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

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.

Information on lower tier methods

MACRO tools

You can conduct higher tier calculations using the:

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.

Background information regarding the development of the higher tier MACRO 5 drainflow tool.

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:

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

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:

Resources

PL0501 Pesticide residues in water - ADAS-SWRC hydrological studies in Brimstone III.
PL0502 Brimstone III - leaching of pesticides to drainage water from a structured clay soil.
PL0503 Brimstone III - Pesticide residues in water.
PL0509 Pesticide behaviour in soils and movement to drainage.
PL0516 Evaluation of the use of preferential flow models to predict the movement of pesticides to water
PL0538 Development of guidance on parameter estimation for MACRO
Updated 2022-06-14