An empirical model based on phenological growth stage for predicting pesticide spray drift in pome fruit orchards

An innovative spray drift model is developed to describe downwind deposits of pesticides applied in an orchard of pome fruit trees (apple, pear). The empirical model is based on 20 years of experimental data of downwind deposits of spray drift for conventional cross-flow spray applications. The model reveals the major factors affecting downwind deposits: wind speed, wind direction, air temperature and density of the tree canopy. Modelling the canopy density of the trees as a continuous function of time is an innovative approach. Canopy density is uniquely related to growth stage through the phenological BBCH index. Observed effects of the mentioned factors on deposits are discussed. Model results and measured deposits show a correlation coefficient of 87%, while covering a range of almost three orders of magnitude. The model forms the basis for risk assessment for exposure of aquatic organisms concerning all edge-of-field water bodies in the Netherlands. Implementation of drift mitigation techniques is straightforward when appropriate experimental data on reductions of downwind spray deposits is available.