R/grass_fuel_moisture.r

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

#' Moisture content Calculation
#'
#' @description Calculation of moisture content for use in the GFMC calculation
#'
#' @param temp Temperature
#' @param rh Relative Humidity
#' @param ws Wind Speed
#' @param prec Precipitation
#' @param isol Insolation
#' @param GFMCold Yesterdays Grass Foliar Moisture Content
#' @param time.step Time step (hour) [default 1 hour]
#' @param roFL The nominal fuel load of the fine fuel layer, default is 0.3
#' kg/m^2
#'
#' @seealso \code{gfmc}

grass_fuel_moisture <- function(
    temp, rh, ws, prec,
    isol, GFMCold, roFL=0.3, time.step=1) {
  # Eq. 13 - Calculate previous moisture code
  MCold <- FFMC_COEFFICIENT * ((101 - GFMCold) / (59.5 + GFMCold))
  # Eq. 11 - Calculate the moisture content of the layer in % after rainfall
  MCr <- ifelse(prec > 0, MCold + 100 * (prec / roFL), MCold)
  # Constrain to 250
  MCr <- ifelse(MCr > 250, 250, MCr)
  MCold <- MCr
  # Eq. 2 - Calculate Fuel temperature
  Tf <- temp + 35.07 * isol * exp(-0.06215 * ws)
  # Eq. 3 - Calculate Saturation Vapour Pressure (Baumgartner et a. 1982)
  eS.T <- 6.107 * 10^(7.5 * temp / (237 + temp))
  # Eq. 3 for Fuel temperature
  eS.Tf <- 6.107 * 10^(7.5 * Tf / (237 + Tf))
  # Eq. 4 - Calculate Fuel Level Relative Humidity
  RH.f <- rh * (eS.T / eS.Tf)
  # Eq. 7 - Calculate Equilibrium Moisture Content for Drying phase
  EMC.D <- ((1.62 * RH.f^0.532 + 13.7 * exp((RH.f - 100) / 13.0))
  + 0.27 * (26.7 - Tf) * (1 - exp(-0.115 * RH.f)))
  # Eq. 7 - Calculate Equilibrium Moisture Content for Wetting phase
  EMC.W <- ((1.42 * RH.f^0.512 + 12.0 * exp((RH.f - 100) / 18.0))
  + 0.27 * (26.7 - Tf) * (1 - exp(-0.115 * RH.f)))
  # RH in terms of RH/100 for desorption
  Rf <- ifelse(MCold > EMC.D, RH.f / 100, rh)
  # RH in terms of 1-RH/100 for absorption
  Rf <- ifelse(MCold < EMC.W, (100 - RH.f) / 100, Rf)
  # Eq. 10 - Calculate Inverse Response time of grass (hours)
  K.GRASS <- 0.389633 * exp(0.0365 * Tf) * (0.424 * (1 - Rf^1.7) + 0.0694 *
    sqrt(ws) * (1 - Rf^8))
  # Fuel is drying, calculate Moisture Content
  MC0 <- ifelse(
    MCold > EMC.D,
    EMC.D + (MCold - EMC.D) * exp(-1.0 * log(10.0) * K.GRASS * time.step),
    MCold
  )
  # Fuel is wetting, calculate moisture content
  MC0 <- ifelse(
    MCold < EMC.W,
    EMC.W + (MCold - EMC.W) * exp(-1.0 * log(10.0) * K.GRASS * time.step),
    MC0
  )
  return(MC0)
}