Improving chemigation efficacy by controlling droplet size distribution of oil-based pesticides

Abstract

For chemigation of nonsoluble pesticides, small oil-pesticides droplets (dmax tend to wash-off from foliage while large droplets tend to stick. Large droplets (dmax are buoyant, tend to rise in the irrigation pipeline and exit at the beginning of the pipeline; thus, uniformity and efficacy are poor. For this research, a new chemigation system was proposed. The system removes water from the irrigation pipeline, injects the oil-pesticide into the water stream, increases dispersion velocity in successively smaller tubing diameters, and finally injects the dispersion back into the irrigation pipeline. The higher velocity flow with high turbulent shear forces breaks the oil-pesticide into desired size droplets. Droplet break-up research was reviewed, and a model developed to predict maximum droplet size and size distribution. A maximum relative error of 40% was observed when dmax predicted by the model was compared against literature data. Equations to predict friction factor in helically coiled pipes and effective viscosity of oil-in-water dispersions were evaluated. The friction factor predicted by the Ito equation was in good agreement with the experimental data. Effective viscosity of soybean oil- and kerosene-in-water dispersions was predicted satisfactorily by the Richardson equation with k₄ = 2.5. Finally, center pivot field experiments were conducted using the new and conventional chemigation systems. For the conventional system, the soybean oil uniformity coefficient along the lateral was 61%, and oil applied over the last tenth of irrigated area was 9% of the initial concentration. For the new system, the uniformity coefficient was 73% and 98% for dmax of 875 mum and 98 mum, respectively; oil applied over the last tenth of the area was 27% and 90% of the initial concentration. Field data were compared with those predicted from a pipeline transport model for nonsoluble pesticides. Agreement between the model and the field data was excellent for both experiments using the new chemigation system. Based on the field results and simulation analyses, droplets < 150 μm should be desirable to keep the discharge uniformity coefficient over 97%, for 0.92 ≤ ρ(d)/ρ(c) ≤ 1.04.