Test and optimization of the water distribution characteristics for end gun combination on circular sprinkler irrigation machine

End guns (also called sprinklers) have been typically used to expand the irrigation area under center pivot irrigation systems, particularly in the four corners of square fields. The irrigation quality can also depend mainly on the irrigation uniformity of end guns that are installed at the end of the overhang. Therefore, it is very necessary to match the length and inlet flow rate of the irrigation system during end gun selection and configuration. This study aims to optimize the water distribution for the end gun combination on the center pivot irrigation systems, in order to improve the irrigation quality and the overall performance. The test platform was taken as the commonly used center pivot irrigation system with a total length of 131.2 m, including two spans and an overhang. Two end guns were also installed at the end of the overhang, such as the AXIS-II rotary damping sprinkler (from XCAD USA Company) and the Twin101 Plus vertical impact sprinkler (from Austria's Komet Company). The water distribution was then evaluated under various combinations of nozzle diameters and operating pressures. The result showed that the irrigation uniformity of the AXIS-II end gun was better than that of the Twin 101 Plus. The radial uniformity coefficient for the AXIS-II ranged from 64.1% to 88.6%, and the circular uniformity coefficient ranged from 80.2% to 88.3%. In contrast, the radial uniformity coefficient for the Twin101 Plus ranged from 49.3% to 64.4%, while the circular uniformity coefficient ranged from 79.7% to 86.6%. The irrigation uniformity coefficient was improved to optimize the combination of nozzle diameter and working pressure. Specifically, the uniformity coefficient along the lateral pipe was improved by approximately 10 percentage points for both types of end guns working separately. Once AXIS-II and Twin101 Plus end guns were combined, the radial and circular uniformity coefficients increased to 79.4% and 89.0%, respectively. Correspondingly, the radial and circular variation coefficients were reduced to 35.1% and 15.8%, respectively. As such, there was a significant improvement in the water distribution uniformity, when the two sprinklers operated in combination. Especially, the irrigation depth increased near the end gun area within 144.2 m of the pivot point, leading to the high uniformity of water distribution. The optimal combination of end guns was recommended as the AXIS-II end gun (with a nozzle diameter of 15 mm, and operating pressure of 200 kPa) and the Twin101 Plus end gun (with a nozzle diameter of 20 mm, and operating pressure of 300 kPa). This configuration was very suitable for the center pivot irrigation systems with two spans and an overhang at a similar overall length. An irrigation uniformity coefficient (CUH) of 85% was achieved, fully meeting the national standards. And an effective range reached 0.89 times, thus exceeding the standard requirement of 0.75 times. Anyway, the combination of end guns effectively improved the irrigation performance. Two types of sprinklers were combined to optimize the nozzle diameter and operating pressure, in order to obtain the high irrigation uniformity of end guns on center pivot irrigation systems. The hydraulic performance of end guns was improved to reduce water waste. The resulting optimal performance of the irrigation system can also be expected to enhance the crop yield and quality. Furthermore, the finding can also provide strong reference and technical support to the end gun selection and optimal configuration in the center pivot irrigation system.