Evaluation of spraying effect using small unmanned aerial vehicle(UAV) in citrus orchard

【Objective】Under the condition of different spray heights and different nozzles,the effect ofdroplet deposition after spraying with small unmanned aerial vehicle(UAV)in Citrus canopy was ex-plored in this work,and the spraying effect was evaluated. The determination of the UAV spray condi-tions in orchard will provide technical support for UAV application in orchard and improvement of spray-ing performance. The research also has significance in reasonable application of pesticide,improving theefficiency of spraying and prevention of plant disease and pest outbreak in large area.【Methods】The 10-year-old Gailiangcheng orange(Citrus sinensis‘Gailiangcheng orange')grafted on Trafoliata [Poncirus trifoliata(L.)Raf.] was selected as experimental material in this work. Ponceau 2R was selected as thechromogenic agent,formulated 300 times solution as the experimental reagent. A QJ-460 small four-rotor UAV was employed to test the aerial spraying effects by changing the spray height and nozzle type,and three working heights which indicated the distance between UAV and the top of the canopy were0.5,1.0 and 1.5 m,respectively;the experiment nozzles included rotating centrifugal nozzle and pres-sure nozzle. The spray experiment was designed as 6 treatments with five replications. In order to avoidthe droplet drift caused by the test error,one extra row of plants was not sprayed as protection. Samplingpoints were arranged in three layers along the altitude direction of citrus growth,denoted as upper,mid-dle and lower layers;along the UAV flying direction,the canopy was divided into three parts at 0.5 m in-tervals,denoted as left,middle and right sides;along the UAV flying direction,the canopy was also di-vided into three parts at 0.5 m intervals,denoted as front,middle and rear sides;samples were taken at9 points in canopy. One blade at a 45 degree angle for each sampling point was chosen,and then onepiece of cardboard was fixed on the leaf using a paperclip for collecting droplets and simulating the effectof droplet deposition on the leaf. The cardboards were collected one by one in ziplock baggies when thespraying finished and the paper cards dried,then they were analyzed in the laboratory. Droplets deposi-tion parameters such as coverage,total deposition,deposition densities,and diameter were extractedand analyzed using digital image processing after scanning.【Results】1)When spraying height was 0.5 m,the spray effect of droplet deposition and uniformity were different at the upper,middle,and bottom ofsweet orange canopies. The droplet deposition parameters in upper canopies were higher than that in themiddle and bottom. Especially to the droplet deposition densities,the densities in upper,middle and low-er were 10.70 droplets per square centimeter,6.44 droplets per square centimeter,and 4.50 droplets persquare centimeter,respectively. It could be seen that droplets mainly deposited on the upper canopy,while the paper jams of lower and inner canopy bore little and tiny droplets. In the left,middle and rightcanopy of UAV flight direction,the average droplet deposition densities were 3.68 droplets per squarecentimeter,8.81 droplets per square centimeter,9.15 droplets per square centimeter respectively;thedroplet coverage rates were 2.20%,5.04% and 5.27% respectively;the droplet size were 0.85,0.72 and0.64 mm respectively;these droplet parameters increased from left to right,but this trend was opposite tothat of the droplet size. The droplet in the side of UAV spraying directions was easily affected by cross-wind which could lead to droplet drift and deposition reduction for the southwest wind. In flight direction,the droplet deposition parameters had no significant difference which indicated steady spraying propertyand uniform droplet deposition. 2)The droplet coverage rate,total droplet deposition and droplet deposi-tion densities were 4.17%,731 droplets,7 droplets per square centimeter when spraying height was 0.5m,respectively;6.71%,2 101 droplets,21 droplets per square centimeter when spraying height was 1.5m,respectively;the droplet deposition on the leaves reached maximum when the working height was 1.0m above the canopy. At this height,the droplet coverage rate,total droplet deposition and droplet deposi-tion densities were 7.75%,2 584 droplets,25 droplets per square centimeter,respectively. These param-eters increased by 85.85%,253.49% and 257.14% compared with spraying height 0.5 m;increased by15.50%,22.99% and 19.05% compared with spraying height 1.5 m,respectively. With the increasing ordecreasing of the working height,the droplet deposition declined. Spraying swath was narrow and drop-lets were large on the upper canopy when spraying height was relatively low,and internal and marginalcanopy often captured less and smaller droplet;higher spraying height caused serious droplet drift andless droplet deposition on the canopy. Different spraying height had greater impact on the droplet deposi-tion,and the spraying height 1.0 m above the canopy produced better droplet deposition. 3)Under thecondition of rotating centrifugal nozzle,the average droplet coverage rate was 7.75%,the average dropletdiameter was 0.53 mm,while the data were 4.56% and 0.26 mm for the pressure nozzle. These parame-ters increased by 69.96% and 103.85% respectively. The droplet deposition densities of rotating centrifu-gal nozzle increased by 117.80% compared with that of the pressure nozzle.【Conclusion】The better drop-let deposition effect with QJ-460 four-rotor UAV spraying was obtained in the upper of canopies,and theatomization performance was relatively stable in the spraying process. The droplets deposition parameterssuch as droplet coverage rate,deposition densities were the optimal when working height was 1.0 m abovethe canopy. The results indicated that the rotating centrifugal nozzle was more suitable for aerial sprayingfor citrus canopy than pressure nozzle.