Selecting the date-of-use in GEM
The Guidance on Protected Crops provides applicants with emission scenarios from protected cultivation so they can calculate emissions with a model. GEM is one model that can be used for this purpose. However, it has been found that the GEM scenarios are very sensitive to the application date.
A specific approach for the date-of-use is recommended for each GEM module (soilless surface water, soilbound surface water and soilbound groundwater). These separate approaches are necessary due to the underlying processes in those modules and the relative effect of the date-of-use on the outcome. Ultimately, the worst-case Predicted Environmental Concentration (PEC) must be reported and used in the risk assessment. The recommended approaches are described below for the various emission scenarios:
Approach for soilless (non-soil-bound) surface water and soil-bound groundwater
Create 12 simulations (or fewer, depending on the application period of use within the GAP) with the date-of-use set to the 15th of each month; this takes account of the temporal effect of the date-of-use. Make two additional simulations, one week before and one week after the date-of-use, that resulted in the highest PEC. A different date may apply to metabolites. Report the highest PEC value in the risk assessment, and report the date on which the highest concentration occurs and which application date was simulated.
Approach for soil-bound surface water
Version 3.3.2 of GEM contains a conceptual error for soil-bound surface water assessments. As an interim solution, the Ctgb proposes that applicants include the worst-case date-of-use (30 May, which is the best option based on a number of test calculations with two different substances) in the use schedule and include an additional safety factor of 2 on the PEC. If the GAP contains multiple uses (for example 4 uses with an interval of 7 days), then May 30 must be set as the last date-of-use. If 30 May does not fall into the application period of the GAP, applicants should still calculate with 30 May because this gives the worst-case PEC value.
If safe use cannot be demonstrated in the outcome with the proposed date of 30 May and safety factor of 2, applicants can demonstrate safe use by submitting an evaluation for all relevant dates-of-use within the GAP period. When all data is included in the simulations, the safety factor of 2 can be ignored. The Ctgb communicated this approach previously (in July 2017).
This procedure will be in force until a new version of GEM becomes available. Applicants can also use their own calculations with a different model.