## 3 Results and Discussion
### Spray Deposition
#### 3.1.4 Characterization of Deposition  
In the test, the WSPs were used to evaluate the characteristics of the deposition, which included droplet size, number of spray deposits and the area of coverage. Influenced by the spray volume and spraying system, the deposition characteristics of different sprayers were quite different (Figure 5). The spray volume of SPB sprayer approached 300 L/ha, which was identical to EAP sprayer belonging to the large volume application. Furthermore, the nozzles of SPB sprayer are similar to EAP sprayer, which are hydraulic nozzles. From the test results of WSPs, the VMD of the droplets of the SPB and EAP sprayer were 272.3 and 254.1 \(\upmu\)m, respectively. They were not significantly different from each other (Figure 5). Although the VMD of droplets and the spray volume of SPB sprayer were similar to EAP sprayer, the area of coverage and the number of spray deposits of SPB sprayer were 75.9% and 73.9% greater than the EAP sprayer with no significant difference (Figure 5). From the CVs results of the total deposition, the greater area of coverage and number of spray deposits of SPB sprayer may be  
Figure 4: Deposition of four sprayers on the different canopies of wheat. Bars with different letters are significant different, [PERSON]’s test, \(p<0.05\). due to the better deposition uniformity. As a conventional knapsack sprayer, the spray volume of KMB sprayer was quite lower than the EAP sprayer of 75 L/ha. With the air assistance, the VMD of droplets of KMB sprayer was significantly lower than the EAP sprayer, which was only 154.7 \(\upmu\)m. Results from previous studies had proved that the area of coverage is proportional to the spray volume [32]. The area of coverage of KMB sprayer was lower than the EAP sprayer with no significant difference (Figure 5). However, with finer droplets, the number of spray deposits of the KMB sprayer was quite similar to the EAP sprayer (Figure 5). The spray volume of UAV sprayer was 5 L/ha, which was lower than the other three sprayers. With the unique atomization method, the droplets were smashed by the high rotational speed of the rotary cup (10,000 rpm). The VMD of the droplets was 124.0 \(\upmu\)m, which was finer than the three other sprayers (Figure 5). Another characteristic of this nozzle, which differed from hydraulic nozzle, is that the width of the droplet spectrum of this nozzle was narrower and the droplet size was more uniform with high rotational speed [33]. The result in this study verified that the CV of the VMD of the droplets was only 17.1%, which was far less than the other sprayers (CV > 35%) (Figure 5). Because of the lower spray volume, the area of coverage of the UAV was 2.2%, which were quite lower than the other sprayers and was only 5.8% to 12.8% of the other three sprayers. The number of spray deposits of the UAV was 28.2 points/cm\({}^{2}\), which was also significantly lower than others (Figure 5). This may go against the control of pests or diseases, especially when the contact pesticides were applied. Therefore, the focus of further studies will be improving the spreading coefficient of the droplets and increasing the area of the coverage, such as adding adjuvant in the tank [34; 35], or changing into electrostatic-charged sprays [36; 37]. Spraying systems had a great effect on the characteristics of deposition, including the area of coverage, number of spray deposits and droplet size. It is difficult to judge the application quality by a single index. Although the lower spray volume of UAV could lead to lower area of coverage and fewer deposits, the dose applied per area was not significant lower than other sprayers, because of the higher concentration of each droplets. Syngenta Crop Protection AG (Basel, Switzerland) recommends  
Figure 5: Characterization of the deposition including droplet size, number of spray deposits and the area of coverage.