R/ET0_PT.R

In CropWaterBalance: Climate Water Balance for Irrigation Purposes

#' Reference Evapotranspiration Using the Preistley-Taylor Method
#'
#' Calculates daily reference evapotranspiration amounts using the
#'   Priestley-Taylor method.
#'
#' @param Rn
#' A vector, 1-column matrix or data frame with daily net radiation in
#'  \eqn{MJ m-2 day-1}.
#' @param Tavg
#' A vector, 1-column matrix or data frame with daily average air temperature.
#' @param G
#' Optional. A vector, 1-column matrix or data frame with daily soil heat flux
#'   in \eqn{MJ m-2 day-1}.
#' May be provided by \code{\link{Soil_Heat_Flux}}
#' @param Coeff
#' Single number defining the Priestley and Taylor coefficient. Default is
#'   1.26.
#
#' @return
#' A matrix object of the daily potential evapotranspiration values in
#'  millimetres.
#'
#' @export
#' @examples
#' # See `?DataForCWB` for more on this data set
#' Tavg <- DataForCWB[, 2]
#' Rn <- DataForCWB[, 6]
#' G <- DataForCWB[, 9]
#' ET0_PT(Tavg = Tavg, Rn = Rn, G = G)
#'
ET0_PT <- function(Tavg, Rn, G = NULL, Coeff = 1.26) {
  Tavg <- as.matrix(Tavg)
  if (!is.numeric(Tavg) || any(is.na(Tavg)) ||
    length(Tavg[Tavg > 70]) != 0 || length(Tavg[Tavg < -70]) != 0 ||
    ncol(Tavg) != 1) {
    stop("Physically impossible or missing Tavg values")
  }
  n <- length(Tavg)
  if (is.null(G)) {
    G <- Soil_Heat_Flux(Tavg)
  }
  G <- as.matrix(G)
  Rn <- as.matrix(Rn)
  if (!is.numeric(Rn) || any(is.na(Rn)) ||
    length(Rn[Rn > 70]) != 0 || length(Rn[Rn < -70]) != 0 ||
    ncol(Rn) != 1 || length(Rn) != n ||
    !is.numeric(G) || any(is.na(G)) ||
    length(G[G > 20]) != 0 || length(G[G < -20]) != 0 ||
    ncol(G) != 1 || length(G) != n || !is.numeric(Coeff)) {
    stop("Physically impossible or missing Coeff, Rn or G values")
  }

  S <- (2503.058 / (Tavg + 237.3)) * exp((17.27 * Tavg) / (Tavg + 237.3))
  ET0 <- Coeff * (Rn - G) * (S / (2.45 * (S + 0.063)))
  colnames(ET0) <- "ET0_PT"
  return(ET0)
}