The field test was conducted in Lvyang Modern Agricultural Park (38° N, 114° E), Hebei Province, China, on August 30 of 2020. The average temperature and humidity in the test environment were 36.3°C and 54%, respectively. The average natural wind speed was 0.315 m/s. The experimental subjects were Fuji apple trees with 5-year-old dwarf rootstock planted in the north–south direction. The spray qualities were measured on free, slender, and high spindles, and fog droplets were detected on water-sensitive paper produced by Liuliu Shanxia Plant Protection Technology Co., Ltd. (China).
The main test instruments were the profiling boom sprayer supported on a tractor, a wind-speed measuring instrument (AS856S, Shanghai Xima Technology (Group) Co., Ltd., Shanghai, China), a temperature and humidity measuring instrument (RC-4, Jiangsu Jingchuang Electric Co., Ltd., Jiangsu, China), water-sensitive paper, double-sided tape, a box ruler, a stopwatch, and a scanner. The water-sensitive paper was cut into 3 cm × 2 cm rectangular units, and its back side was pasted to the apple trees of the test target with a small amount of double-sided tape. Facing the east, south, west, and north directions, papers sprayed by the inner, middle and outer rings were pasted on the top, middle and bottom layers of the fruit tree canopy (Dong et al., 2018, Fig.5). To avoid disturbance from spray drift, six fruit trees were selected as the test targets at intervals of their tree shapes, and 648 water-sensitive papers in total were pasted.
After starting the tractor, the parameters of the profile-spray-bar sprayer were adjusted to the optimal spray conditions. The spray nozzle adopted an ARAG imported from Italy with a spray angle of 110°. The sprayers were separated by 0.39 m and their pressure was 0.74 MPa, giving a spray distance of 1.49 m. The tractor was operated at 3.6 km·h-1.
The water-sensitive papers were detached from the trees and pasted onto A4-sized paper sheets. As an example, Fig. 6 shows the water-sensitive paper from one of the test targets.
To collect the fog drop information, the water-sensitive papers were scanned using Image-master software. The sigma and weight parameters were both set to 5, and the analysis area was selected and extracted. After adjusting the foreground and background pixels in the area, the foreground background stripping, foreground removal, and noise-reduction processing were adjusted using the "threshold adjustment" functionality. Finally, the total number of droplets, droplet deposition coverage rates, and droplet deposition densities were derived from the processed water-sensitive paper (see Fig. 7).
Analysis of test results
The test results were analyzed in terms of spraying quality of the plant protection equipment. For convenience, the top, middle and bottom layers are represented by T, M, and B, respectively, the east, west, south, and north directions are represented by E, W, S, and N, respectively, and the inner, center, and outer rings are represented by I, C, and O, respectively. To analyze the droplet distribution in the outer ring of the target, the droplet deposition coverage rates from that ring were counted at different heights (Fig. 8).
The droplet deposition coverage rates were subjected to single-factor analysis of variance (ANOVA) and the results are shown in Table 2. All p-values were greater than 0.05, indicating no significant differences in the droplet deposition coverage rates at different heights in different directions. The standard deviations of the droplet deposition coverage rates of the outer rings on the free, slender, and high spindles at different heights were 4.43, 2.82, and 5.29, respectively, indicating small variations in the droplet deposition coverage rates for a particular tree shape. Clearly, the outer-ring droplets expelled from the profiling boom sprayer were evenly distributed over the height of the test target.
The droplet deposition densities on the target of the outer ring were counted at different heights, and the results are presented in Fig. 9. From these results, the effect of the medicine application on the leaves of the outer-ring target was analyzed.
Table 3 lists the deposition densities on the targets of the outer ring, obtained by single-factor ANOVA. All p-values exceeded 0.05, indicating no significant difference in the droplet deposition densities at different heights in any direction. The standard deviations of the droplet deposition densities of the free, slender, and high spindles from the outer rings at different heights were 5.97, 4.98, and 6.15, respectively, indicating small differences in the deposition densities on a particular tree shape. Clearly, the outer-ring blades of the profiling boom sprayer uniformly coated the test target.
Figure 10 shows the droplet deposition coverage rates on the targets of the inner, center, and outer rings in different directions. From these results, the distribution characteristics of the inner, center, and outer rings were analyzed. In any given region, the droplet deposition coverage rates in each direction were similar to those of the outer ring; specifically, the average coverage was higher in the E and W directions (charging side) than in the S and N directions (non-charging side). Averaged over the three tree shapes, the droplet deposition coverage rates of the inner, center, and outer rings were 2.59%, 4.63%, and 7.50% respectively.
The droplet deposition coverage rate suddenly increased between the inner and outer rings. Averaged over the four directions, the coverage rate in the outer ring was 41.46% higher than in the center ring, and 90.87% higher than in the inner ring. Meanwhile, the average coverage rate was 34.93% higher in the center ring than in the inner ring, indicating that fog reached the outer and center rings. The growths of the droplet deposition coverage rates were similar, and the droplet penetrations in different rings were consistent. The average droplet deposition coverage rates of the inner rings of the free, slender, and high spindles were 37.41%, 36.69% and 35.47%, respectively, lower than in the outer rings but still exceeding 33%. Although the droplet penetration of the inner ring was poorer in the horizontal than center and outer ring in the vertical direction, the blades of the inner ring were sprayed sufficiently to meet both the quality assessment of plant protection operations and the design operating requirements of the profiling boom sprayer (NY/T 992-2006).
The deposition densities on the targets of the inner, center, and outer rings were analyzed by single-factor ANOVA, and the results are listed in Table 4.
The droplet deposition densities on the targets of the inner, middle, and outer rings were calculated in different directions (Fig. 11). From the results, the drug loadings on the blades of the inner, center, and outer rings were analyzed.
The standard deviations of the droplet deposition densities on the free, slender, and high spindles were 3.50, 5.25, and 3.38, respectively (inner ring) and 6.83, 6.63, 4.25, respectively (center ring). The droplet deposition densities of the inner and center rings were almost independent of direction. Meanwhile, the droplet deposition densities over the four directions of the outer ring were 10.75, 9.69, and 11.41, respectively, larger than in the inner and center rings. The E and W sides, which were exposed to drug administration from the outer ring, presented a higher density of deposited droplets than the S and N sides. This asymmetry is explained by the smaller spray distance in the E and W directions of the outer ring than in the S and N directions, so the droplet deposition density is more obvious in the E and W directions. However, the spray distances in the inner and middle rings are similar and blocked by layers of blades; accordingly, their droplet deposition densities are smaller and more directionally similar than in the outer ring. The droplet deposition density gradually increased from the inner to the outer ring (by 52.36% from the inner to center ring, and by 91.27% from the inner to outer ring). The average droplet deposition densities in the middle and outer rings exceeded 150 grains·cm-2. According to this result, applying the medicine to the blades of the middle and outer rings will improve the droplet penetration into regions between the canopy. Although the penetrability of the inner ring was relatively poor, the average droplet deposition density was 100.60 grains·cm-2, exceeding the 70 grains·cm-2 stipulated in plant protection operating standards; therefore, it meets the operating requirements of profiling boom sprayer design (GB/T 17997-2008). In summary, the developed profiling boom sprayer provides good droplet penetration from its inner, center, and outer rings, thereby homogenizing the distribution of droplets in the top, middle and bottom layers of the canopy, and achieving the goal of a profiling spray.