Spray drift modelling using field-measured PDI laser techniques

Abstract

The modeling of airborne spray drift and ground deposition to off-target areas requires a knowledge of the initial source of particles such as the emission droplet size, velocity and flux spectrum (the particle cloud “available” to drift after any deposition on the intended target), the transport conditions of the spray such as the wind speed, direction, temperature and relative humidity as well as atmospheric stability, and any spray interception by foliage, vegetation and other structures. Established models such as AGDISP are widely used in North America and Australia for such modelling of conventional sprays, but currently include mainly aerial application platforms with some preliminary ground boom sprayer options. The spray release direction for these aerial and ground boom sprayer scenarios is downwards. Tree and vine crops sprayers involve spray release in several directions including down, sideways and upwards. AGDISP does not offer a way to model the latter and there are no “buttons” to press in the models to cover many drift reduction technologies (DRTs) such as addition of a hood, cover, shroud, shield as an equipment modification or the addition of an electrostatic charge. Certain other DRTs such as evaporation reduction systems or narrow droplet size spectra atomizers can be accommodated in the AGDISP model through input of these variables in the appropriate model screens. A modified ground modeling system called WTDISP was described by Hewitt (2008). However, the model is limited to use with wind tunnel data only and has not been used for spray deposition modeling because the inputs cannot all be measured using the preferred single measurement system of monofilament lines. A new approach to data collection for this model is discussed in the present paper, based on a Phase Doppler Inteferometer (PDI) developed for the measurement of sprays in the field and in wind tunnels. The measured droplet size and flux data can be processed to distances of up to 100 m. Validation against field deposition data to distances up to 200 m will allow future extensions of the model range to encompass most DRT interest in Australia and New Zealand.