G. J. Dorr

The entrapment of particles by windbreaks

A theory is developed for calculating the entrapment of particles by a windbreak, with four results. (1) The fraction of particles in the oncoming flow which pass through the windbreak, or transmittance of the windbreak for particles (sigma), is related to the optical porosity (tau). The very simple approximation sigma=tau works well for most applications involving the interception of spray droplets by windbreaks. Results from a field experiment agree with the theoretical predictions. (2) A new equation for the bulk drag coefficient of a windbreak is tested against numerical, wind tunnel and field experiments. This enables the bleed velocity for the flow through the windbreak to be predicted in terms of the screen pressure coefficient (k) of the barrier. (3) The relationship between k and tau is different for a vegetative barrier than for a screen across a confined duct, implying a lower Fc for given tau. (4) The total deposition of particles to a windbreak is determined by a trade-off between particle absorption and throughflow, implying an optimum value of tau for maximum total deposition. For particles larger than 30 mum and vegetation elements smaller than 30 mm, this occurs near tau = 0.2

Spray deposition on plant surfaces: a modelling approach

For pesticides to effectively manage pests, they must first be deposited on the target (typically a plant surface) in a manner in which the active ingredient(s) can be readily taken up by the target organism. A plant architectural model that enables the location of various plant components in 3-D space combined with a particle trajectory model has been used to study the interception of spray droplets by various vegetative elements. Results from the simulation are compared with wind tunnel studies of glyphosate deposition on cotton (Gossypium hirsutum L. var. Sicala), sow thistle (Sonchus oleraceus L.) and wild oats (Avena ludoviciana Durieu). An air induction flat fan nozzle (AI110015 at 500 kPa pressure) and an extended range flat fan nozzle (XR11002 at 280 kPa pressure) were predicted to have similar glyphosate deposition on cotton and sow thistle plants, whereas the extended range nozzle resulted in higher deposit on wild oats. Spray deposition (µg cm-2) on wild oat plants at the 5-leaf stage was more than double the amount of deposition on sow thistle or wild oat plants at the 2-leaf stage. The model was in good agreement with the experimental data except that it tended to over predict deposition on sow thistle plants.

A simple guide to predicting aircraft spray drift

A sensitivity analysis has been performed on a simple Gaussian Diffusion Model used to predict spray drift deposition from agricultural aircraft at far wake distances. It is intended that the model be used as a training tool to demonstrate clearly the basic effects of windspeed, turbulence intensity, release height, droplet size and uniformity on far downwind spray drift deposition i.e. at distances greater than 50 m. Overlapping the model outputs has also provided a useful benchmark against which experimentally obtained spray drift values may be compared. This information has assisted in the preparation of guidelines regarding required buffer zone distances and the best ways in which aircraft pesticide spray drift can be effectively reduced and managed.

Impaction of spray droplets on leaves: influence of formulation and leaf character on shatter, bounce and adhesion

This paper combines experimental data with simple mathematical models to investigate the influence of spray formulation type and leaf character (wettability) on shatter, bounce and adhesion of droplets impacting with cotton, rice and wheat leaves. Impaction criteria that allow for different angles of the leaf surface and the droplet impact trajectory are presented; their predictions are based on whether combinations of droplet size and velocity lie above or below bounce and shatter boundaries. In the experimental component, real leaves are used, with all their inherent natural variability. Further, commercial agricultural spray nozzles are employed, resulting in a range of droplet characteristics. Given this natural variability, there is broad agreement between the data and predictions. As predicted, the shatter of droplets was found to increase as droplet size and velocity increased, and the surface became harder to wet. Bouncing of droplets occurred most frequently on hard-to-wet surfaces with high-surface-tension mixtures. On the other hand, a number of small droplets with low impact velocity were observed to bounce when predicted to lie well within the adhering regime. We believe this discrepancy between the predictions and experimental data could be due to air layer effects that were not taken into account in the current bounce equations. Other discrepancies between experiment and theory are thought to be due to the current assumption of a dry impact surface, whereas, in practice, the leaf surfaces became increasingly covered with fluid throughout the spray test runs.

Surface reconstruction of wheat leaf morphology from three-dimensional scanned data

Realistic virtual models of leaf surfaces are important for several applications in the plant sciences, such as modelling agrichemical spray droplet movement and spreading on the surface. In this context, the virtual surfaces are required to be smooth enough to facilitate the use of the mathematical equations that govern the motion of the droplet. Although an effective approach is to apply discrete smoothing D2-spline algorithms to reconstruct the leaf surfaces from three-dimensional scanned data, difficulties arise when dealing with wheat (Triticum aestivum L.) leaves, which tend to twist and bend. To overcome this topological difficulty, we develop a parameterisation technique that rotates and translates the original data, allowing the surface to be fitted using the discrete smoothing D2-spline methods in the new parameter space. Our algorithm uses finite element methods to represent the surface as a linear combination of compactly supported shape functions. Numerical results confirm that the parameterisation, along with the use of discrete smoothing D2-spline techniques, produces realistic virtual representations of wheat leaves.

