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A model for the capture of aerially sprayed pesticide by barley S.J.Cox, D.W.Salt, B.E.Lee & M.G.Ford University of Portsmouth, U.K.

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Introduction §Chemicals in agriculture - problem of growing environmental concern §Wind drift of spray chemicals off target §Knowledge of spray deposition patterns on plants can reduce the volume of chemical used §Role of mathematical modelling

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Introduction §This work aims to develop an improved method of treating the capture of pesticide spray by a crop for use with a trajectory model of droplet transport §Previous methods have relied upon treating the crop in an averaged, homogeneous manner

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Introduction §Here we use a realistically modelled crop which should allow a more detailed consideration of particular elements of the crop structure and capture process than previously

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Transport §the simulation follows the fate of individual droplets at evenly spaced points in time §movement of droplet controlled by gravity and airflow l mean wind profile logarithmic above crop exponential within crop l statistical treatment of turbulence §initial transport modelled by a combination of ballistic and random-walk motion (Mokeba et al., 1997)

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Ballistic Model §Marchant (1977) specifies the instantaneous acceleration of a droplet, §the Runge-Kutta algorithm is simultaneously applied to velocity and diameter of the droplet to obtain the ballistic velocity components

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Random Walk Model §simple Markov Chain model for droplets moving with the airflow except for the addition of their sedimentation speed, §additional terms required to correct for aspects of non- physical behaviour (Legg, 1983) §parameters from Walklate (1987)

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Combined Model the ballistic and random-walk models are combined via a weighting parameter §this weighting accounts for the increasing influence of turbulence on the droplet as it slows to its sedimentation speed

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Crop Model §realistic barley plants determined by a series of parameters taken from measurements on real plants §this allows the effect of real properties of the crop to be investigated in an intuitive manner and produces detailed results

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Crop Effect on Airflow §mean wind profile - affected by the density of the crop normal to the wind and its height (Raupach, 1994), relative magnitude determined by the friction velocity §turbulence statistics (Walklate, 1987) related to the crop via its height and the friction velocity

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Crop Effect on Airflow §Reynolds stress gradient with height - affected by the density of the crop normal to the wind at each height (Raupach & Thom, 1981)

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Droplet Trajectories 10 x 150 m 10 x 100 m

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Capture §Interception - comparison of positions in space of droplet and crop §Deviation - around plant elements caused by local deviation of airflow, impaction efficiencies of May & Clifford (1967) are used

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Capture §Rebound - droplets of the size considered here are prone to rebound rather than to be retained by the leaf, critical speed approach of Lake & Marchant (1983) is used,

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Plant Distribution

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Field Distribution §off-target drift when spraying a square area of field with wind angles of 0° and 60°

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Field Distribution §amount of drift outside the spraying area and 5 m buffer zone §the direction of the wind affects more than just the location of off- target drift; it can also affect the total amount of this drift

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Conclusions §Possible improvements to model: l use more refined transport model l include more interaction between crop and airflow including coherent crop waving l use of a more widely applicable model of rebound would allow greater confidence in the results for the smallest droplets and perhaps allow better account to be taken of leaf characteristics

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Conclusions §Possibilities opened up by method: l an investigation of the detailed effects of plant structure l to see detail of droplet distribution on plant - use as input to further models of pesticide action §The role of rebound in the preferential penetration of certain droplet sizes through the crop may be as important as differences in impaction efficiencies

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