The adoption of modern precision agriculture technologies is crucial for sustainable orchard management. Among these, the agricultural drone, or unmanned aerial vehicle (UAV), presents a transformative tool for plant protection. Unlike traditional high-volume, high-pressure ground sprayers which are labor-intensive, inefficient, and prone to significant chemical runoff, agricultural drones offer a low-volume, targeted application method. Their advantages include high operational efficiency, adaptability to complex terrains and tall canopies, and substantial reductions in water and chemical usage, thereby minimizing environmental impact and operator exposure. While significant research exists on agricultural drone applications in field crops like cotton and cereals, its optimized use in tall-tree orchards, such as walnuts, remains underexplored. Walnut trees, often reaching 6-10 meters in height, pose a unique challenge for spray coverage and penetration. This study investigates the optimal operational parameters for an agricultural drone in a mid-sized walnut orchard, focusing on the factors influencing droplet deposition density and distribution within the complex canopy structure.
The core objective was to identify the combination of flight and application parameters that maximize effective droplet coverage on walnut foliage. The study was conducted in a mature walnut orchard with tree heights of 6-7 meters and a planting spacing of 3m x 5m. A multi-rotor agricultural drone (MG-1P) was employed for all aerial applications. To systematically evaluate the influence of key parameters, a three-factor, three-level orthogonal experimental design (L9 array) was implemented. The factors and their levels are detailed in the table below.
| Treatment | Spray Volume (L/ha) | Flight Height (m above canopy) | Flight Speed (m/s) |
|---|---|---|---|
| 1 | 22.5 | 1.5 | 1.2 (Slow) |
| 2 | 30.0 | 2.0 | 1.1 (Slow) |
| 3 | 37.5 | 2.5 | 1.1 (Slow) |
| 4 | 37.5 | 2.0 | 2.1 (Medium) |
| 5 | 22.5 | 2.5 | 2.1 (Medium) |
| 6 | 30.0 | 1.5 | 1.6 (Medium) |
| 7 | 30.0 | 2.5 | 3.0 (Fast) |
| 8 | 37.5 | 1.5 | 2.2 (Fast) |
| 9 | 22.5 | 2.0 | 3.0 (Fast) |
The spray solution contained a tracer dye (Allura Red AC) at 450 g/ha for quantitative analysis. Water-sensitive papers and filter paper cards (9 cm diameter) were positioned at 19 distinct locations within the canopy of three sample trees per treatment before spraying. The canopy was stratified into upper, middle, and lower layers, and each layer was further divided into outer and inner (within the canopy) positions. After the agricultural drone completed its application, the cards were collected. Droplet density (deposits/cm²) was analyzed using DepositScan software. The tracer deposition on filter papers was eluted and quantified via spectrophotometry to calculate the deposition amount per unit area (μg/cm²). The deposition was calculated using the formula:
$$ d = \frac{c \times V}{a} $$
where \(d\) is the deposition (μg/cm²), \(c\) is the tracer concentration (μg/mL), \(V\) is the elution volume (mL), and \(a\) is the sampling area (cm²).
The primary statistical analysis focused on the main effects of the three operational parameters. Analysis of variance (ANOVA) revealed that flight speed was the dominant factor significantly affecting droplet density across almost all canopy positions (upper, middle, lower, inner, outer). Flight height had a significant effect on droplet density in the upper canopy, while spray volume’s effect was less pronounced statistically but still influential. Regarding chemical deposition, flight speed significantly affected the lower canopy deposition, and flight height significantly influenced inner canopy deposition. In summary, for operations with an agricultural drone, the parameter hierarchy affecting coverage and deposition is: Flight Speed > Flight Height > Spray Volume.
The detailed results from the nine treatments are consolidated in the table below, showing average droplet density and deposition across different canopy strata and positions.
| Trt | Avg. Droplet Density (deposits/cm²) | Avg. Deposition (μg/cm²) | Canopy Distribution (Density & Deposition) |
|---|---|---|---|
| 1 | 12.34 | 0.15 | Upper > Middle > Lower; Outer > Inner |
| 2 | 18.21 | 0.18 | Upper > Middle > Lower; Outer > Inner |
| 3 | 15.67 | 0.17 | Upper > Middle > Lower; Outer > Inner |
| 4 | 19.85 | 0.24 | Upper > Middle > Lower; Outer > Inner |
| 5 | 16.02 | 0.16 | Upper > Middle > Lower; Outer = Inner |
| 6 | 22.41 | 0.21 | Upper > Middle > Lower; Outer > Inner |
| 7 | 28.73 | 0.29 | Upper > Middle > Lower; Outer > Inner |
| 8 | 24.15 | 0.19 | Upper > Middle > Lower; Outer > Inner |
| 9 | 37.94 | 0.24 | Upper > Middle > Lower; Outer > Inner |
Treatment 9 (22.5 L/ha, 2.0 m height, 3.0 m/s speed) achieved the highest average droplet density (37.94 deposits/cm²), while Treatment 7 (30.0 L/ha, 2.5 m height, 3.0 m/s speed) yielded the highest average deposition (0.29 μg/cm²). A consistent pattern emerged across nearly all treatments: droplet density and deposition were highest in the upper canopy, decreased in the middle, and were lowest in the lower canopy. Similarly, the outer periphery of the canopy received significantly greater coverage than the inner, shaded sections. This vertical and horizontal gradient is a critical consideration for agricultural drone operations targeting pests and diseases that may inhabit all parts of the tree.