Economics of improved spatial distribution of herbicide for weed control in crops

Abstract There are various causes of unintended spatial variability of herbicide dose in a crop, but each causes a reduction in profitability. The specific problem of spatial variability of the herbicide diclofop-methyl in postemergence control of ryegrass ( Lolium rigidum ) is examined. A model of wheat yield response to herbicide application has been implemented in a microcomputer and used to analyse the economics of spatial variability. The economically optimal herbicide dose was found to be insensitive to spatial variability, but marked effects on profitability were observed. The costs of spatial variation of herbicide are estimated to be as high as 25% of net returns in some scenarios. For this case study, profit was found to be more sensitive to variance of herbicide dose within the path of the spray boom (due to nozzle design, wind, boom roll, etc.) than to variance due to overlap or underlap of the boom in successive circuits of the field.

Determining the drift potential of Venturi nozzles compared with standard nozzles across three insecticide spray solutions in a wind tunnel

BACKGROUND Previous research has sought to adopt the use of drift-reducing technologies (DRTs) for use in field trials to control diamondback moth (DBM) Plutella xylostella (L.) (Lepidoptera: Plutellidae) in canola (Brassica napus L.). Previous studies observed no difference in canopy penetration from fine to coarse sprays, but the coverage was higher for fine sprays. DBM has a strong propensity to avoid sprayed plant material, putting further pressure on selecting technologies that maximise coverage, but often this is at the expense of a greater drift potential. This study aims to examine the addition of a DRT oil that is labelled for control of DBM as well and its effect on the drift potential of the spray solution. The objectives of the study are to quantify the droplet size spectrum and spray drift potential of each nozzle type to select technologies that reduce spray drift, to examine the effect of the insecticide tank mix at both (50 and 100 L ha(-1) ) application rates on droplet size and spray drift potential across tested nozzle type and to compare the droplet size results of each nozzle by tank mix against the drift potential of each nozzle. RESULTS The nozzle type affected the drift potential the most, but the spray solution also affected drift potential. The fine spray quality (TCP) resulted in the greatest drift potential (7.2%), whereas the coarse spray quality (AIXR) resulted in the lowest (1.3%), across all spray solutions. The spray solutions mixed at the 100 L ha(-1) application volume rate resulted in a higher drift potential than the same products mixed at the 50 L ha(-1) mix rate. The addition of the paraffinic DRT oil was significant in reducing the drift potential of Bacillus thuringiensis var. kurstkai (Bt)-only treatments across all tested nozzle types. The reduction in drift potential from the fine spray quality to the coarse spray quality was up to 85%. CONCLUSION The addition of a DRT oil is an effective way to reduce the spray solution drift potential across all nozzle types and tank mixes evaluated in this study. The greatest reduction in drift potential can be achieved by changing nozzle type, which can reduce the losses of the spray to the surrounding environment. Venturi nozzles greatly reduce the drift potential compared with standard nozzles by as much as 85% across all three insecticide spray solutions. Results suggest that a significant reduction in drift potential can be achieved by changing the nozzle type, and can be achieved without a loss in control of DBM.

Deposition of aerially applied spray to a stream within a vegetative barrier.

Drift of aerially applied forest herbicides can result in chemical deposition to streams. Riparian vegetation is expected to attenuate drift, but there is little corresponding data. A field study was conducted in the Coast Range west of Corvallis, Oregon, to evaluate the effectiveness of forested riparian buffers. The buffers studied are typical of those used for small and medium fish-bearing streams in western Oregon as mandated by the Oregon Forest Practices Act. A helicopter sprayed two tracers over four transects. Twenty trials were conducted, resulting in over 1400 tracer samples. Results confirm that these vegetative barriers are effective at reducing deposition into streams. Reduction of deposition on artificial foliage samplers placed immediately above the stream surface ranged from 37% to 99% and averaged 92%. Reductions were less clear in stable atmospheric conditions due to low wind speed and highly variable wind directions. Low wind speed conditions are not generally high-drift scenarios, but there is evidence that drift of suspended droplets beyond the barrier, comprising a small fraction of the total mass, increases in stable conditions.

Predicted Deposition Variability Due to Fluctuations in Release Height and Drop Size Distribution

An extensive field study has been undertaken to quantify the aerial release of spray material through the changes in meteorology as the day progresses. An important subset of these collected data is one-second interval data of the aircraft behavior and the mechanical release systems. These unique data provide an excellent source of information on bounding the variability in the expected deposition patterns, and how this variability might impact any error bounds established around the time-averaged predictions generated by the AGDISP model. This paper quantifies the variability in aerial application parameters and makes suggestions with regard to possible implications of this variability on the variability of deposition predictions in the flight line direction.

Preliminary investigation of Phase Doppler derived flux measurements in a wind tunnel for the sampling of orchard spray drift

Introduction Hand-held spray guns and lances are the most widely used methods of crop protection in greenhouses despite the heavy workload and high risk of operator exposure associated with these techniques (Foque 2012).