Modelling spray drift in precision spray applications

In general, precision application of plant protection products (PPPs) results in a reduction of the amount applied compared to a full field application. Therefore, a lower exposure to PPPs of different off-field areas is expected. At present, precision applications are not adequately acknowledged or assessed in the authorisation process of PPPs.
Typically, variable-rate and spot-spraying applications can be distinguished. Variable-rate application is a whole field treatment where the minimally effective dose is applied based on the conditions for each location in the field. In a spot-spraying application the decision is made whether or not a (full dose) spray application is required for each location. Finally, a hybrid form exists where variable-rate and spot-spraying are combined.
The effects of precision spray applications in arable crops are assessed regarding the downwind off-field deposits of spray drift. A spray drift model was developed that takes into account the non-uniform dose rates on the crop field. In the model, field sections receiving a certain PPP dose rate are identified as polygons, which act as (very) small crop fields, treated independently. Each polygon contributes to downwind deposits of spray drift, depending on its area, the applied dose rate and the effective distance to the off-field area. The total off-field deposits of spray drift are computed as the sum of the contributions of the polygons. Test cases are elaborated with edge-of-field watercourses as the off-field areas exposed to PPP. Watercourses are imaginarily divided into smaller segments, for each of which spray drift deposits are determined. Example computations indicate that drift reductions can be higher or lower than the reduction merely based on the lower amount of PPP used in precision application. This depends not only on the location and distribution of treated polygons, but also on wind direction.