To determine the optimal level for each factor, marginal mean estimations were calculated. The analysis confirmed that a higher flight speed (2.2 – 3.0 m/s) consistently resulted in greater droplet density and deposition, likely due to improved air turbulence and droplet penetration into the canopy. A medium flight height of 2.0 m above the canopy generally provided the best balance, optimizing coverage across layers without excessive drift or poor canopy penetration. For spray volume, low to medium rates (22.5 – 30.0 L/ha) were sufficient and often superior to higher volumes, which may lead to droplet coalescence and runoff. Therefore, the recommended optimal operational parameters for an agricultural drone in a walnut orchard of this scale are: Flight Speed: 2.2 – 3.0 m/s, Flight Height: 2.0 – 2.5 m, Spray Volume: 22.5 – 30.0 L/ha.
Beyond the core flight parameters, the choice of nozzle type on the agricultural drone significantly influences spray characteristics. A subsequent experiment evaluated three flat-fan nozzles: Fine (Teejet 11001), Medium (Teejet 110015), and Coarse (Teejet 11002), under a fixed parameter set (30 L/ha, 2.0 m, 2.0 m/s). The results are summarized below.
| Nozzle Type | Avg. Droplet Density (deposits/cm²) | Avg. DV0.5 (μm) | Avg. Deposition (μg/cm²) |
|---|---|---|---|
| Fine (11001) | 23.90 a | 239.04 b | 0.108 |
| Medium (110015) | 16.52 ab | 258.41 ab | 0.139 |
| Coarse (11002) | 9.28 b | 361.37 a | 0.080 |
The fine nozzle produced the highest droplet density and the smallest volume median diameter (DV0.5), while the coarse nozzle produced the lowest density with the largest droplets. Although the medium nozzle did not have the highest density, it resulted in the greatest average chemical deposition, with no statistically significant difference in deposition among the three types. Considering the high evaporation and potential drift risks associated with very fine droplets in arid climates, the medium nozzle (Teejet 110015) is recommended as the optimal choice for agricultural drone spraying in walnut orchards, offering a balance between coverage, droplet size, and effective deposition.
A critical advantage of using an agricultural drone for low-volume application is the drastic reduction in environmental contamination due to ground loss. To quantify this, the pesticide loss rate to the ground was compared between the agricultural drone (using optimal parameters) and a conventional ground-based high-pressure sprayer. Ground deposition cards were placed under the tree trunk, within the canopy drip line, and between rows. The ground loss rate (\(D\)) was calculated as:
$$ D = \frac{m_2}{M} \times 100\% $$
where \(m_2\) is the pesticide deposition per unit ground area (μg) and \(M\) is the total pesticide applied per unit area (μg).
| Application Method | Avg. Ground Deposition (μg/cm²) | Pesticide Ground Loss Rate (%) |
|---|---|---|
| Agricultural Drone | 0.16 b | 3.61 b |
| Ground Sprayer (Control) | 1.07 a | 23.69 a |
The results are striking. The ground sprayer lost nearly a quarter (23.69%) of the applied pesticide to the soil, whereas the agricultural drone lost only 3.61%. This more than six-fold reduction in ground loss demonstrates a significant environmental benefit of agricultural drone technology, minimizing soil and water pollution and improving the efficiency of chemical use.
In conclusion, this study provides a validated framework for optimizing agricultural drone operations in walnut orchards. The key findings are: 1) Flight speed is the most critical operational parameter governing droplet deposition; 2) A combination of moderate speed (2.2-3.0 m/s), a flight height of 2.0-2.5 m above the canopy, and a low-volume application rate (22.5-30.0 L/ha) yields optimal coverage and deposition; 3) Canopy deposition follows a predictable gradient (upper>middle>lower; outer>inner), which must be considered for comprehensive pest control; 4) A medium spray nozzle offers the best practical compromise for deposition performance; and 5) Agricultural drone application drastically reduces pesticide ground loss compared to traditional methods, offering a clear path toward more sustainable orchard management. These parameters serve as a crucial reference for deploying agricultural drones effectively in tall-canopy fruit trees, enhancing spray efficacy while promoting environmental stewardship.