A NUMERICAL-MODEL FOR FORESTRY AERIAL SPRAYING

A numerical model for the aerial application of pesticides in forestry aerial spraying is described. The model predicts canopy and ground de position and drift patterns for evaporating droplets released over a f orest canopy by an aircraft flying at right angles to the wind directi on. Droplet trajectories are initially calculated from aircraft wake v elocities. Wake velocities are generated from a second-order closure m odel for the turbulent transport and dissipation of aircraft vortices. When the aircraft wake has decayed to background levels of turbulence , droplet trajectory calculations are continued with a Markov type of Lagrangian model for particle dispersion in a turbulent atmosphere. Mo delled predictions for off-target drift, and foliage and ground deposi t within a 400 m fetch are compared with field trial data for six diff erent spray experiments each for insecticide application (foliage area index: 3.2 m(2) foliage per m(2) ground) and for herbicide applicatio n (foliage area index: 0.48 m(2) foliage per m(2) ground). Model predi ctions of the downwind location of peak canopy deposit are within 18% and 20% of the experimental values for the insecticide and herbicide c ases, respectively, on average. Canopy deposit predictions are 10% hig her and 26% lower on average than experimental values, for insecticide and herbicide tests, respectively. Ground deposit predictions for the herbicide tests are 12% higher than experimental values, on average, but for the insecticide tests, computed values were significantly high er than measured values partly because the measurements in that case a re not accurate. Model predictions in general agree very well with exp eriments for a wide range of atmospheric stability values and it is co ncluded that the model is an ideal method of evaluating and optimizing aerial spray applications.