The United Nations expects the population to grow by two billion people over the next 30 years (United Nations, 2022), for which agriculture must provide food, fiber, and fuel. The estimated increase in calorie consumption that comes with the economic growth of developing countries will require an expansion of approximately 70% in agricultural production (Searchinger et al., 2018). Increasing agricultural yield is not an easy task, especially in the context of climate change, which is expected to cause a significant reduction in precipitation in the Mediterranean region, one of the main agricultural areas in the world (Masson-Delmotte et al., 2019). Increasing the agricultural area at the expense of natural ecosystems is not sustainable, so the increased agricultural yield must come from sustainable intensification. Moreover, there is increasing concern about the environmental impact of PPPs. This concern has resulted in strategies, such as the Farm to Fork initiative, that aims to reduce the use of PPPs in Europe by 50% over the next decade.

Some technologies with the potential to contribute to solving this challenge have already been developed. Unmanned aerial vehicles (UAVs) are becoming increasingly popular in the agricultural industry due to their adaptability and versatility. UAVs are now being utilized for various agricultural tasks, including the precise application of PPPs with a high spatial resolution (Huang et al., 2009). In some situations, spraying UAVs might be more suitable than conventional spraying systems since they can spray areas that are difficult for workers or machinery to access, such as hilly or muddy plots. Some studies suggest that the use of spraying UAVs has several advantages over conventional terrestrial spraying systems, especially when compared to orchard (Sánchez-Fernández et al., 2023) and backpack sprayers (Wang et al., 2018; Sarri et al., 2019; Xiao et al., 2020).

Spraying UAVs can perform variable and spot spraying, reducing the application’s environmental impact and contributing to the sustainability of agricultural systems. They are more time efficient than conventional terrestrial sprayers, and they can spray approximately 4ha·h^-1, a significantly higher surface than a knapsack sprayer (Giles and Billing, 2015), while reducing the exposition to the operator. Furthermore, several studies suggest that the downwash airflow generated by the UAVs’ rotors may contribute to the penetration of the spray into the crop (He et al., 2017; Tang et al., 2017). Moreover, batteries power most commercial spraying UAVs, and their use might contribute to reducing the dependency on fossil fuels. However, there are still unknown aspects of the flight and structural parameters of UAVs that might affect the spray depositions, such as the distance between the rotors, their number, or the lifted weight (da Cunha et al., 2021). More studies in this field are needed to fully understand how all these parameters affect spraying UAVs’ depositions.

Spraying UAV technology, while promising, still faces significant limitations. The limited payload capacity of these systems restricts their application to ultra-low volume rates. Technologically, the payload limitation is being overcome by developing larger and heavier UAVs. However, one of the most vital limitations of this technology, especially in Europe, is the strict legal framework. UAV spraying is considered aerial spraying in most parts of Europe and, consequently, banned, except under exceptional circumstances and with minimal active ingredients approved. Comparative studies such as this are needed to determine if, under specific scenarios, UAV spraying systems can help reduce environmental exposure to PPPs compared to conventional terrestrial orchard sprayers.

Olive tree (Olea europaea) is the main permanent crop worldwide and one of the main crops in the Mediterranean region, covering 11.5 million hectares. Olive production is restricted to the Mediterranean climate areas, but a globally dispersed growing demand exists. As a result, the surface dedicated to olive orchards increases steadily to 162,000 hectares yearly (Vilar et al., 2018). Recently, superhigh-density orchards have gained prominence and represent most new plantations. Moreover, some traditional olive orchards are converted to superhigh-density or hedgerow orchards yearly due to their reduced human labor requirements, earlier returns on investment, consistency in yield, and efficient management (Lindell et al., 2023). Furthermore, some studies suggest that super high-density orchards may have some environmental benefits over traditional production systems, and they exhibit a lower impact on climate change per ton of production (Ben Abdallah et al., 2021). Currently, the spraying of PPPs in super high-density olive orchards is performed by terrestrial mist blowers. Introducing spraying UAVs in olive orchards has the potential to mitigate the environmental impact of these operations under specific conditions, supporting the sustainability of agricultural systems. Some studies assessing the drift generated by UAV sprayers have been published (Liu et al., 2020; Wang et al., 2021; Dengeru et al., 2022; Li et al., 2022). Nevertheless, the deposition parameters and airborne drift associated with UAV sprayers in superhigh-density olive orchards are still unknown. Our study is the first to compare the airborne drift, soil, and crop depositions generated by a UAV sprayer to those caused by a conventional orchard sprayer in super-high density olive orchards.

