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diff --git a/pkg/R/PEC_sw_drift_NL.R b/pkg/R/PEC_sw_drift_NL.R
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+# Copyright (C) 2016  Johannes Ranke
+# Contact: jranke@uni-bremen.de
+# This file is part of the R package pfm
+
+# This program is free software: you can redistribute it and/or modify it under
+# the terms of the GNU General Public License as published by the Free Software
+# Foundation, either version 3 of the License, or (at your option) any later
+# version.
+
+# This program is distributed in the hope that it will be useful, but WITHOUT
+# ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+# FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more
+# details.
+
+# You should have received a copy of the GNU General Public License along with
+# this program. If not, see <http://www.gnu.org/licenses/>
+
+#' Calculate predicted environmental concentrations in surface water due to drift
+#'
+#' This is a basic, vectorised form of a simple calculation of a contaminant
+#' concentration in surface water based on complete, instantaneous mixing
+#' with input via spray drift.
+#'
+#' @param rate Application rate in units specified below
+#' @param applications Number of applications for selection of drift percentile
+#' @param drift_percentages Percentage drift values for which to calculate PECsw
+#' @param water_depth Depth of the water body in cm
+#' @param rate_units Defaults to g/ha
+#' @param PEC_units Requested units for the calculated PEC. Only µg/L currently supported
+#' @return The predicted concentration in surface water
+#' @export
+#' @author Johannes Ranke
+#' @examples
+#' PEC_sw_drift_NL(100)
+PEC_sw_drift_NL <- function(rate, 
+                            applications = 1,
+                            water_depth = 30, 
+                            drift_percentages = c(1),
+                            rate_units = "g/ha",
+                            PEC_units = "\u00B5g/L")
+{
+  rate_units <- match.arg(rate_units)
+  PEC_units <- match.arg(PEC_units)
+  water_volume <- 100 * 100 * (water_depth/100) * 1000   # in L (for 1 ha)
+  PEC_sw_overspray <- rate * 1e6 / water_volume          # in µg/L
+  PEC_sw_drift <- PEC_sw_overspray * drift_percentages / 100
+  names(PEC_sw_drift) <- paste(drift_percentages, "%")
+  return(PEC_sw_drift)
+}