Modelling spray drift from boom sprayers

A random-walk model, IDEFICS, is described to compute downwind spray drift from conventional boom sprayers in chemical crop protection. Spray deposits are computed downwind from the sprayed crop field. The model basically is two-dimensional (2D), but close to the spray nozzle the model is 3D, incorporating driving speed and entrained air-currents below the nozzle. Input parameters are related to the geometry of the field, to the boom sprayer settings and to environmental factors. The model was calibrated with a set of field trials using an experimental single-nozzle sprayer in a cross wind. In the trials, tap water containing a fluorescent dye was used as the spraying liquid, and downwind deposits were measured by fluorimetry. Variations of boom height, spray nozzle size, driving speed and liquid pressure were examined, at varying wind speeds. Both experiments and simulations showed that boom height, wind speed and nozzle size were the major factors affecting spray drift. Surprisingly, liquid pressure did not affect downwind spray deposits at all. A comparison between model results and results of a practical field trial showed a good agreement if field trials were averaged over several replications. The variation between individual replications is however too large to use single trials for model verification. Further calibration trials are needed to investigate the effect on drift of multi-nozzle arrangements, crop height and medium- and coarse-sized spray nozzles.