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.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 <- 147.27723 * (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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