MACRO higher tier drainflow modelling for pesticide registration in Great Britain and Northern Ireland


Applicants are encouraged to use the new version of the HSE HTDF MACRO tool (v2.1) in their modelling from now on which corrects a minor bug issue in v2.0.

HSE will stop accepting applications received with modelling conducted with v1.1 of the HSE HTDF tool after 31st May 2023.

When you apply for registration of a pesticide in Great Britain and Northern Ireland, you need to submit an assessment of surface water exposure via drainflow. If you cannot show an acceptable risk using lower tier methods, you will need to submit a higher tier drainflow exposure assessment.

MACRO tools

You can conduct higher tier calculations using the:

  • ‘MACRO 4 PECsw drainflow (higher tier MACRO)’ tool - only operates on Windows 7 and earlier operating systems
  • ‘MACRO 5 PECsw drainflow (higher tier MACRO)’ tool  - operates on Windows 7 and Windows 10 operating systems

Step-by-step user guides are included in the tools download folders.

As far as possible the MACRO 5 tool has been developed to be aligned with the MACRO 4 tool.

Both tools have the same appearance and functionality. They include 4 soil types linked to 30-year datasets for dry, medium and wet weather climates. Each of the 4 soils is combined with the climate datasets to give 12 different soil-climate scenarios. You can model both an active substance and a primary metabolite.

Testing of a range of active substances and metabolites for a variety of crops and application timings has shown that in the vast majority of cases the regulatory decision would not be impacted by the version used. However, due to fundamental differences between MACRO 4 and MACRO 5, it has not been possible to create versions of the higher tier tools that are completely aligned in all cases.

HSE will continue to accept MACRO higher tier drainflow (HTDF) assessments performed using either the MACRO 4 HTDF or MACRO 5 HTDF tool during the transition period lasting until 31 May 2023. After this date, HSE will only accept HTDF modelling that use the MACRO 5 HTDF tool. This transition period will be used by HSE to gather information and build knowledge on the performance of the MACRO 5 HTDF tool.

You do not need to provide assessments using both models. Due to the ease of use of MACRO 5 on Windows 10, we suggest using the MACRO 5 HTDF tool immediately if possible.

If you want to use any methods that deviate from this guidance please contact HSE.

Presentation of higher tier MACRO drainflow modelling in pesticide risk assessment

When you are preparing higher tier MACRO modelling for an application for pesticide registration in Great Britain and Northern Ireland, there are two ways to present information:

The main method (Approach 1) uses the concept of exceedance years. This approach does not need any information on the distribution of crops within each scenario. You must always present approach 1 in your submission.

The other method (Approach 2) calculates a weighted level of exceedance and does need information on crop distributions. The use of Approach 2 is optional but may provide additional context to the information presented in Approach 1.

Approach 1: Individual exceedance years

This presentation method uses the largest annual concentration each year and the Regulatory Acceptable Concentration (RAC). When the RAC is from a higher tier effects study such as a mesocosm, you need to consider the exposure profile. Provide a case for why the RAC is appropriate for the drainflow risk assessment.

Follow these steps:

  • compare the largest annual concentration each year to the RAC
  • determine the number of years where the largest concentration exceeds the RAC. This is the number of exceedance years
  • repeat for each scenario
  • consider the significance of the number of exceedance years within each scenario

HSE differentiates between the following groups when considering the significance of the number of exceedance years:

  • Algae and aquatic plants
    For aquatic plants and algae, there must be no more than 60% of exceedance years in each scenario. The risk is acceptable if there are no more than 18 years out of 30 exceeding the RAC based on first tier data. You do not need to provide any further information in this case.

  • Fish and aquatic invertebrates
    For aquatic invertebrates and fish there is a lower limit threshold value. The risk is acceptable if there are no more than 10% of exceedance years in each scenario. This equates to no more than 3 years out of 30 exceeding the RAC. You do not need to provide any further information in this case

    If the exceedance years are above 10% for any scenario it may still be possible to show an acceptable risk. This will need a more detailed case-by-case assessment. This should consider the size, frequency and the duration of exceedance events. Applicants must consider all scenarios where exceedances are above 10%. Use the following metrics:

    • the size of the maximum exposure peak in relation to the RAC
    • duration of exceedance events
    • the number of exposure peaks above the RAC within each year

Approach 2: Weighted level of exceedance

This presentation method uses information on the extent of each scenario within the crop. You can only calculate this where you have access to soil, crop and climate data for each scenario from SEISMIC. The weighted level of exceedance should not exceed 10%. If you do not have access to SEISMIC, use the individual exceedance years approach above.

