Update of the definition of vegetative filter strip scenarios for Europe w/ appendices

Environmental risk assessment procedures for pesticides in the context of approval and authorization at EU and national level must keep up with scientific developments and therefore have to be updated regularly. 
The standardised FOCUS surface water approach for the estimation of predicted environmental concentrations of pesticides in surface water and sediment consists of a total of four tiers (Steps 1 to 4). Mitigation measures for spray drift and surface runoff can be included in Step 4. While spray drift inputs are typically mitigated with drift-reducing nozzles and no-spray zones, the most widely implemented mitigation measure to reduce the input of pesticides to surface waters via surface runoff and erosion are vegetative filter strips (VFS).
The SWAN tool for higher-tier surface water exposure assessment implements fixed runoff and erosion reduction efficiencies according to the report of the FOCUS Landscape and Mitigation Working Group. Since SWAN version 3 the tool also includes the option to apply mitigation using the model VFSMOD, a numerical, mechanistic, dynamic and event-based model for predicting runoff reduction, sediment deposition, and pesticide trapping across a vegetative filter strip (VFS). VFSMOD was applied by Brown et al. (2012) to develop realistic worst case VFS scenarios for calculating the reduction of pesticide input to surface water for the four FOCUS runoff scenarios. The resulting scenarios were subsequently included in SWAN. 
Since then, with the publication of substantial scientific developments, VFSMOD has been improved in its underlying assumptions and process descriptions. This requires also updating the original VFS scenarios of Brown et al. (2012) using the new VFSMOD version. Moreover, new EU soil profile and land cover data have become available, and the United Kingdom has left the EU. Hence, the population of the spatial cumulative distribution functions (CDFs) of the relative reduction of incoming pesticide load (∆P) by VFS needed to be updated. The objective of this study was therefore to update and improve the SWAN-VFSMOD scenarios for EU pesticide risk assessment.
In this study, 64 new spatial CDFs of ∆P have been created. Out of these, the four CDFs corresponding to the most relevant combination of input factors in the context of FOCUS were selected for the new SWAN-VFSMOD scenarios. From those, the soil profiles corresponding to the 10th percentile of ∆P (thus, the 90th percentile worst case in space) were selected for parameterization of VFSMOD in SWAN 5.2.
A comparison of the new spatial CDFs with predicted trapping efficiencies ∆P obtained with the previous SWAN-VFSMOD scenarios and the old VFSMOD version yielded that the new SWAN-VFSMOD scenarios are overall more conservative than the old ones. Analysis of the new CDFs with regard to soil texture confirmed that the approach of selecting overall 90th percentile worst case soil profiles for the VFS scenarios is reasonable. As expected, some texture classes have higher vulnerability than others. The general ranking of texture classes with respect to ∆P (Coarse > Medium > Fine > Medium Fine > Very Fine) reflects the soil hydraulic properties; notably the predicted saturated conductivity is on average lowest for soils with texture classes Medium Fine (very high silt content) and Very Fine (very high clay content).
In conclusion, this study has derived a set of updated VFSMOD scenarios for FOCUS Step 4 simulations. The new 90th percentile worst case profiles for SWAN-VFSMOD correspond to the state of the art of VFS scientific knowledge and the most recent available soil profile databases and land cover data. The updated SWAN-VFSMOD scenarios are both more realistic and more protective than the old ones. While the derived SWAN-VFSMOD scenarios represent the best estimate of a 90th percentile worst case given the available data, this report may also offer a blueprint for revising the scenarios as new data become available. Potential future updates of the SWAN-VFSMOD scenarios should include first a revision of the FOCUS scenario maps because of climate change.