An empirical model has been developed for computing downwind deposits of spray drift after spray applications in fruit orchards. A database of downwind spray deposits from experiments during 20 years of field trials was used in regression analysis to show the relevant parameters: wind speed, wind direction, ambient temperature, growth stage (BBCH code), the sprayer’s fan setting and orchard size. The basic regression model was modified into a generic drift model for fruit crops like apple and pear, using known or logical relationships between in the input parameters and their effect on spray drift. A major advantage of the new model with respect to current practice in regulations is the possibility to estimate drift as a continuous function of growth stage, rather than using only a few fixed growth stages. This study also describes the implementation of the spray drift model into a risk assessment model to evaluate the risk for aquatic organisms regarding exposure to pesticides in watercourses surrounding fruit orchards. The risk assessment model takes account of regional differences with respect to presence of fruit orchards, types of water bodies and weather conditions. Multiple spray applications during the growing season are accounted for into the risk model. Both the spray drift model and the risk assessment model are still under development. Preliminary results show a good correlation between modelled deposits and measured deposits: a correlation coefficient of 90% was obtained. The risk assessment model indicates that different regulatory measures may be required for fast-degrading and persistent pesticides.
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