Irrigation performance of centre pivot end-guns operating in windy conditions

Irrigation performance of centre pivot end-guns operating in windy conditions

Sprinkler irrigated areas are increasing rapidly around Australia and the world. Wind is the main factor that affects sprinkler irrigation performance of all systems, especially those sprinklers that throw into higher elevations above the ground or crop surface. Two sprinkler irrigation systems that are most prone to being impacted by wind are travelling big gun irrigation machines, and end-guns on centre pivot machines.

A lot of research has been conducted to identify the effect of wind for the large impact sprinklers, for solid set sprinkler systems, and movable sprinkler system, using different approaches. The TravGUN irrigation model was developed to simulate big-gun irrigation performance, and has the capability to calculate the radial leg and calibrate the model, from three measured sets of data, one in quiescent wind, and two in different wind speed and directions, and then simulate the wind effect for any wind speed and direction, for a limited combination of wetted sector and side sector angles.

End-guns are a large impact sprinkler which are commonly attached on the distal end of the centre pivot machine to extend the irrigated radius by up to 50 metres. The end-gun sprinkler configurations are very similar to travelling big guns. But, there are two main differences between them. The first difference is that the end-gun side sector angle is perpendicular and asymmetrical to the direction of travel, while big guns have side sector angles parallel to the direction of travel. The second difference is that end-gun on centre pivot travels on curved path, while big guns travel on straight paths.

The literature review addressed low irrigation performance under end-gun irrigated areas, comparing to the irrigated area under regular sprinklers on the centre pivot machine. In addition, there is a lack of studies that investigate the end-gun performance as a separate pattern from the regular sprinkler pattern on the centre pivot under different windy conditions. Therefore, the research problem that is addressed in this dissertation, aims to develop a new methodology and software tools in order to simulate end-gun performance under different wind speed and directions. The special focus is on a new mathematical model and a new methodology for data collection, to overcome research limitations and answer the research question: Does TravGUN have the capability to simulate end-gun performance in windy conditions?

Six research objectives have been identified to achieve the main aim of this dissertation:

- First, there is a need to develop a new mathematical model (for inclusion in TravGUN V3.1) to produce radial leg data for end-gun and big gun sprinklers from transect data of any combination of wetted sector and side sector angles.
- Second, there is a need to develop a new methodology for data collection for an end-gun on centre pivots under different wind conditions.
- Third, any new model should be validated by comparing simulated radial leg data for different transect data sets from a range of wetted sector and side sector angles, with measured radial leg data of the same configuration.
- Fourth, simulate end-gun patterns under a constant wind speed and direction for the eight cardinal points of the compass, to represent a full irrigated rotation for all end-gun travel directions on a centre pivot machine, and calculate the application uniformity at each of these eight end-gun travel directions.
- Fifth, the effect of the curved movement on the simulated water depth in the transect data, need to be corrected according to the value of the centre pivot radius.
- Finally, the combined and overlapped normalised end-gun and regular sprinkler patterns from zero wind conditions should be analysed, to evaluate the irrigation performance of both patterns. Then the combined results from eight simulated and corrected end-gun patterns under constant wind speed and direction with eight simulated and normalised regular sprinkler patterns for a centre pivot in the same wind conditions can be simulated, in order to calculate the application uniformity for the entire irrigation system at each the eight cardinal compass points.

The novelty of research lies in developing a new version of TravGUN software to simulate end-gun pattern in different wind conditions. The study concluded that the empirical simulation result under zero wind speed from TravGUN V3.1 for the combination of a wetted sector angle of 130° and side sector angle of 60°, provides the optimum transect for an end-gun on a centre pivot. These simulation results for a centre pivot operating with an end-gun at the eight cardinal positions of the compass under constant speed and direction revealed that the application patterns were distorted, the length of the transect was greatly shortened, and the coefficient of uniformity decreased when the end-gun was travelling crosswind, and upwind. The simulated results adjusted, by the correction factor for curved movement, revealed that the applied water depth decreased. This depth decrease depended on the radius of the centre pivot. The results of the correction factor for three different pivot radii revealed that the curved movement of pivots with radii equal to and less than 400 m have a significant effect, of up to 8.8 % decrease, in the applied water depths