Effects of downwash airflow and droplet size on unmanned agricultural aerial sprayer (UAAS) spray deposition and ground loss in citrus orchards

Precise agrochemical application by unmanned agricultural aerial sprayers (UAASs) in dense orchard canopies challenges smart agriculture, demanding intelligent optimization to balance uniform deposition and drift. This study investigated how downwash intensity and droplet size affect spray deposition and ground loss on both sides of citrus leaves. An integrated smart sensing approach combined a wireless wind speed measurement system (WWSMS) for real-time canopy downwash characterization and an oligonucleotide tracking/dot blotting (OSTDB) method for quantifying deposition. Three UAAS models with tank capacities of 20 L, 30 L, and 40 L were evaluated. Results showed that downwash strength increased with take‑off weight. Stronger downwash enhanced deposition and penetration in the middle‑lower canopy, but excessive downwash caused localized over‑penetration and the highest ground loss. Very Fine droplets showed the highest deposition on the upper canopy among the tested conditions, although they also resulted in the greatest ground loss. In contrast, Fine droplets coupled with strong downwash provided the most balanced stratification across all canopy layers, with lower ground loss. Medium droplets primarily concentrated in the lower canopy and on the ground. The 40 L’s capacity and Very Fine droplets combination achieved the highest coverage and density on both upperside and underside leaf surfaces, yet also the greatest ground loss. Therefore, effective UAAS application requires a careful trade‑off between downwash intensity and droplet size. Stronger downwash with finer droplets improves targeted deposition, but precise adjustment of flight parameters according to operational conditions, such as flight height, speed, or swath width, is essential to prevent overpenetration and ground loss, thereby balancing efficacy with environmental sustainability. This provides a replicable sensor-based methodology for smart, environmentally responsible precision spraying across crop systems.