**4. Discussion**

The study results indicate that the SSCDS configured modified irrigation microemitters resulted in comparable canopy deposition and coverage against that of hollow cone nozzles. The modified irrigation micro-emitter configured treatment resulted in 28.0%

and 19.5% higher mean spray deposition and coverage, respectively, compared to hollow cone nozzle configured SSCDS treatment. Despite a considerable numerical difference in deposition and coverage between the modified irrigation micro-emitter and hollow cone nozzle SSCDS treatment, the corresponding difference was statistically non-significant, perhaps due to high variability in deposition and coverage data and a relatively small number of samples. Such variability is very typical in agrochemical application scenarios [20,31,32] and could be reduced by increasing the number of replicates in the experiment. The bottom zone deposition and coverage for both the tested treatments were highest compared to the other sampling zones. The spray droplets that missed the target and spray run-off from the top and mid canopy zone, settled to the bottom, would have resulted in a higher deposition in later zone. The modified irrigation micro-emitter treatment resulted in 59.4% and 33.1% higher bottom zone deposition and coverage, respectively, compared to the hollow cone nozzle treatment. Similarly, mid zone deposition and coverage was higher than the hollow cone nozzle treatment.

Micro-emitter modification also improved the leaf surface deposition and coverage. The abaxial and adaxial deposition of modified irrigation micro-emitter treatment were 38% and 21.6% higher than hollow cone nozzle treatment, respectively. Likewise, the modified treatment had 38.3% and 12.4% higher abaxial and adaxial coverage compared to hollow cone nozzle treatments. Additionally, modified SSCDS treatment resulted in lower CV in spray deposition and coverage between the abaxial and adaxial leaf surfaces. Results indicate that the micro-emitter modification also augmented the leaf level spray uniformity. Previous SSCDS configuration test studies have reported that shower down arrangemen<sup>t</sup> with only one nozzle/micro-emitter atop tree canopy was the simplest and the most economical SSCDS configuration [20,22]. However, poor bottom zone and abaxial leaf surface deposition were perceived as a major constraint with such configuration [33]. The presented 3-tier configured SSCDS with modified emitters indicates a substantial increase in the bottom zone and abaxial leaf surface deposition and coverage. Furthermore, the modified SSCDS configuration uses a low-cost micro-emitter (1.5 \$/unit) (Table 1) that assisted in reducing the system installation cost by ~12 times, compared to expensive offthe-shelf hollow cone nozzles (20 \$/unit) configured SSCDS. Such cost savings is expected to improve its economic viability.

The off-target drift data trends indicate that the modified irrigation micro-emitter treatment (T1) had numerically lower aerial, ground run-off and drift losses compared to treatment configured with hollow cone nozzle (T2). The adjacent mid-row ground deposition and coverage evaluated at 1.5 m downwind were, respectively, 258.5% and 326.5% lower for the modified micro-emitter treatment. Likewise, overall mean mid-row ground deposition and coverage in the modified treatment were 267.3% and 327% lower than hollow cone nozzle treatment. A similar trend was observed for downwind aerial drift. The aerial deposition for T1 recorded at 3 m and 6 m downwind was, respectively, 298% and 373% lower than T2. The overall mean aerial deposition of modified treatment was 395% lower compared to hollow cone nozzle SSCDS treatment. Furthermore, the ground spray coverage at 4.5 m and 7.5 m and aerial coverage at 3 m and 6 m downwind for modified treatment were almost negligible (<0.1%). The hollow cone nozzles produced fine size droplets (Table 3) that are highly susceptible to drift due to longer air suspension time and lighter weight [34–36]. In contrast, the modified micro-emitter produced medium sized droplets (Table 3) that would have reduced the off-target drift potential [37]. Additionally, wind speed during the modified micro-emitter trials was 1.5 times higher than hollow cone nozzle treatment trials (Table 2). With similar wind direction trend during both treatments, higher wind speed would cause higher off-target drift for spray with modified irrigation micro-emitters. Nonetheless, modification in droplet size spectrum would have curtailed pertinent off-target drift losses. This might be one of the reasons why modified micro-emitter configured treatments would have resulted in lesser ground and aerial drift. Therefore, pertinent modification could assist in minimizing the environmental contamination and subsequent hazards [6–8]. Furthermore, the reduced drift for modified

SSCDS treatment would have resulted in improved overall as well as zone and leaf specific canopy deposition and coverage [38].

While a modified irrigation micro-emitter provided a low-cost alternative to the hollow cone nozzles in SSCDS configuration, existing pneumatic spray delivery reservoir drains the residual spray mix onto the soil through auto drain valve for cleaning. During pesticide application, such residues can contaminate the soil [39] and can cause up to 25% pesticide losses to the ground. To overcome this problem, our lab is working on the modification of the existing pneumatic spray delivery reservoirs, which allows partial passage of the air to the nozzle feed line towards the end of the spray cycle so that residues can be sprayed into the canopy instead of draining onto the soil. Pertinent self-cleaning ability would eliminate the need of the auto-drain valve in the reservoir and ground chemical losses.
