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+# Calculate PEC surface water due to drainage as in Exposit 3
+
+This is a reimplementation of the calculation described in the Exposit
+3.02 spreadsheet file, in the worksheet "Konzept Drainage". Although
+there are four groups of compounds ("Gefährdungsgruppen"), only one
+distinction is made in the calculations, between compounds with low
+mobility (group 1) and compounds with modest to high mobility (groups 2,
+3 and 4). In this implementation, the group is derived only from the
+Koc, if not given explicitly. For details, see the discussion of the
+function arguments below. It is recommened to specify the arguments
+`rate`, `Koc`, `DT50`, `t_drainage`, `V_ditch` and `V_drainage` using
+[units::units](https://r-quantities.github.io/units/reference/units.html)
+from the `units` package.
+
+## Usage
+
+``` r
+PEC_sw_exposit_drainage(
+  rate,
+  interception = 0,
+  Koc = NA,
+  mobility = c(NA, "low", "high"),
+  DT50 = set_units(Inf, "d"),
+  t_drainage = set_units(3, "days"),
+  V_ditch = set_units(30, "m3"),
+  V_drainage = set_units(c(spring = 10, autumn = 100), "m3"),
+  dilution = 2
+)
+```
+
+## Source
+
+Excel 3.02 spreadsheet available from
+<https://www.bvl.bund.de/SharedDocs/Downloads/04_Pflanzenschutzmittel/zul_umwelt_exposit.html>
+
+## Arguments
+
+- rate:
+
+  The application rate in g/ha
+
+- interception:
+
+  The fraction intercepted by the crop
+
+- Koc:
+
+  The sorption coefficient to soil organic carbon used to determine the
+  mobility. A trigger value of 550 L/kg is used in order to decide if
+  Koc \>\> 500.
+
+- mobility:
+
+  Overrides what is determined from the Koc.
+
+- DT50:
+
+  The soil half-life in days
+
+- t_drainage:
+
+  The time between application and the drainage event, where degradation
+  occurs, in days
+
+- V_ditch:
+
+  The volume of the ditch is assumed to be 1 m \* 100 m \* 30 cm = 30 m3
+
+- V_drainage:
+
+  The drainage volume, equivalent to 1 mm precipitation on 1 ha for
+  spring/summer or 10 mm for autumn/winter/early spring.
+
+- dilution:
+
+  The dilution factor
+
+## Value
+
+A list containing the following components
+
+- perc_drainage_total:
+
+  Gesamtaustrag (total fraction of the residue drained)
+
+- perc_peak:
+
+  Stoßbelastung (fraction drained at event)
+
+- PEC_sw_drainage:
+
+  A matrix containing PEC values for the spring and autumn scenarios. If
+  the rate was given in g/ha, the PECsw are in microg/L.
+
+## See also
+
+[`perc_runoff_exposit`](https://pkgdown.jrwb.de/pfm/reference/perc_runoff_exposit.md)
+for runoff loss percentages and
+[`perc_runoff_reduction_exposit`](https://pkgdown.jrwb.de/pfm/reference/perc_runoff_reduction_exposit.md)
+for runoff reduction percentages used
+
+## Examples
+
+``` r
+  PEC_sw_exposit_drainage(500, Koc = 150)
+#> $perc_drainage_total
+#> spring autumn 
+#>    0.2    1.0 
+#> 
+#> $perc_peak
+#> spring autumn 
+#>   12.5   25.0 
+#> 
+#> $PEC_sw_drainage
+#> Units: [µg/L]
+#>   spring   autumn 
+#> 1.562500 4.807692 
+#> 
+```