R/ffmcCalc.R

In cffdrs: Canadian Forest Fire Danger Rating System

.ffmcCalc <- function(ffmc_yda, temp, rh, ws, prec) {
  #############################################################################
  # Description: Fine Fuel Moisture Code 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: ffmc_yda:   The Fine Fuel Moisture Code from previous iteration
  #           temp:   Temperature (centigrade)
  #             rh:   Relative Humidity (%)
  #           prec:   Precipitation (mm)
  #             ws:   Wind speed (km/h)
  #       
  #
  # Returns: A single ffmc value
  #
  #############################################################################
  #Eq. 1
  wmo <- 147.2 * (101 - ffmc_yda)/(59.5 + ffmc_yda)
  #Eq. 2 Rain reduction to allow for loss in 
  #  overhead canopy
  ra <- ifelse(prec > 0.5, prec - 0.5, prec)
  #Eqs. 3a & 3b
  wmo <- ifelse(prec > 0.5, 
                ifelse(wmo > 150, wmo + 0.0015 * (wmo - 150) * (wmo - 150) * 
                         sqrt(ra) + 42.5 * ra * exp(-100 / (251 - wmo)) * 
                         (1 - exp(-6.93 / ra)), 
                       wmo + 42.5 * ra * exp(-100 / (251 - wmo)) * 
                         (1 - exp(-6.93 / ra))), 
                wmo)
  #The real moisture content of pine litter ranges up to about 250 percent,
  # so we cap it at 250
  wmo <- ifelse(wmo > 250, 250, wmo)
  #Eq. 4 Equilibrium moisture content from drying
  ed <- 0.942 * (rh^0.679) + (11 * exp((rh - 100) / 10)) + 0.18 * 
    (21.1 - temp) * (1 - 1 / exp(rh * 0.115))
  #Eq. 5 Equilibrium moisture content from wetting
  ew <- 0.618 * (rh^0.753) + (10 * exp((rh - 100) / 10)) + 0.18 * 
    (21.1 - temp) * (1 - 1 / exp(rh * 0.115))
  #Eq. 6a (ko) Log drying rate at the normal
  #  termperature of 21.1 C
  z <- ifelse(wmo < ed & wmo < ew, 
              0.424 * (1 - (((100 - rh) / 100)^1.7)) + 0.0694 * 
                sqrt(ws) * (1 - ((100 - rh) / 100)^8), 
              0)
  #Eq. 6b Affect of temperature on  drying rate
  x <- z * 0.581 * exp(0.0365 * temp)
  #Eq. 8
  wm <- ifelse(wmo < ed & wmo < ew, ew - (ew - wmo)/(10^x), wmo)
  #Eq. 7a (ko) Log wetting rate at the normal
  #  termperature of 21.1 C    
  z <- ifelse(wmo > ed, 0.424 * (1 - (rh/100)^1.7) + 0.0694 * sqrt(ws) * 
                (1 - (rh/100)^8), z)
  #Eq. 7b Affect of temperature on  wetting rate
  x <- z * 0.581 * exp(0.0365 * temp)
  #Eq. 9
  wm <- ifelse(wmo > ed, ed + (wmo - ed)/(10^x), wm)
  #Eq. 10 Final ffmc calculation
  ffmc1 <- (59.5 * (250 - wm))/(147.2 + wm)
  #Constraints
  ffmc1 <- ifelse(ffmc1 > 101, 101, ffmc1)
  ffmc1 <- ifelse(ffmc1 < 0, 0, ffmc1)
  return(ffmc1)
}