Optimization of flight operation parameters for supplementary pollination in Kuerlexiangli pear using an unmanned aerial vehicle

Abstract: 【Objective】The self- pollination fruiting set rate of Kuerlexiangli pear was low, and in the production the artificially assisted pollination is needed in order to improve the yield and quality. However, the traditional method of hand-assisted pollination suffers from low efficiency and high cost. In recent years, with the rapid development of modern information technology and its widespread application in the field of agriculture, the technology of unmanned-aerial-vehicle (UAV) -assisted pollination has been widely applied due to its multiple advantages, such as high efficiency, high fogging effect, low cost and high disease blocking rate. To investigate the distribution pattern of droplets deposited on the canopy layer of Kuerlexiangli pear trees under different operating parameters of UAV-assisted pollination, the present experiment was undertaken to screen the optimal operational parameters, so as to improve the pollination quality and operational efficiency.【Methods】Five factors, including different flight routes (up-row flight and inter-row flight), spray volume (15 L×hm^-2 , 30 L×hm^-2 and 45 L×hm^{- 2} ), droplet size (70 μm, 100 μm, 130 μm and 170 μm), flight height (1 m, 2 m and 3 m from the top of the canopy), and flight speed (3 m×s^-1 , 5 m×s^-1 and 7 m×s^-1 ) were selected in this study. Three measurement indexes, including the droplet density, deposition volume and coverage rate, were analyzed during theblooming period of Kuerlexiangli pear.【Results】The droplet density, deposition volume and coverage rate decreased from the upper to lower layers on the trees when the UAV flew along the upper row. The droplet density was significantly different between the upper and lower layers, and the amount of deposition and coverage rate were not significantly different among the upper, middle and lower layers. In the inter-row flight, the droplet density, deposition volume and coverage rate on the upper layer were at the least leve and the three indexes on the same layer were lower than those in the up-row flight. The droplet density, deposition and coverage on all layers of the canopy increased with increasing spray volume. At a spray volume of 15 L×hm^-2 , there was no significant difference in droplet density, deposition and coverage between the upper and lower layers of the canopy, and at spray volumes of 30 L×hm^-2  and 40 L×hm^{- 2} , droplet density and deposition decreased significantly from the upper to lower layers of the canopy, with progressively significant differences. With the increase of droplet size, droplet density showed a gradual decrease on the same tree canopy. In addition, with the same drop size, droplet density gradually decreased from the upper layer to the lower layer, and the deposition amount and coverage rate gradually decreased from the upper layer to the lower layer when the droplet size was 70 μm-130 μm. The deposition amount and coverage tended to increase sequentially from the upper layer to the lower layer when the droplet size was 170 μm, but the difference did not reach the significant level. As flight height increased, a significant difference in droplet density, deposition and coverage was only detected on the upper canpoy layer, but not on the middle and lower layers. At the same flight height, droplet density, deposition and coverage decreased from upper layer to lower layer of the tree canopy. In the upper canopy layer, there were no significant differences in droplet density, deposition and coverage at different flight speeds. Moreover, on the middle and lower layers, the three indexes gradually decreased with increasing flight speed. When the flight speed was 3 m×s^-1 , the droplet density, deposition amount and coverage rate showed a first decreasing and then increasing trend from the upper, middle and lower layers, but the differences did not reach a significant level. When the flight speed was 5 m×s^-1 , the droplet density, deposition amount and coverage rate showed a decreasing trend from the upper layer to the lower layer, the droplet density on the upper layer differed significantly from that on the lower layer, and the difference in the deposition amount and coverage rate was not significant among different layers. When the flight speed was 7 m×s^-1 , the droplet density, deposition amount and coverage rate showed a first decreasing and then increasing trend from the upper, middle and lower layers, the droplet density and coverage rate on the upper layer showed very significant differences on the middle and lower layers, and the deposition amount of the upper layer showed very significant differences with the middle layer and significant differences with the lower layer. There were no significant differences in droplet density, deposition amount and coverage between the middle and lower layers.【Conclusion】The comprehensive analysis suggested that the UAV was more effective when it flew along the top of the row, with a spray volume of 45 L×hm^-2 , a droplet size of 100 μm, a flight height of 1 m above the top of the canopy and a flight speed of 3 m×s^-1 . This paper provides technical reference for UAV-assisted pollination technology, and also provide theoretical basis for futher completing the relevant technical specifications.