Fix UK drainage for very early applications

Create a function `drainage_date_UK` that does not only respect the
beginning of the drainage period on 1 October, but also the end of the
drainage period on 30 April, and use it for determining the degradation
time. Applications early in the year before 1 May will now correctly be
calculated without degradation time.

7 files changed, 59 insertions, 18 deletions

diff --git a/NAMESPACE b/NAMESPACE
index 2daa348..499e471 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -28,6 +28,7 @@ export(SSLRC_mobility_classification)
 export(TOXSWA_cwa)
 export(TSCF)
 export(chent_focus_sw)
+export(drainage_date_UK)
 export(drift_percentages_rautmann)
 export(endpoint)
 export(geomean)
diff --git a/NEWS.md b/NEWS.md
index 53a84c8..41a3da7 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,4 +1,6 @@
-## version 0.6.4
+## version 0.6.5
+
+- R/PEC_sw_drainage_UK.R: Create a function `drainage_date_UK` that does not only respect the beginning of the drainage period on 1 October, but also the end of the drainage period on 30 April, and use it for determining the degradation time. Applications early in the year before 1 May will now correctly be calculated without degradation time.
 
 - R/PEC_sw_drift.R: Vectorise the function not only with respect to distances, rates and water depths, but also with respect to crop groups. Closes issue #2 reported by Julian Klein (@juklei).
 
diff --git a/R/PEC_sw_drainage_UK.R b/R/PEC_sw_drainage_UK.R
index d5f0bab..40835b2 100644
--- a/R/PEC_sw_drainage_UK.R
+++ b/R/PEC_sw_drainage_UK.R
@@ -1,7 +1,8 @@
 #' 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 before the start of the drainage period is taken into account if
+#' 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`.
 #'
 #' @param rate Application rate in g/ha or with a compatible unit specified
@@ -18,12 +19,12 @@
 #' @return 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)
-#'   \url{https://www.hse.gov.uk/pesticides/topics/pesticide-approvals/pesticides-registration/data-requirements-handbook/fate/active-substance-uk.htm}
-#'   accessed 2019-09-27
+#'   \url{https://www.hse.gov.uk/pesticides/data-requirements-handbook/fate/pecsw-sed-via-drainflow.htm}
+#'   accessed 2026-02-13
 #'
-#'   Drainage PECs Version 1.0 (2015) Spreadsheet published at
-#'   \url{https://www.hse.gov.uk/pesticides/topics/pesticide-approvals/pesticides-registration/data-requirements-handbook/fate/pec-tools-2015/PEC\%20sw-sed\%20(drainage).xlsx}
-#'   accessed 2019-09-27
+#'   PECsw/sed spray drift and tier 1 drainflow calculator Version 2.1.1 (2025) Spreadsheet published at
+#'   \url{https://www.hse.gov.uk/pesticides/assets/docs/PEC%20sw-sed%20(spraydrift).xlsx)}
+#'   accessed 2026-02-13
 #' @export
 #' @author Johannes Ranke
 #' @examples
@@ -61,12 +62,13 @@ PEC_sw_drainage_UK <- function(rate,
     latest <- as.Date(paste(latest_application, ref_year), "%d %b %Y")
     if (is.na(latest)) stop("Please specify the latest application in the format '%d %b', e.g. '01 July'")
     tmp <- Sys.setlocale("LC_TIME", lct)
-    degradation_time <- as.numeric(difftime(as.Date(paste0(ref_year,"-10-01")), units = "days", latest))
+    
+    drainage_date <- drainage_date_UK(latest)
+    degradation_time <- as.numeric(difftime(drainage_date, latest, units = "days"))
+    
     if (degradation_time > 0) {
       if (!missing(soil_DT50)) {
         k = log(2)/soil_DT50_d
-        as.Date(paste(latest_application, "1999"), "%d %B %Y")
-
         amount_available <- amount_available * exp(-k * degradation_time)
         if (!missing(model)) stop("You already supplied a soil_DT50 value, implying SFO kinetics")
       }
@@ -82,3 +84,22 @@ PEC_sw_drainage_UK <- function(rate,
   PEC = set_units(1e6 * (percentage_lost/100) * amount_available / volume, "\u00B5g/L")
   return(PEC)
 }
+
+#' @rdname PEC_sw_drainage_UK
+#' @param application_date Application date
+#' @export
+#' @examples
+#' drainage_date_UK("2023-07-10")
+#' drainage_date_UK("2020-12-01")
+#' drainage_date_UK(as.Date("2022-01-15"))
+drainage_date_UK <- function(application_date) {
+  year <- substr(application_date, 1, 4)
+  drainage_end <- as.Date(paste0(year, "-04-30"))
+  drainage_start <- as.Date(paste0(year, "-10-01"))
+  if (application_date <= drainage_end | application_date >= drainage_start) {
+    drainage_date <- application_date
+  } else {
+    drainage_date <- drainage_start
+  }
+  return(drainage_date)
+}
diff --git a/log/build.log b/log/build.log
index fb1f5c2..4b09656 100644
--- a/log/build.log
+++ b/log/build.log
@@ -3,6 +3,7 @@
 * checking DESCRIPTION meta-information ... OK
 * checking for LF line-endings in source and make files and shell scripts
 * checking for empty or unneeded directories
+Removed empty directory ‘pfm/tests/testthat/_snaps’
 * re-saving image files
 * building ‘pfm_0.6.5.tar.gz’
 
diff --git a/log/check.log b/log/check.log
index 8dfd612..8ba020c 100644
--- a/log/check.log
+++ b/log/check.log
@@ -14,7 +14,7 @@
 * checking CRAN incoming feasibility ... NOTE
 Maintainer: ‘Johannes Ranke <johannes.ranke@agroscope.admin.ch>’
 
