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Developments to the Silsoe Spray Drift Model: investigations of air flows. - PS2030

The Silsoe Spray Drift model was developed from an earlier model as part of the BREAM project aimed at producing a bystander and resident exposure assessment model. The model has been successful in predicting airborne spray and ground deposits under relatively limited conditions: a single nozzle design and size spraying over a short crop or bare ground. There is a need to ensure that the model can be used more widely with some confidence and therefore this work will aim to improve the model in three specific areas:

1. Detailed investigations of the airflows within a spray and their interaction with forward speed and cross wind.
This component of the research will allow nozzles to be better parameterised in the model andl enable a wider range of nozzles to be included in the model without the need for extensive field trials to 'calibrate' model output.

A previous project (PS2025) explored techniques for investigating air velocities within and around a spray plume and identified an appropriate methodology using the Oxford Lasers Visisizer for determining entrained air velocotoes when small droplets are present, i.e. within the plume. Measurements of entrained air flow within the spray of a multi-nozzle boom have been made successfully, and the findings are soon to be incorporated into an updated model. There is a need to continue investigating techniques for measurement of such air flows with a moving boom in a cross flow. This will require the development of new techniques - probably based on the Visisizer imaging system, together with visualisation, passive spray collection and other novel methods for assessing the interactions that might influence spray drift.

2. Drift from applications to a tall crop
Limited resources were available in BREAM to address the filtering mechanism of a tall (>0.5 m) crop on spray drift. Field work will be undertaken to establish (a) air flows within a crop canopy, (b) the movement of droplets through a crop canopy and (c) drift profiles downwind of a crop canopy. It is proposed that two crop types are considered - a mature cereal canopy and an oilseed rape canopy.

3. Measurements of wind characteristics
The BREAM model provides a probabilistic assessment of the potential exposure by considering variability in two model inputs - boom height and wind speed. The statistical description of the wind in the BREAM model was based on atypical data sets - short bursts of measurements that are made during a spray drift trial. While there is a wealth of published information relating to air flows in the atmospheric boundary layer, project PS2025 found that there was a focus on extreme conditions where spraying would not occur and a shortage of data relating to moderate and typical wind conditions over UK arable fields. A further element of this project will aim to provide more appropriate wind speed data to input into the model
Measurements will be made of the wind characteristics in arable fields over up to ten typical spraying days throughout the spraying season. The data will be analysed to determine the probability distribution of wind speeds that can be used in predictions of spray drift, as a function of a nominal mean wind speed that might be measured by a spray operator as well as parameters that characterise turbulence that are currently used in the model.

4. Analysis and modification of model
Following parts 1-3, the need for the model to be revised and updated will be assessed, the model will be modified appropriately and tested. Where possible, it will be validated against existing experimental data.
Main objectives are:
• Characterisation of air velocities with the spray of a moving nozzle in an air flow
• Characterisation of wind over agricultural fields during typical spraying days
• Improvements in the description of droplet movements within a crop canopy or field margin vegetation
• Improvements to the model based on the findings and including additional nozzles where possible
Project Documents
• EVID4 - Final project report : PS2030 final report   (1169k)
Time-Scale and Cost
From: 2011

To: 2014

Cost: £75,456
Contractor / Funded Organisations
The Arable Group Limited
Pesticide Residues              
Fields of Study
Pesticide Safety