Optimization of UAV Spraying in Mountainous Nanguo Pear Orchards: Effects of Canopy Size and Operational Parameters on Droplet Deposition and Penetration.

The use of uniform spray volume rates for fruit trees with different canopy sizes is common in orchard spraying with plant protection unmanned aerial vehicles (UAVs), whereas the applicability of Leaf Wall Area (LWA)- and Tree Row Volume (TRV)-based methods to UAV spraying remains insufficiently validated. This study evaluated canopy size-based spray volume optimization and droplet deposition and penetration along the vertical canopy profile in a mountainous Nanguo pear orchard. The results showed that, under a uniform spray volume rate, small canopy trees exhibited significantly higher droplet deposition and ground deposition than large canopy trees, indicating greater potential spray losses. LWA- and TRV-based adjustment reduced the spray volume rate for small canopy trees by 43.0% and 49.0%, respectively, while maintaining comparable deposition in the upper and middle canopy layers. However, deposition in the lower canopy and on abaxial leaf surfaces remained limited, indicating that conventional LWA and TRV methods do not fully account for the top-down deposition characteristics of UAV spraying. Along the vertical canopy profile, finer atomization levels generally favored droplet penetration into the lower canopy, whereas increasing the spray volume rate increased overall deposition but did not significantly improve vertical penetration. Flight speed showed no consistent effect on penetration within the tested range. The results highlight canopy size as a key factor in UAV spray deposition. Canopy size-based variable-rate application can reduce spray volume rate while maintaining effective deposition, but further optimization should consider rotor-induced airflow and canopy structure.