## 3 Results and Discussion
### Spray Deposition
#### 3.1.3 Droplets Penetrability  
In the test, filter papers were used to measure the tracer deposition on the different canopies of wheat. An analysis of the deposition on the wheat heads showed that the deposition of the low volume sprayer of the UAV and KMB sprayer were significantly higher than the SPB and EAP sprayer (Figure 4). The greatest deposition was achieved with the conventional KMB sprayer with 1.46 \(\upmu\)g/cm\({}^{2}\), which was 18.7%, 80.2% and 111.6% more than the UAV, EAP and SPB sprayer, respectively (Figure 4). From the droplet size test results presented in Section 3.1.4, the VMD of droplets of KMB and UAV sprayer were lower than the other two sprayers. The main reason for higher wheat head deposition of UAV and KMB sprayer may be that fine droplets were better retained in the upper canopy [30].  
The deposition of four sprayers on the flag leaf (top canopy) ranged from 0.57 to 1.02 \(\upmu\)g/cm\({}^{2}\) (Figure 4). The UAV sprayer had the highest deposition on the flag leaf, which was 41.7-78.9% higher than the other sprayers. However, influenced by the great variability, the depositions on the flag leaf were not significantly different. In the study by [PERSON] et al. [31], the spray deposits decreased dramatically from the top to the bottom of the canopies and also tended to linearly decrease as the leaf area index increase. Due to the overlap and the block of the blades at the later growth stage of the wheat, the depositions on the lower parts were far less than top parts. The depositions of different sprayers were influenced by many factors, such as spraying pressure, spraying height and spraying pattern. The deposition of the UAV sprayer on the third top leaf (bottom canopy) was only 0.26 \(\upmu\)g/cm\({}^{2}\), which was not significantly different from the other two conventional sprayers. However, it was 50.0% compared to the SPB sprayer and the deposition of SPB sprayer on the bottom was 0.52 \(\upmu\)g/cm\({}^{2}\).  
To improve the adhesive rate of solution and control efficacy, it is crucial to enhance the droplets penetrability and obtain a homogeneous deposition distribution [31], especially since many pests and diseases occur on the bottom of plants. Although many studies [18; 19] have proven that the downwash airstream generated by the UAV is conducive to the disturbance of leaves and droplets penetration, the results from our study proved that the penetration of the droplets of UAV is still worse than boom sprayer. This result is similar to the results of [PERSON] et al. [9] and [PERSON] et al. [7]. There are many factors accounting for the poor droplet penetrability. One of the most important reasons is that the droplets were sprayed by the rotary cup atomizers, which lack the downward kinetic energy in comparison with hydraulic nozzles. [PERSON] et al. [9] compared four kinds of UAV sprayer and found the flight height and the flight speed had a pronounced impact on droplet penetrability. In their study, the penetrability of the droplets was inversely proportional to the flight speed and the flight height. The flight parameter had an effect on the downwash flow, which further affected the droplets penetrability. [PERSON] et al. [18] testified that the vertically downward wind had a significant effect on the penetrability. To improve the droplet penetrability of the UAV sprayer, the fly height and speed should be lower, thus ensuring operating efficiency, and suitable nozzles should be chosen to improve the droplets penetrability.  
The deposition uniformity on different parts of the canopy was consistent with the total deposition. The SPB sprayer had the best deposition uniformity (CVs < 56%) on different canopies and other sprayers had lower uniformity (CVs > 70%).