Standard Spray Drift Risk Assessment Scenarios

The APVMA uses a series of standard spray drift risk assessment scenarios to tailor its risk assessments to the most common situations.

The APVMA has developed these standard scenarios to address the most common agricultural and forestry spray practices used in Australia.  They are intended to help readers understand how the APVMA assesses spray drift risk.

The standard scenarios are divided into three major groups - ground applications, aerial agricultural applications and aerial forestry applications. Each standard scenario is available in an Excel file to download below and contains spray drift deposition data for a particular type of spray operation.

Note: Background discussion about these scenarios can be found in section 7.3 of the spray drift risk operating principles.

Each Excel file contains the deposition data as a fraction of applied field rate listed every two metres downwind from the application area. Deposition from ground applications is projected to approximately 300 metres downwind and for aerial applications to approximately 800 metres. Each file also contains a graph of the deposition data as well as a summary of the input parameters (on separate worksheets following the graph).

The 19 standard scenarios are grouped according to method and introduced below with links to each scenario.

Scenarios in Each Group

Ground Application

The ground deposition data provided in this section is based on a large reference data set from studies conducted in the USA in conjunction with the US Environmental Protection Agency.

The studies were grouped into the 11 categories shown in these scenarios and a ‘best fit’ curve was applied for each group. These reference curves were built into a spray drift assessment software program called AgDRIFT from which the deposition data can be taken.

Orchard and Vineyard Airblast

Broadacre High Ground Boom

Broadacre Low Ground Boom

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Aerial Agricultural Application

These aerial application scenarios rely on modeling software called AGDISP developed by the US Government. AGDISP is a 'true model' based on first principles that allows the use of different parameters to examine spray release height, wind speed, temperature, humidity and many others parameters including relevant features of different aircraft. Summaries of the input parameters are included with each scenario. The scenarios use the AT502 as the representative fixed wing aircraft and the Bell 206BIII as the representative helicopter. Each scenario displays downwind deposition data for ASAE droplet size categories each modeled for three different wind speeds.

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Aerial Forestry Application

The great majority of forestry applications are herbicides applied pre-planting and during establishment, up to a time when the trees are about two metres high. There are many fewer insecticide applications.

There are five aerial forestry application scenarios in this section; three for level ground and two for sloping ground. The two for sloping ground will be the ones the APVMA uses most often to assess and manage risk. The level ground versions will mainly be used for permit assessments when the APVMA can be assured that a use would be limited to level forestry plantings.

In addition to these five helicopter scenarios, there is one aerial fixed-wing scenario specifically for applications to bananas. It is included in the forestry section because the forestry features of AGDISP were used in its modeling.

Summaries of the input parameters are included with each scenario.

Read how the APVMA uses this information to determine the size of protective no-spray zones.

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Further Information about each Major Group of Scenarios

Ground Application Standard Spray Drift Scenarios

The deposition data in the standard ground scenarios are based on a large reference data set from studies conducted in the US in conjunction with the US Environmental Protection Agency. The various studies were grouped into the categories shown in these scenarios and a ‘best fit’ reference curve was applied to the collected data for each group. These reference curves were built into a spray drift assessment software program called AgDRIFT that was used to produce the detailed deposition list presented in these standard ground scenarios.

Because these ground data are not produced by a true model, meaning a mathematical model that can change its output as a result of changed input parameters, the ground scenario sets do not contain detailed input parameters as do the aerial application scenarios where a ‘true model’ has been used. Although the ground scenarios cannot be modeled to reflect changing wind speeds and other relevant variables, the large number of high quality reference studies provides a rigorous basis for the common situations the APVMA needs to assess.

The ground studies were done under real-world conditions including the higher wind speeds that the APVMA would use in a true model. The ground scenarios display downwind deposition to a distance of approximately 300 metres.

Note that a previously included standard scenario called “Normal Orchard” has been deleted. This scenario has been re-examined and determined to be misnamed.  The designation “Normal Orchard” originally assigned by the US Spray Drift Task Force is inappropriate because this spray drift deposition set is based on a major component of vineyard data making it not appropriate for risk assessment of orchard applications.  It will not be used and has been deleted from the APVMA’s group of standard scenarios.  (A separate set specifically for vineyards remains for vineyard assessment.)  For spray drift risk assessment of orchards, the APVMA will use, “Composite Orchard”, “Dense Orchard” or “Sparse Orchard” depending on the particular situation.

A figure that combines the standard ground scenarios into one graph is presented here for easy comparison of the different application methods. This figure shows only the first 60 metres of deposition so that differences between the various scenarios are more apparent.

Ground scenarios graph

Graph showing spray drift deposition from ground scenarios.

