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Webfram: higher tier refinement of drainflow exposureWebfram is a web based risk assessment tool that can be used as a higher tier refinement option. In refining the risk it uses the current risk assessment models but incorporates both uncertainty and variability. Webfram performs multiple simulations (sometimes millions) to produce outputs as probability distributions. The outputs are different from conventional risk assessments and therefore the aim of these information and the step by step guide is to outline how to carry out a Webfram assessment and what information to include in a regulatory submission. In addition it provides some guidance on how to interpret outputs and use them to make regulatory decisions. All modules have been peer reviewed, validated, are freely available and user friendly. The tool has been available for use in regulatory submissions since 2009 and an updated version was released in 2014. The updated version has significantly greater functionality to aid refinement of drainflow exposure. The drainflow module is likely to be the most frequently encountered part of Webfram and is therefore the focus of this guidance. When addressing variability and uncertainty within the drainflow module, the key factors considered are as follows: timing of application, amount of spray intercepted by the crop, degradation and sorption properties, soil pH and organic matter content, field capacity period and clay content (as an indicator of vulnerability to preferential flow). The drainage module is based on current risk assessment methods and utilises a meta model of MACRO v4.3. Links to the underlying R that supported the development of Webfram and that provide more background information on all of the modules are included at the end of the guidance and also at Research development.

Role of Webfram in regulation

Webfram is only one of a range of options available and an Applicant is free to choose which option is the most appropriate for their assessment. Advantages over alternative approaches (eg stand alone higher tier assessments using MACRO see Higher tier drainflow from MACRO) are that Webfram contains a set of standardized scenarios and outputs, where the user only selects the substance properties and GAP. The ability to quantitatively account for variability1 and uncertainty2 in a fully probabilistic manner is also an advantage over alternative deterministic models. However the incorporation of uncertainty and variability in environmental regulatory risk assessment is still relatively new and care should always be taken when using and interpreting Webfram outputs to ensure we maintain the same degree of consistency between assessments and maintain the appropriate level of protection or conservatism. Further guidance on this is provided in the sections below.

Since Webfram is only one of the options available it may be used on its own to refine a Tier 1 drainflow exposure assessment, or may also be provided alongside a standalone MACRO assessment. Based on limited current experience, there is no standard tiered approach to refining drainflow. What this means is that for some substances and use patterns, Webfram will provide a greater level of refinement (ie higher tier) than standalone MACRO, and in other cases Webfram will provide a lower level of refinement (ie lower tier). Since either approach is accepted, provided that the subsequent aquatic risk assessment is acceptable, the use of either approach would be accepted by CRD. It is not necessary to pass both methods. Based on limited experience, Webfram may give a greater level of refinement for moderately sorbed substances with relatively large regulatory datasets for DT50 and/or Koc. It may give a lower level of refinement for more mobile substances with standard data sets. This latter finding may be linked to the ability to quantify variability and uncertainty.

Basic model guide

Further Information on running the model can be obtained via the available links within the Webfram model itself, or clicking on the various ‘here’ or the ‘?’ symbols throughout. Further detailed guidance on specific aspects of the model are provided in the additional sections below.

How to deal with uncertainty

Currently in regulatory risk assessment uncertainty is not dealt with explicitly as it is assumed to be incorporated in the uncertainty factors applied to the ecotoxicological effects endpoints. These do not account for any uncertainty in the exposure estimate. However, the output from Webfram highlights the degree of certainty via the use of confidence intervals,
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ie it gives an indication of how sure we are about any of the outputs. The key issue here is how should we deal with this issue?

It is proposed that the median of any chosen exposure value should be used in regulatory risk assessment. In addition it is proposed to not use the confidence intervals provided by the Webfram models directly in making a regulatory decision. However since the confidence intervals do provide useful information on the level of certainty in any particular assessment, it is proposed that these are always reported as part of a regulatory submission. These may provide useful supporting information when making a regulatory decision. For example, an exceedance of the Regulatory Acceptable Concentration (RAC) in one scenario of 25% with confidence intervals of 23 to 27% would be different to an exceedance of 25% with confidence intervals of 2 to 98%.

How to present and use in a regulatory submission

The following outlines what should be submitted and considered when a regulatory submission includes a Webfram based risk assessment.

Input data: It is necessary for Applicants to submit all the input data that has been used in the risk assessment. Details of the GAP assessed and how this relates to the label should be provided (ie confirmation that the modelled GAP represents a reasonable worst case use pattern as per the label). As regards the choice of application date, this should be justified in terms of the disease, weed or pest being treated and should generally reflect the most vulnerable timing of application with regards likely drainflow exposure. Where a wide application window may be expected based on the label recommendations, it may be necessary to run multiple simulations to cover both early and late applications. Confirmation of all other parameters (eg percentiles, number of iterations used in each calculation, whether any relationship between sorption, degradation and pH was assumed, relevant RAC etc.) should be provided automatically as part of the input data submission.

All relevant outputs used to make a regulatory decision should be submitted with any submission, along with underlying studies unless these have been previously evaluated (eg at EU level or as part of another UK product assessment in which case COP number references are required). The key output from the drainage model is most likely to be the details of the Exceedance Statistics for each scenario. This broadly matches the standard scenario years type approach detailed in higher tier drainflow from MACRO. Here it will be critical to report the individual level of exceedance for each scenario for comparison against regulatory triggers (for example, where the RAC is based on effects against primary producers a maximum of 60% exceedance for any single scenario must not be breached). The overall exceedance must also be less than 10%.

For risk assessments where the RAC is based on effects against fish or aquatic invertebrates the maximum 60% exceedance level for any single scenario is not appropriate. This is consistent with the approach to assessing risks to these groups in other forms of higher tier drainflow modelling eg using MACRO. However an alternative approach based on exceedances in Webfram modelling can be derived based on the individual 90th percentile PEC values for each scenario. Given that the risk from spray drift is regulated on the 90th percentile PECs, it is considered that the same approach can be used to assess the risk from drainflow exposure on these soils, ie consider the 90th percentile PECs against the RAC. It should be noted that Webfram calculates a single maximum PECsw value after the first drainflow event in each model iteration so the distributions represent maximum values that effectively account for both the spatial and temporal aspects in an appropriately conservative manner. The use of a 90th percentile value for direct use in the risk assessment is therefore considered reasonable in this case. If the 90th percentile PEC in Webfram is less than the RAC, the risk may be considered acceptable. If the 90th percentile PEC in Webfram is above the RAC, it does not necessarily mean that the use is unacceptable. However additional modelling and analysis (eg using MACRO) may be necessary in order to determine the acceptability of the use.

In addition copies of the key exposure distributions or risk graphs may be useful for further interpretation of the outputs. One drawback of the Webfram approach is that it is not possible to provide a detailed assessment of exposure profiles (eg to consider duration of exposure events or intervals between exceedances and potential for recovery). However the advantage of the Webfram simulation in providing a quantitative estimate of uncertainty and variability is useful when considering the outputs in a regulatory decision making context.

Where the regulatory submission provided is insufficient to allow HSE CRD to validate all aspects of the submission, the missing information will be requested.

How to download input data

A summary of the input data can be obtained by going to ‘Saved Models’, locating the relevant model and right clicking on ‘data’ (see red circle),
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click on ‘save target as’ and save this as word document.