Effects of operation height and tree shape on droplet deposition in citrus trees using an unmanned aerial vehicle

•The effects of operation height and tree shape on droplet deposition in citrus trees were studied.•An operation height of 1.2 m exhibits the best spraying performance.•An inverted triangle shape can greatly improve droplet deposition, especially at lower layers.•The most uniform droplet distribution occurs at the lower layer of the inverted triangle shape. The effects of operation height and tree shape on droplet deposition in citrus trees using an unmanned aerial vehicle (UAV) were investigated in this study. When the operation height was 1.2 m and the flight speed was 3.5 m s−1, the droplet density and droplet coverage rate were maximized in all the canopies. The droplets exhibited the most uniform distribution (CV = 32.44%) in the lower layer of inverted triangle-shaped trees. The droplet density in the lower layer of inverted triangle-shaped trees was 48.04% higher than that in triangle-shaped trees. There was no statistically significant difference between the middle and lower layers of inverted triangle-shaped trees. The uniformity of the droplet deposition distribution was studied in six parts of the citrus trees at an operation height of 1.2 m. The results indicated that the front, middle, rear, left and centre parts displayed uniform distributions, but the right part of the inverted triangle-shaped trees did not. This difference might have been caused by a deviated flight route due to manual control error or by droplet drift from the wind originating from the right.