How does SAFE work?#
SAFE takes a pre-existing GEM assessment---referred to as the base assessment---and generates a set of subassessments with shifted application dates. These asessments are run and relevant output is extracted and presented in a report.
Application scheme generation#
The application dates of the subassessments should be spread out over the full calendar year. This is achieved by dividing the year into a set of periods of a user-selected interval. For each period in the set, an application scheme with a corresponding subassessment is created. The number of periods and their length depends on which interval is selected. Five choices are possible, as listed in the table below.
| Interval | Length | No. of subassessments | Maximum offset | Default offset |
|---|---|---|---|---|
| Quarterly | 3 months | 4 | 90 | 46 |
| Monthly | 1 month | 12 | 28 | 15 |
| Biweekly | 14 days | 26 | 14 | 8 |
| Weekly | 7 days | 52 | 7 | 4 |
| Daily | 1 day | 365 | 1 | 1 |
Overview of the supported intervals in SAFE.
Offset in interval#
For each subassessment, the first application in the application scheme is set on the Nth day of the corresponding period, where N is a fixed offset (days) within that period. For example, for interval Monthly with an offset of 5 days, 12 subassessments are created with dates for the first application: 5 January, 5 February, 5 March, etc. The maximum value of the offset depends on the selected interval (see above table).
For schemes with multiple applications, the others are set relative to the first, according to the spacing in the base application scheme.
Note
For Quarterly and Monthly intervals, the maximum offset is determined by the shortest period in a non-leap year. This means that offsets 29--31 for Monthly and 91--92 for Quarterly cannot be selected.
Application window#
The applications can be limited to a specific window within the calendar year, specified as a start and end day-of-year (DOY). SAFE removes any application that occurs outside of this window. For application schemes with multiple applications, this means that some assessments might have fewer applications than the base scheme. If all applications of a scheme fall outside the window, the entire subassessment is excluded from the set.
Wrapping application dates#
For schemes with multiple applications, it may happen that one or more applications of the subassessments occur in the following year. Since applications schemes in GEM are specified for a single year, SAFE "wraps" these applications to the start of the year. As an example, consider a scheme with three applications spaced 18 days apart. For a subassessment where the first application occurs on 29 December, the second and third application would occur on 16 January and 3 February in the following year. Because SAFE wraps the applications, the generated scheme includes applications on 16 January, 3 February, and 29 December. These applications occur for every year of the simulation.
Output#
For each subassesment a predicted environmental concentration (PEC) is calculated by GEM. For soil-bound groudwater assessments, the PEC is defined as the 90th percentile over all simulation years of the average annual concentration in pore water at 1m depth. For soilless assessments, two PECs are reported for the ditch simulated by TOXSWA: the target perecentile (50th or 90th--selected in GEM) over all simulation years of the annual maximum dissolved concentration in the water layer and of the annual maximum content in the top 10cm of the sediment.
After the simulations have finished, SAFE collects the PECs from all subassessments and aggregates them to an overall PEC value. How this is done depends on the assessment type. For soil-bound groundwater assessments, the overall PEC is the the maximum value over all subasessments. For soilless assessments, the overall PEC is is defined as the 90th percentile over the PECs for the different subassessments.
Note
For Quarterly intervals, the 90th percentile is in fact the maximum of the PECs of the subassessments, since the 90th percentile cannot be calculated for four values.
Addtionally, for soilless assessments with monthly intervals, corrected overall PECs (both water and sediment), based on a safety factor determined by the scenario of the waterstreams model and the reference half-life (DT50) of the substance in the recirculation water as specified in the table below1.
| DT50 < 50 d | DT50 ≥ 50 d | |
|---|---|---|
| WSM scenario ≠ 40 | 1.5 | 1.2 |
| WSM scenario = 40 | 5 | 1.2 |
PEC safety factors for soilless assessments.