Example of Approach 1 and Approach 2

This example uses a risk assessment based on an aquatic plant RAC of 5.0 µg/l for an active substance used on wheat.

Table 1: Number of exceedance years. These are the years when the largest concentration is greater than the RAC of 5.0 µg/l for each scenario. Total years modelled = 30; values in parentheses are percentages of exceedance years. In the Excel tool very wet climate scenarios (>850 mm rainfall) are not modelled. Use the wet scenario results as a surrogate for results from these very wet scenarios.

Soil Dry (<625 mm per annum) Medium (625-750 mm per annum) Wet (750-850 mm per annum) Very wet (> 850 mm per annum)
Denchworth 5/30 (16.7) 10/30 (33.3) 8/30 (26.7) 8/30 (26.7)
Hanslope 4/30 (13.3) 6/30 (20.0) 5/30 (16.7) 5/30 (16.7)
Brockhurst 1/30 (3.3) 2/30 (6.7) 1/30 (3.3) 1/30 (3.3)
Clifton 0/30 (0) 1/30 (3.3) 0/30 (0) 0/30 (0)
Quorndon 0/30 (0) 0/30 (0) 0/30 (0) 0/30 (0)

Table 2: Data on the extent of total wheat growing land for each scenario. The figures below are from Brown et al, 2004 for England and Wales.

Soil type Extent of soil within each scenario (%) Total extent
Dry Medium Wet Very wet (%)
Undrained - - - - 45.9
Peaty soils - - - - 3.5
Denchworth 2.7 3.0 0.7 0.5 7.0
Hanslope 9.0 5.6 0.5 0.4 15.5
Brockhurst 4.8 7.6 1.8 0.9 15.1
Clifton 1.5 5.2 1.6 0.9 9.2
Quorndon 2.5 0.9 0.3 0.2 3.8
Total 20.4 22.3 4.9 2.9 100

You can use this information to weight the percentage of exceedance years. Weight each scenario according to the extent of the crop they represent. Using 'Denchworth dry' as an example, weight the exceedance years (16.7%) by the extent of the scenario for the crop (2.7%). So 16.7% * 2.7% = 0.5%.

Table 3: Sum of all scenarios where exceedances occur

Soil type Extent of soil within each scenario (%) Total extent
Dry Medium Wet Very wet (%)
Denchworth 16.7 * 2.7%
= 0.5
33.3 * 3.0% = 1.00 26.7 * 0.7%
= 0.2
26.7 * 0.5%
= 0.1
= 1.8
Hanslope 13.3 * 9.0%
= 1.2
20.0 * 5.6%
= 1.1
16.7 * 0.5%
= 0.1
16.7 * 0.4
= 0.1
= 2.5
Brockhurst 3.3 * 4.8%
= 0.2
6.7 * 7.6%
= 0.5
3.3 * 1.8%
= 0.1
3.3 * 0.9%
= 0.0
= 0.8
Clifton 0 * 1.5%
= 0
3.3 * 5.2%
= 0.2
0 * 1.6%
= 0
0 * 0.9%
= 0
= 0.2
Quorndon 0* 2.5%
= 0
0 * 0.9%
= 0
0 * 0.3%
= 0
0 * 0.2%
= 0
= 0

Summing values for each scenario (1.8 + 2.5 + 0.8 + 0.2) shows exceedances occurred in a combined total of 5.3% of wheat growing land. The concentration is below the RAC in 100 - 5.3 = 94.7% of combined scenario years.

In conclusion:

  • The weighted level of exceedance is less than 10% (i.e. only 5.3%)
  • The largest number of exceedance years is less than 60% (i.e. 10 out of 30 years or 33% in the Denchworth medium scenario  and the RAC is based on effects on aquatic plants)
  • The risk is acceptable


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Updated 2023-03-29