R/FMCcalc.r

In cffdrs: Canadian Forest Fire Danger Rating System

.FMCcalc <- function(LAT, LONG, ELV, DJ, D0) {                                                                                               # if D0, date of min FMC, is not known then D0 = NULL.
  #############################################################################
  # Description:
  #   Calculate Foliar Moisture Content on a specified day.
  #
  #   All variables names are laid out in the same manner as 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.
  #
  # Args:
  #   LAT:    Latitude (decimal degrees)
  #   LONG:   Longitude (decimal degrees)
  #   ELV:    Elevation (metres)
  #   DJ:     Day of year (offeren referred to as julian date)
  #   D0:     Date of minimum foliar moisture content
  #   
  # Returns:
  #   FMC:    Foliar Moisture Content
  #
  #############################################################################
  #Initialize vectors
  FMC <- rep(-1, length(LAT))
  LATN <- rep(0, length(LAT))   
  #Calculate Normalized Latitude
  #Eqs. 1 & 3 (FCFDG 1992)
  LATN <- ifelse(D0 <= 0,
                 ifelse(ELV <= 0, 
                        46 + 23.4 * exp(-0.0360 * (150 - LONG)),
                        43 + 33.7 * exp(-0.0351 * (150 - LONG))), 
                        LATN)
  #Calculate Date of minimum foliar moisture content
  #Eqs. 2 & 4 (FCFDG 1992)
  D0 <- ifelse(D0 <= 0, 
               ifelse(ELV <= 0, 
                      151 * (LAT / LATN), 
                      142.1 * (LAT / LATN) + 0.0172 * ELV),
                      D0)
  #Round D0 to the nearest integer because it is a date
  D0 <- round(D0,0) 
  #Number of days between day of year and date of min FMC
  #Eq. 5 (FCFDG 1992)
  ND <- abs(DJ-D0)
  #Calculate final FMC
  #Eqs. 6, 7, & 8 (FCFDG 1992)
  FMC <- ifelse(ND < 30, 85 + 0.0189 * ND^2,
                ifelse(ND >= 30 & ND < 50, 
                       32.9 + 3.17 * ND - 0.0288 * ND^2, 
                       120))
  
  return(FMC)
}