Drift Calculator


About Drift Calculator


General Information

The FOCUS drift calculator is embedded in the SWASH shell and is run automatically run in the background to provide drift values for use in TOXSWA simulations of aquatic fate.

The equations in the drift calculator were developed by Denis Yon (Dow Agrosciences, UK) and Mark Russell (DuPont Crop Protection, USA) and the model was programmed by Alterra, in close co-operation with WISL, the Wageningen Software Labs, both located in Wageningen, the Netherlands.


Short Model Description

Main function of the Drift Calculator

The drift calculator requires five input values in order to determine the correct drift loading:

  1. Application rate (g ai/ha)
  2. Number of applications per season
  3. Crop type
  4. Water body type
  5. Distance between crop and water body

For Step 3 FOCUS calculations, the types of water bodies associated with each scenario and default distances between crop and water have been set by the FOCUS SW group. The user enters the crop type, the application rate and the number of applications per season in SWASH and the SWASH shell automatically runs the drift calculator and generates a report for each TOXSWA run in the file "projectname_report.txt" located in the SWASH PROJECTS directory under the selected project name.

The Drift Calculator uses a series of drift regressions based on BBA, 2000 drift data [1]. Each drift regression curve is a function of the crop type as well as the number of applications. The calculator determines the mean drift loading over the width of the receiving water body and the drift loading is reported with units of mg/m2 of water body surface area.

A detailed description of the equations used in the drift calculations is provided in the FOCUS Surface Water Scenarios Final Report in section 5.2.


Role and Place in FOCUS scenarios calculation

For Step 3 calculations

The Drift Calculator runs in the background of SWASH and automatically provides TOXSWA with the correct drift values for each model run. The user can check the drift values provided by SWASH for use in TOXSWA in three ways:

  1. Using Windows Explorer, open the file "projectname_report.txt" located in the SWASH PROJECTS directory under the selected project name
  2. Within SWASH: select the Information tab in SWASH and then click on the Drift button.
  3. Within TOXSWA: select project, select "OK", at the top of the screen, select "Edit scenarios", then "Application Schemes". In this screen, select "Spray drift edit/view" and then "FOCUS drift calculator".

It should be noted that the drift values used for ditches and ponds are identical in all of the listed sources. For streams, the drift value reported in the Drift Calculator in SWASH (and also accessible within TOXSWA) is multiplied by a factor of 1.2 to account for the 20% of the upgradient area that is assumed to be chemically treated. Therefore, the "projectname_report.txt" file will list drift loadings that are 1.2 times higher than the Drift Calculator in SWASH.

For Step 4 calculations

In some cases, the Step 3 PECsw and PECsed values calculated by TOXSWA may not result in acceptable TERsw and TERsed values. If drift is a primary contributor to the total aquatic loading, it may be necessary to increase the default buffer widths between crop and water that have been set by FOCUS in order to demonstrate aquatic safety.

To perform Step 4 calculations using modified spray drift values, the user will need to work outside of the SWASH shell and run TOXSWA using a batch file. Due to time and funding constraints, it is not currently possible to perform Step 4 calculations using adjusted drift values in the TOXSWA GUI.

Step 4 calculations with modified drift values can be performed as follows:

  1. Exit the SWASH GUI.
  2. Create a new subdirectory in the desired project location in SWASHPROJECTS.
    Example: C:\SWASHPROJECTS\testcase\TOXSWA\Step 4
  3. Copy the entire contents of the TOXSWA subdirectory to the new Step 4 subdirectory.
    Example: copy C:\SWASHPROJECTS\testcase\TOXSWA\*.* to C:\SWASHPROJECTS\testcase\TOXSWA\Step 4\*.*
    Three files are needed in the Step 4 subdirectory in order to run TOXSWA for a specific scenario:
    • (a) TOXSWA input file: e.g. 12345s_pa.txw
    • (b) Batch file: toxswa.bat
    • (c) Appropriate *.met files (these will be present if you copied the entire TOXSWA subdirectory to the Step 4 subdirectory)
  4. Determine the correct drift value for the desired crop, application rate, number of applications, type of water body and distance between crop and water using the FOCUS drift calculator.

    For ditches and ponds, use the "mg/m2" drift value listed at the bottom of the calculator.

    For streams, multiply the "mg/m2" drift value by a factor of 1.2 to account for the upgradient drift that is assumed to occur in the stream watershed.

  5. Edit the TOXSWA input files (e.g. 12345s_pa.txw), replacing the Step 3 drift values in Section 4 in each file with the correct value for the Step 4 calculation. The spray drift values are listed as "mldsd" which stands for mass load of spray drift, in units of mg/m2.
  6. Edit the batch file, creating a separate line for each run that is desired. The existing line can simply be duplicated and the run_id values edited for each TOXSWA input file in the Step 4 subdirectory.
  7. Execute the toxswa.bat file and the new runs are then calculated using the adjusted drift values.


Current use outside FOCUS

The FOCUS Drift Calculator was designed for use within the SWASH shell and is not a separate piece of software. As described above, it is also useful for Step 4 calculations for the FOCUS surface water scenarios. Any use of this calculator outside of FOCUS is the responsibility of the user.


Excerpt of existing model documentation

  • FOCUS Surface Water Scenarios Final Report, Section 5.2 

Download Drift Calculator

Download Drift_Calculator

The Drift Calculator is part of the SWASH software package and can therefore not be downloaded seperately.

There are however the following Test Results available : HTML, Word


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