R/saturatedVP.R

In CentreForHydrology/CRHMr: Pre- and post- processing for CRHM

#' Converts calculates saturated vapour pressure from air temperature
#'
#' @description This is an internal \pkg{CRHMr} function and should normally not
#' need to be called directly.
#' @param airTemp Required. Air temperature in \eqn{^\circ}{ }C.
#'
#' @return If successful, returns the saturated vapour pressure in kPa.
#' If unsuccessful, returns the value \code{FALSE}.
#' @author Kevin Shook
#' @export
#' @keywords internal
#' @examples
#' estar <- saturatedVP(-5)
#'
saturatedVP <- function(airTemp){
  #check parameter

  if (length(airTemp) == 0) {
    cat('Error: air temps missing\n')
    return(FALSE)
  }

  if (max(airTemp) > 200) {
    cat('Error: air temps must be in C NOT K\n')
    return(FALSE)
  }

  if (length(airTemp) == 1) {
    if (airTemp <= 0)
      estar <- 0.611 * exp((21.88 * airTemp) / (airTemp + 265.5))
    else
      estar <- 0.611 * exp((17.27 * airTemp) / (airTemp + 237.3))
  }
  else{
    estar <- 0.611 * exp((21.88 * airTemp) / (airTemp + 265.5))
    estar[airTemp > 0] <- 0.611 * exp((17.27 * airTemp[airTemp > 0]) /
                                        (airTemp[airTemp > 0] + 237.3))
  }

  return(estar)
}