R/initial_spread_index.r

In nrcan-cfs-fire/cffdrs: Canadian Forest Fire Danger Rating System

#' Initial Spread Index Calculator
#'
#' @description Computes the Initial Spread Index From the FWI System. Equations
#' are from Van Wagner (1985) as listed below, except for the modification for
#' fbp taken from FCFDG (1992).
#'
#' Equations and FORTRAN program for the Canadian Forest Fire
#' Weather Index System. 1985. Van Wagner, C.E.; Pickett, T.L.
#' Canadian Forestry Service, Petawawa National Forestry
#' Institute, Chalk River, Ontario. Forestry Technical Report 33.
#' 18 p.
#'
#' Forestry Canada  Fire Danger Group (FCFDG) (1992). Development and
#' Structure of the Canadian Forest Fire Behavior Prediction System."
#' Technical Report ST-X-3, Forestry Canada, Ottawa, Ontario.
#'
#' @param ffmc Fine Fuel Moisture Code
#' @param ws Wind Speed (km/h)
#' @param fbpMod TRUE/FALSE if using the fbp modification at the extreme end
#'
#' @returns ISI - Intial Spread Index
#'
#' @noRd

initial_spread_index <- function(
    ffmc,
    ws,
    fbpMod = FALSE) {
  # Eq. 10 - Moisture content
  fm <- FFMC_COEFFICIENT * (101 - ffmc) / (59.5 + ffmc)
  # Eq. 24 - Wind Effect
  # the ifelse, also takes care of the ISI modification for the fbp functions
  # This modification is Equation 53a in FCFDG (1992)
  fW <- ifelse(
    ws >= 40 & fbpMod == TRUE,
    12 * (1 - exp(-0.0818 * (ws - 28))),
    exp(0.05039 * ws)
  )
  # Eq. 25 - Fine Fuel Moisture
  fF <- 91.9 * exp(-0.1386 * fm) * (1 + (fm^5.31) / 49300000)
  # Eq. 26 - Spread Index Equation
  isi <- 0.208 * fW * fF
  return(isi)
}

.ISIcalc <- function(...) {
  .Deprecated("initial_spread_index")
  return(initial_spread_index(...))
}