R/fwiCalc.R

In cffdrs: Canadian Forest Fire Danger Rating System

.fwiCalc <- function(isi, bui) {
  #############################################################################
  # Description: Fire Weather Index Calculation. All code
  #              is based on a C code library that was written by Canadian
  #              Forest Service Employees, which was originally based on
  #              the Fortran code listed in the reference below. All equations
  #              in this code refer to that document.
  #
  #              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.
  #
  #              Additional reference on FWI system
  #
  #              Development and structure of the Canadian Forest Fire Weather 
  #              Index System. 1987. Van Wagner, C.E. Canadian Forestry Service,
  #              Headquarters, Ottawa. Forestry Technical Report 35. 35 p.
  #  
  #
  # Args:   isi:    Initial Spread Index
  #         bui:    Buildup Index
  #
  # Returns: A single fwi value
  #
  #############################################################################
  #Eqs. 28b, 28a, 29
  bb <- ifelse(bui > 80, 0.1 * isi * (1000/(25 + 108.64/exp(0.023 * bui))), 
               0.1 * isi * (0.626 * (bui^0.809) + 2))
  #Eqs. 30b, 30a
  fwi <- ifelse(bb <= 1, bb, exp(2.72 * ((0.434 * log(bb))^0.647)))
  return(fwi)
}