 50% efficiency is considered poor and should be investigated.

Delivery system The best way to test the efficiency of a delivery system is to assess the application uniformity, which describes how evenly an irrigation system applies water over the area being irrigated. Application uniformity was measured using the distribution uniformity (DU) method where a higher percentage indicates greater evenness between the depths of water applied across the field. The minimum industry standard for DU of drip irrigation systems is 85%, while the minimum standard for overhead systems is 75%. Other tests included pressure variation along mainlines and laterals, pressure differential across filters (if applicable), and flow variation between emitters.

Results Assessment results were provided to growers in a ‘traffic light’ format that rated the performance of major system components as; Good, Acceptable, or Poor. These ratings indicate whether the performance exceeded, met, or undershot the industry minimum standard.

· Drip systems had an average pump efficiency of 61%, resulting in ratings of either ‘Acceptable’ or ‘Good’.

· Overhead systems had an average pump efficiency of 55%, which is within the normal operating range. However three individual overhead systems had pump efficiencies below 50% resulting in a rating of ‘Poor’.

· Drip systems had an average uniformity score of 91%, which exceeds the minimum industry standard of 85%. This resulted in all the drip systems receiving a uniformity rating of either ‘Acceptable’ or ‘Good’.

· Overhead systems had an average uniformity score of 70%, which is below the minimum industry standard of 75%. This resulted in more than half the overhead systems receiving a uniformity rating of ‘Poor’.

Unlike drip systems, uniformity tests on overhead systems are highly impacted by wind conditions. This makes one-off tests somewhat unreliable because a system may score significantly higher or lower depending on the wind speed

Above: A typical market garden in the Wanneroo area. Conducting tests and inspection of pumping system

amount of over watering required to ensure the crop water requirement is met for the whole crop.No assessed systems had the capacity to regulate pressure at the valves. Pressure regulation can significantly improve system efficiency by limiting pressure fluctuation that can reduce uniformity. With increasing emphasis on water and energy conservation, many systems are being operated at lower pressures. As a result, pressure variations have become a larger percentage of the total, requiring more precise regulation.

Above: Distribution uniformity and pump efficiency scores for drip and overhead systems

and direction at the time of the test. However, the average wind speed in Wanneroo during summer is 11km/hr, while the average maximum wind speed is 45km/hr (DPIRD weather station data). This means that significant wind speeds should be considered the norm and systems should be designed and operated to account for these conditions.

Conclusions It was not within the scope of this project to provide recommendations to individual growers. However we can make some general recommendations for improving systems based on our observations of the systems that were assessed.

The spacing of sprinklers in overhead systems was often too wide resulting in poor irrigation uniformity, particularly in windy conditions. A high level of application uniformity is one of the key areas for increasing water use efficiency as it reduces the None of the pumping systems assessed used variable frequency drives (VFDs). Combining the strength of submersible pumps with the efficiency of variable frequency drives creates a tremendous opportunity for pump users to reduce energy costs, improve performance, regulate the output pressure at the pump and extend equipment life.

None of the systems assessed incorporated evaporationbased irrigation scheduling in their management plan. Evaporation-based scheduling is part of a systematic approach to efficient irrigation that considers plant and environmental factors to determine crop water requirements. Matching irrigation to crop demand ensures that water is delivered when required and does not exceed the storage capacity of the soil. This method works best when combined with soil moisture monitoring to confirm irrigation effectiveness.