Unmanned Aerial Vehicle (UAV)-Mediated Spraying in Arecanut Plantations: Impact of Operational Parameters on Droplet Distribution

With the increasing adoption of unmanned aerial vehicles (UAVs) for precision spraying, optimization of spray parameters is essential to enhance spray efficacy and reduce environmental losses. This study evaluated the effects of UAV flight speed, flight height, and spray volume on droplet characteristics and spray performance in arecanut plantations. Nine treatment combinations were assessed, and key parameters including volume mean diameter (VMD), droplet density, penetrability, distribution uniformity, and ground losses were measured at three canopy positions: crown, inflorescence, and fruit bunch. Among the treatments, UAV operation at 2.5 m above the canopy, 3 m s⁻¹ flight speed, and 75 L ha⁻¹ spray volume produced the coarsest droplets (VMD > 430 µm), highest droplet density (∼95 drops cm⁻²), greatest canopy penetration (∼42%), and highest spray uniformity (∼82%), although it also resulted in the highest ground losses (∼51 drops cm⁻²). In contrast, operation at 2.5 m height, 5 m s⁻¹ speed, and 75 L ha⁻¹ generated finer droplets (∼250 µm) with lower ground losses and improved penetration, but reduced droplet density and uniformity. Droplet density and VMD declined markedly from the upper to lower canopy layers, indicating limited deposition within inner canopy zones. Spray volume exerted the strongest influence on VMD, penetration, and uniformity, whereas lower flight speed and moderate flight height enhanced deposition. Although manual spraying achieved higher penetrability and fruit bunch coverage, it exhibited greater non-uniformity, chemical wastage, and ground losses. Overall, UAV spraying offers a safer, faster, and more environmentally sustainable alternative for crop protection in tall-growing arecanut plantations.