This study assesses the potential advantages of UAV spraying systems compared to conventional terrestrial systems concerning airborne drift and crop and soil depositions within specific conditions, particularly in superhigh-density hedgerow olive orchards. This study involves a comparative analysis of the airborne drift, crop, and soil depositions resulting from using a UAV sprayer and conventional terrestrial mist blower. Our trials were conducted in a representative commercial superhigh-density olive orchard whose characteristics and agricultural practices are common to orchards of the same type in the Mediterranean region. Our hypothesis states that UAV sprayers may generate less airborne drift while maintaining similar soil depositions to conventional terrestrial atomizers. Conventional orchard sprayers typically project the spray horizontally onto the crop’s canopy, resulting in a substantial portion of the applied volume passing through the canopy as drift.

Conversely, UAVs spray vertically, directing the spray downward over the crop with the assistance of the downwash airflow generated by the rotors. This downward airflow might promote spray penetration into the canopy, mitigating airborne drift. In specific scenarios, adopting UAV sprayers can help reduce the use of PPPs, consequently safeguarding and expanding areas of environmental interest adjacent to agricultural areas.

The chosen crop for our study is superhigh density olive-orchards. These orchards hold significant importance in the region, accounting for 46.7% of the total agricultural surface (SIGPAC, 2021). Andalusia contains 61% of the total olive surface of Spain, the main olive producer in the world.

We evaluated airborne drift and mass balance distribution to assess spray deposition and drift. In our analysis of swath width, we observed a decrease in the coefficient of variation (CV) as the height increased. This might suggest that the spray’s distribution trended to become more homogenized at higher altitudes, potentially due to the effect of the turbulence generated by the downwash airflow. In our study, the flight height had a limited influence on the UAV’s swath width. This suggests that, within the range of 1 to 3 m, the swath width was more dependent on other factors, such as the spacing between the rotors and the characteristics of the downwash airflow generated by these rotors.

Our results suggest that UAV sprayers generate significantly less airborne drift than conventional orchard sprayers. These results align with the findings of previous work (Sánchez-Fernández et al., 2023), which observed reduced sedimented spray drift from a UAV sprayer compared to a conventional terrestrial orchard sprayer. Our experiments observed no statistically significant differences in the airborne spray drift among the various heights tested for any of the studied sprayers. However, our data shows a trend of increasing airborne spray drift as height decreases; this phenomenon seemed stronger for the terrestrial orchard sprayer at 10 m downwind from the spraying area. This trend is consistent with observations in similar studies (van de Zande et al., 2014; Torrent et al., 2017; Gil et al., 2018; Wang et al., 2021).

The influence of the vortex generated by rotor blades interacting with the air and the interplay between the UAV’s wind field and the canopy can affect droplet deposition, as suggested in previous studies (Xue et al., 2014; Guo et al., 2019). Spraying at lower altitudes facilitates a more efficient projection of the spray onto the crop below, whereas, at higher altitudes, the vortex can emerge, leading to increased drift. Our airborne spray drift results support our hypothesis that downward spraying of the UAV sprayer results in less drift than a terrestrial orchard sprayer, even at heights of 1.5 m above the olive hedgerow, corresponding to a spraying height of 4 m above the ground. The mentioned vortex occurs at this height and induces significantly more drift than lower-altitude spraying. Despite the presence of the vortex and its potentially harmful effects, the UAV sprayer still generated significantly lower airborne spray drift than the terrestrial orchard sprayer.

