Spray Model to Predict Deposition in Air-Carrier Sprayer Applications

Abstract

On-target deposition and distribution of pesticide spray material within target tree canopies are of critical importance in air-carrier spray applications. In order to maximize canopy spray deposition, a model has been developed to predict on-target deposition of spray material, accounting for evaporation, drift, and ground deposition. The model was developed as a compartment model where, at a discrete position in the direction of the sprayer travel, the plume or spray cloud is handled as passing through several connected compartments of equal thickness but increasing cross section, in the direction of application. The model simulates the mass dispersion of spray material, assuming no slip between spray droplets and airstream and no contribution to the sprayer air velocity from spray droplets. The tree canopy sub-model accounts for the foliage distribution in the direction of spray application, which in turn represents the canopy resistance to spray transport and deposition. The model response has been observed under no-tree (which concerns just dispersion in the free space without a target) and with-tree (includes both dispersion in the free space and deposition inside a target canopy) conditions. Using spray volume rate, air velocity, sprayer ground speed, target canopy distance, and canopy foliage density as input factors, each with three levels, 243 simulated deposition values were obtained from a complete factorial experiment. Global sensitivity analysis on this data showed that target canopy distance contributes the most to the variation in the mean deposition data. Also, spray volume rate and canopy density were also found to contribute significantly, but sprayer ground speed and air velocity effects were not found to be significant. The model has the potential to assist spray applicators in effectively planning spray programs to achieve high on-target deposition.