#' Calculate the direct storm runoff (Q) from P and CN
#'
#' @param P precipitation
#' @param CN curve number, in the range [0,100]
#' @param PQunits units in which P and Q are expressed (default="mm")
#'
#' @return list containing two elements: S () and Q (direct storm runoff).
#'
#' @export
#'
#' @examples
#' \dontrun{
#' SQ <- DirectStormRunoff(P = 30, CN = 80, PQunits = "mm")
#' }
#'
DirectStormRunoff <- function(P, CN, PQunits = "mm"){
if (PQunits=="mm"){
S <- 25400/CN - 254
}
if (PQunits=="inches"){
S <- 1000/CN - 10
}
if (P > 0.2*S) {
Q <- ((P - 0.2*S)^2) / (P + 0.8*S)
}else{
Q <- 0
}
return(list("S" = round(S, 3), "Q" = round(Q, 3)))
}
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