In terms of soil depositions, our study revealed significantly higher depositions for the terrestrial orchard sprayer in the intra-row sampled positions. However, no statistically significant differences were observed in soil depositions for the inter-row sampled positions. Additionally, our results suggest a higher spray variability when using the UAV sprayer than the conventional orchard sprayer. These soil spray deposition results align with previous studies, suggesting that UAV sprayers tend to have lower droplet density and deposition uniformity. The spray drift generated by the conventional orchard sprayer while spraying adjacent rows generated soil depositions comparable to those generated by the UAV sprayer. Although no statistically significant differences were observed in the soil depositions collected from the inter-row positions, this result holds significant meaning, especially considering that the terrestrial orchard sprayer was a mist blower whose spraying area was adapted to the canopy’s surface, that it did not spray the soil, and that the swath width of our UAV sprayer was wider than the hedgerow width. In hedgerow crops, the swath width of the UAV sprayer becomes especially important, ideally it should be equal to or narrower than the hedgerow width to prevent the spraying of the crop’s interrow space. In our study, the swath width of the UAV sprayer was 2.25 m, while the hedgerow width was, on average, 1.66 m. This excessive swath width may be why we did not find significant differences in soil depositions between the two tested sprayers. Further optimization of this parameter may reduce the UAV’s soil depositions. Maintaining an appropriate swath width is essential to prevent off-target spray drift, which can decrease spraying efficiency and increase environmental exposure to plant protection products (PPPs). Commercial UAV sprayers cannot adapt their swath width to woody orchards’ heterogeneous and changing characteristics. Further research is needed in this aspect to ensure the precision of this technology in spraying applications.

Regarding crop depositions, our results show that the UAV sprayer’s crop depositions are significantly limited compared to the crop depositions generated by the conventional orchard sprayer. This behavior can be attributed to the distinct spraying parameters employed by the two spraying systems: the terrestrial orchard sprayer applies a high volume laterally to the canopy, while the UAV sprayer projects an ultra-low volume spray downward from the top of the canopy. The higher spray volume of the terrestrial orchard sprayer promotes better penetration but also leads to increased run-off from the canopy to the soil, which explains the higher soil depositions detected when using the conventional orchard sprayer. The terrestrial orchard sprayer generated more crop depositions in the middle section of the hedgerow, while the UAV sprayer produced more crop depositions at the top part of the canopy. This characteristic crop deposition pattern observed in the7nbsp;UAV sprayer is expected, given that drones spray from above the canopy. The phenomenon has been previously documented (Zhang et al., 2016; Martinez-Guanter et al., 2020; Li et al., 2021). The UAV sprayer did not exhibit statistical differences in crop depositions at the middle and lower levels of the canopy. This crop deposition pattern suggests that the downwash airflow associated with the UAV sprayer promotes better penetration and homogenizes the spray distribution at the middle and lower sections of the canopy.

This study demonstrates that UAV sprayers offer a significant advantage in reducing airborne drift compared to conventional orchard sprayers, particularly in super-high-density olive orchards. Our results also suggest that UAV sprayers generate soil depositions similar to conventional orchard sprayers. However, the practicality of using UAV sprayers may vary depending on the specific context and the desired crop depositions. Our findings suggest that UAV sprayers may be particularly well-suited for systemic or bait products that do not require extensive crop coverage for their efficacy. Moreover, in scenarios where spot spraying is necessary, UAV sprayers could prove valuable and efficient. Introducing UAV sprayers for systemic or bait PPPs, particularly in cases where ultra-low volume or spot spraying is effective, seems promising for reducing environmental exposure.

The findings presented in this work can be useful for quantifying the implications and the impact of the introduction of UAV sprayers in the Mediterranean agricultural environment and assessing the benefits of this technology to reduce the use and mitigate the consequences of plant protection product spraying. Introducing UAV sprayer technology might offer advantages in reducing the usage of PPPs and addressing their environmental impacts. However, further research is needed to fully understand how the spraying parameters affect the performance of UAV sprayers and to find ways of adapting UAV sprayers’ swath width to the requirements of each crop.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

LS-F: Data curation, Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review & editing. MB-B: Data curation, Investigation. JM-G: Conceptualization, Funding acquisition. MP-R: Funding acquisition, Investigation, Writing – original draft.