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+# Calculate initial predicted environmental concentrations in surface water due to drainage using the UK method
+
+This implements the method specified in the UK data requirements
+handbook and was checked against the spreadsheet published on the CRC
+website. Degradation between the end (30 April) and the start (1
+October) of the drainage period is taken into account if
+`latest_application` is specified and the degradation parameters are
+given either as a `soil_DT50` or a `model`.
+
+## Usage
+
+``` r
+PEC_sw_drainage_UK(
+  rate,
+  interception = 0,
+  Koc,
+  latest_application = NULL,
+  soil_DT50 = NULL,
+  model = NULL,
+  model_parms = NULL
+)
+
+drainage_date_UK(application_date)
+```
+
+## Arguments
+
+- rate:
+
+  Application rate in g/ha or with a compatible unit specified with the
+  units package
+
+- interception:
+
+  The fraction of the application rate that does not reach the soil
+
+- Koc:
+
+  The sorption coefficient normalised to organic carbon in L/kg or a
+  unit specified with the units package
+
+- latest_application:
+
+  Latest application date, formatted as e.g. "01 July"
+
+- soil_DT50:
+
+  Soil degradation half-life, if SFO kinetics are to be used, in days or
+  a time unit specified with the units package
+
+- model:
+
+  The soil degradation model to be used. Either one of "FOMC", "DFOP",
+  "HS", or "IORE", or an mkinmod object
+
+- model_parms:
+
+  A named numeric vector containing the model parameters
+
+- application_date:
+
+  Application date
+
+## Value
+
+The predicted concentration in surface water in µg/L
+
+## References
+
+HSE's Chemicals Regulation Division (CRD) Active substance PECsw
+calculations (for UK specific authorisation requests)
+<https://www.hse.gov.uk/pesticides/data-requirements-handbook/fate/pecsw-sed-via-drainflow.htm>
+accessed 2026-02-13
+
+PECsw/sed spray drift and tier 1 drainflow calculator Version 2.1.1
+(2025) Spreadsheet published at
+<https://www.hse.gov.uk/pesticides/assets/docs/PEC%20sw-sed%20(spraydrift).xlsx)>
+accessed 2026-02-13
+
+## Author
+
+Johannes Ranke
+
+## Examples
+
+``` r
+PEC_sw_drainage_UK(150, Koc = 100)
+#> 8.076923 [µg/L]
+PEC_sw_drainage_UK(60, interception = 0.5, Koc = 550,
+  latest_application = "01 July", soil_DT50 = 200)
+#> 0.8388303 [µg/L]
+drainage_date_UK("2023-07-10")
+#> [1] "2023-10-01"
+drainage_date_UK("2020-12-01")
+#> [1] "2020-12-01"
+drainage_date_UK(as.Date("2022-01-15"))
+#> [1] "2022-01-15"
+```