An Experimental Study of Momentum and Heavy Particle Transport in a Row-Oriented Agricultural Canopy

The transport of particles through the air by turbulence plays an important role in plant ecosystems. One example is the spread of airborne plant pathogens. Without turbulent dispersion, disease proliferation would be limited to spread from vectors or gravity thereby limiting epidemic severity. The interaction of turbulence with plant canopy architecture and the effect this has on particle dispersion in and above the canopy is not well understood. To better understand the underlying turbulent transport mechanisms associated with row-oriented agricultural canopies, a series of field experiments were performed in a grape vineyard in Oregon's Willamette Valley during fall 2010 and 2011. Turbulence in the vineyard canopy was measured using an array of four sonic anemometers deployed at various heights in and above the canopy. A series of point release particle dispersion experiments were conducted simultaneously using inert particles with the approximate size and density of grape powdery mildew (Erysiphe Necator) spores. Particle concentrations were measured using a roto-rod impaction trap array. Measurements from the sonic anemometers demonstrate that first- and second-order statistics of the wind field are dependent on the orientation of the mean wind with respect to the vineyard row orientation. This dependence is attributed to channeling within the canopy that transfers energy between the velocity components when the mean wind direction is not aligned with the rows. In contrast, spectral analysis indicates that the structure of the turbulent flow is not fundamentally altered by the interaction between mean wind direction and row direction. Examination of a number of particle release events indicates that the wind turning and channeling observed in the momentum field does indeed impact particle dispersion characteristics. For row-aligned flow, particle dispersion in the direction normal to the flow is decreased relative to the plume spread predicted by a standard Gaussian plume model. For flow that is not aligned with the row direction, the plume is found to rotate in the same manner as the momentum field thus causing spread to areas not predicted by the Gaussian model.