R/ISIcalc.r

Defines functions .ISIcalc

.ISIcalc <- function(ffmc, ws, fbpMod=FALSE){
  #############################################################################
  # 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
  #   takene 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 ReportST-X-3, Forestry Canada, Ottawa, Ontario.
  #
  # Args:
  #   ffmc:   Fine Fuel Moisture Code
  #     ws:   Wind Speed (km/h)
  # fbpMod:   TRUE/FALSE if using the fbp modification at the extreme end
  #
  # Returns:
  #   ISI:    Intial Spread Index
  #
  #############################################################################
  #Eq. 10 - Moisture content
  fm <- 147.2 * (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)
}

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cffdrs documentation built on July 1, 2020, 6:04 p.m.