-Size of tarball: 8536724 bytes
+Size of tarball: 8537003 bytes
 * checking package namespace information ... OK
 * checking package dependencies ... OK
 * checking if this is a source package ... OK
@@ -62,7 +62,7 @@ Size of tarball: 8536724 bytes
 * checking data for non-ASCII characters ... OK
 * checking LazyData ... OK
 * checking data for ASCII and uncompressed saves ... OK
-* checking examples ... [17s/11s] OK
+* checking examples ... [20s/12s] OK
 * checking for unstated dependencies in ‘tests’ ... OK
 * checking tests ... SKIPPED
 * checking PDF version of manual ... OK
diff --git a/man/PEC_sw_drainage_UK.Rd b/man/PEC_sw_drainage_UK.Rd
index af020c6..b641114 100644
--- a/man/PEC_sw_drainage_UK.Rd
+++ b/man/PEC_sw_drainage_UK.Rd
@@ -2,6 +2,7 @@
 % Please edit documentation in R/PEC_sw_drainage_UK.R
 \name{PEC_sw_drainage_UK}
 \alias{PEC_sw_drainage_UK}
+\alias{drainage_date_UK}
 \title{Calculate initial predicted environmental concentrations in surface water due to drainage using the UK method}
 \usage{
 PEC_sw_drainage_UK(
@@ -13,6 +14,8 @@ PEC_sw_drainage_UK(
   model = NULL,
   model_parms = NULL
 )
+
+drainage_date_UK(application_date)
 }
 \arguments{
 \item{rate}{Application rate in g/ha or with a compatible unit specified
@@ -32,29 +35,35 @@ days or a time unit specified with the units package}
 "DFOP", "HS", or "IORE", or an mkinmod object}
 
 \item{model_parms}{A named numeric vector containing the model parameters}
+
+\item{application_date}{Application date}
 }
 \value{
 The predicted concentration in surface water in µg/L
 }
 \description{
 This implements the method specified in the UK data requirements handbook and was checked against the spreadsheet
-published on the CRC website. Degradation before the start of the drainage period is taken into account if
+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
 \code{latest_application} is specified and the degradation parameters are given either as a \code{soil_DT50} or a \code{model}.
 }
 \examples{
 PEC_sw_drainage_UK(150, Koc = 100)
 PEC_sw_drainage_UK(60, interception = 0.5, Koc = 550,
   latest_application = "01 July", soil_DT50 = 200)
+drainage_date_UK("2023-07-10")
+drainage_date_UK("2020-12-01")
+drainage_date_UK(as.Date("2022-01-15"))
 }
 \references{
 HSE's Chemicals Regulation Division (CRD) Active substance
 PECsw calculations (for UK specific authorisation requests)
-\url{https://www.hse.gov.uk/pesticides/topics/pesticide-approvals/pesticides-registration/data-requirements-handbook/fate/active-substance-uk.htm}
-accessed 2019-09-27
+\url{https://www.hse.gov.uk/pesticides/data-requirements-handbook/fate/pecsw-sed-via-drainflow.htm}
+accessed 2026-02-13
 
-Drainage PECs Version 1.0 (2015) Spreadsheet published at
-\url{https://www.hse.gov.uk/pesticides/topics/pesticide-approvals/pesticides-registration/data-requirements-handbook/fate/pec-tools-2015/PEC\%20sw-sed\%20(drainage).xlsx}
-accessed 2019-09-27
+PECsw/sed spray drift and tier 1 drainflow calculator Version 2.1.1 (2025) Spreadsheet published at
+\url{https://www.hse.gov.uk/pesticides/assets/docs/PEC\%20sw-sed\%20(spraydrift).xlsx)}
+accessed 2026-02-13
 }
 \author{
 Johannes Ranke
diff --git a/tests/testthat/test_UK_drainage.R b/tests/testthat/test_UK_drainage.R
index 75d9c98..0286eb4 100644
--- a/tests/testthat/test_UK_drainage.R
+++ b/tests/testthat/test_UK_drainage.R
@@ -1,4 +1,5 @@
 library(pfm)
+library(units)
 context("UK drainage PEC calculations")
 
 test_that("The mobility classification and the drained percentage are correct", {
@@ -41,6 +42,12 @@ test_that("UK drainflow PECs are correct", {
     latest_application = "01 July",
     soil_DT50 = 200), 2), 
     as_units(0.84, "\u00B5g/L"))
+  
+  # Check and example with early application before the end of the drainage period
+  expect_equal(round(PEC_sw_drainage_UK(90, interception = 0, Koc = 10,
+    latest_application = "01 February",
+    soil_DT50 = 200), 4),
+    as_units(13.1538, "\u00B5g/L"))
 
   expect_error(round(PEC_sw_drainage_UK(60, interception = 0.5, Koc = 550,
                                         latest_application = "100 July",