Open a larger scale image of the All Ground Scenarios Graph

In this graph, zero on the horizontal axis represents the downwind edge of the application area. The wind is blowing from left to right. The numbers along the horizontal axis represent the distance in metres downwind from the application area.

The airblast scenarios are based on standard radial airblast applications where the machine was set up optimally for each situation. For the ground boom situations, ‘high boom’ refers to a boom set 1.27 metres above the ground and ‘low boom’ refers to one placed 50 centimetres above the ground. Coarse, Medium and Fine refer to the standard ASAE droplet size categories.

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Aerial Application Standard Spray Drift Scenarios

All the aerial application standard scenario data sets for both agriculture and forestry contain predicted downwind deposition from a validated mathematical spray drift model called AGDISP. AGDISP is a true model based on first principles and allows examination of differing conditions such as spray release height, wind speed, temperature, humidity and other factors including relevant features of different aircraft. The various scenario outputs of the model are achieved by using different sets of input parameters tailored for each situation.

The APVMA has chosen specific input parameters for each standard scenario in consultation with the aerial industry to relect typical and current industry practices and aircraft configurations. Summaries of the APVMA's input parameters are found in each Excel file on separate worksheets following the graph. After consultation with industry, the APVMA has chosen the AT502 as its representative fixed wing aircraft for agriculture and the Bell 206BIII as its representative helicopter for both agriculture and forestry. Note that the input parameters used for the active ingredient, non-volatile component and specific gravity (selected as typical values for these examples) would be changed to be those of actual products during real assessments. Those changes could have effects on outcomes of actual assessments but would be expected to be small in most cases.

Each scenario displays downwind deposition data for appropriate droplet size spectra with each modeled for 8, 14 and 20 km/hr wind speeds, the upper limit of each of three wind speed ranges used for aerial no-spray zones. The downwind deposition is displayed to a distance of 794 metres, the maximum distance for which the model has been validated by extensive field studies

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Forestry Aerial Application Standard Spray Drift Scenarios

The great majority of forestry applications are herbicides applied pre-planting and during the establishment phase of a stand until a time when the trees are approximately two metres high. Only a much smaller number of applications are of insecticides.

Forestry land that is relatively level and accessible is more frequently treated with ground application machinery. Aerial applications are typically done on difficult to reach, often sloping land. For pre-plant and new plantings on level land, forestry helicopters can safely apply herbicides at approximately 5 metres above the ground. For irregular or sloping ground, 15 metre application heights are usually needed for pilot safety.

The APVMA uses five aerial forestry standard application scenarios; three are for level ground and two are for sloping ground. The two for sloping ground will be the ones used most often by the APVMA. The level ground versions will mainly be used for permit assessments when the APVMA can be assured that a use would be limited to level forestry plantings.

The sloping ground scenarios are both modeled for a situation where the helicopter is applying pesticide high on a slope and the wind is blowing directly down the slope. A slope angle of 20 degrees is used as representative for risk assessment. This downslope wind situation is a higher risk scenario than a level ground situation. Three of the five forestry scenarios are specifically for herbicide applications. These are modeled only for ASAE Coarse, Very Coarse and Extremely Coarse droplet size categories. The APVMA would consider finer droplet spectra for forestry herbicide applications only when justified for very unusual circumstances and after special modeling outcomes had been analysed.

The remaining two insecticide scenarios are modeled for the standard three droplet spectra, Fine, Medium and Coarse. Both insecticide scenarios are modeled for established tree stands 12 metres in height. Spray release height from the helicopter is set at 27 metres above the ground, a height that is 15 metres above the tree canopy. This scenario is regarded as representing the higher end of risk for typical insecticide applications in forestry.

For these two high canopy insecticide scenarios, the APVMA is currently using the ‘Height’ setting in the ‘Canopy’ section of AGDISP. An alternative setting available in the model is called ‘LAI’ which refers to a calculation method based on leaf area index that uses reference data to simulate the properties of a canopy. Compared to the ‘Height’ setting, the ‘LAI’ method takes into account in a more sophisticated way the interaction of turbulent eddies with the tree canopy. The result is that the ‘LAI’ method predicts much lower levels of downwind spray drift than does the ‘Height’ setting. However, the ‘LAI’ method has not yet been fully validated. For the time being, the APVMA will continue to use the more conservative ‘Height’ setting until more information becomes available for the ‘LAI’ method.

For banana applications, greater than 90% of aerial application is done by fixed-wing aircraft applying fungicides mainly over plantations on level ground.  The standard scenario for aerial application to bananas was modeled for application release 8 metres above the ground over banana plants averaging 4 metres in height.

Last updated on 17 November, 2010

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