R/DirectStormRunoff.R

In cvitolo/curvenumber: An implementation of the USDA Soil Conservation Service Curve Number method

#' 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)))

}