Spray deposition efficiency of unmanned aerial spraying systems in hillside vineyards with variable slope

Abstract

The adoption of unmanned aerial spraying systems (UASS) represents an innovative approach to sustainable plant protection in vineyards, particularly in areas with irregular topography. This study evaluated the spraying efficiency of a four-rotor UASS with 30 L tank capacity and centrifugal nozzles in a vineyard trained to a vertical shoot position (VSP) system, focusing on two droplet size settings, T1 (170 μm) and T2 (305 μm), across three terrain slope zones. These included Z1 (<5 %), representing flat areas, Z2 (10–15 %) for moderate slopes, and Z3 (>15 %) covering steep sections of vineyard. Water-sensitive papers (WSP) were placed in both the upper and bottom canopy layers, as well as at ground level, to assess deposition. Vegetation indices, including the normalized difference vegetation index (NDVI) and canopy area (CA), were derived from unmanned aerial vehicle (UAV)-based multispectral imagery to characterize vine vigor and canopy growth variability. Spray coverage (%) varied significantly with both droplet size and topographic zones (Z1-Z3), reflecting the influence of spatial heterogeneity. T1 consistently achieved higher canopy coverage than T2. Regardless of the treatment and slope zone, deposition was higher in the upper canopy than in the bottom layers. The flat area, which exhibited higher NDVI and CA values, showed greater spray deposition compared to sloped areas, where vine growth was more limited. In the upper canopy, T1 coverage showed strong positive correlations with NDVI (R² = 0.78) and CA (R² = 0.64). In contrast, in the lower canopy, T2 showed significant inverse correlations, vines with higher NDVI and CA values exhibited lower spray coverage. Interestingly, the results of the two-way analysis of variance (ANOVA) indicated that terrain slope did not have a significant effect on spray coverage or relative span (RS) parameters at ground level (p > 0.05), suggesting that topographic variability does not have a substantial impact on deposition losses. This supports the conclusion that vineyard canopy structure, rather than slope, is the main factor determining spray distribution patterns. Overall, this study highlights the potential of UASS equipped with centrifugal nozzles for precision spraying in vineyards, while also identifying limitations in achieving uniform coverage within multilayered canopies.