TSCF estimation equations, update docs

Briggs et al. (1982) and Dettenmaier et al. (2009)

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+#' Estimation of the transpiration stream concentration factor
+#'
+#' The FOCUS groundwater guidance (FOCUS 2014, p. 41) states that a reliable measured
+#' log Kow for neutral pH must be available in order to apply the Briggs
+#' equation. It is not clarified when it can be regarded reliable, but the
+#' equation is stated to be produced for non-ionic compounds, suggesting that
+#' the compound should not be ionogenic (weak acid/base)
+#' or ionic.
+#'
+#' The Dettenmaier equation is given to show that other views on the subject exist.
+#' @references FOCUS (2014) Generic Guidance for Tier 1 FOCUS Ground Water Assessments.
+#'  Version 2.2, May 2014
+#' Dettenmaier EM, Doucette WJ and Bugbee B (2009) Chemical hydrophobicity and uptake
+#' by plant roots. Environ. Sci. Technol 43, 324 - 329
+#' @param log_Kow The decadic logarithm of the octanol-water partition constant
+#' @param method Short name of the estimation method.
+#' @export
+#' @examples
+#' plot(TSCF, -1, 5, xlab = "log Kow", ylab = "TSCF", ylim = c(0, 1.1))
+#' TSCF_2 <- function(x) TSCF(x, method = "dettenmaier09")
+#' curve(TSCF_2, -1, 5, add = TRUE, lty = 2)
+#' legend("topright", lty = 1:2, bty = "n",
+#'   legend = c("Briggs et al. (1982)", "Dettenmaier et al. (2009)"))
+TSCF <- function(log_Kow, method = c("briggs82", "dettenmaier09")) {
+  method <- match.arg(method)
+  TSCF <- switch(method,
+    briggs82 = 0.784 * exp(- (log_Kow - 1.78)^2 / 2.44),
+    dettenmaier09 = 11 /( 11 + 2.6^log_Kow)
+  )
+  return(TSCF)
+}