hffmcRaster {cffdrs}

Raster-based Hourly Fine Fuel Moisture Code

Description

hffmcRaster is used to calculate hourly Fine Fuel Moisture Code (FFMC) based on hourly weather observations of screen level (~1.4 m) temperature, relative humidity, 10 m wind speed, and 1-hour rainfall. This implementation of the function includes an optional timestep input which is defaulted to one hour, but can be reduced if sub-hourly calculation of the code is needed. The FFMC is in essence a bookkeeping system for moisture content and thus it needs to use the last timestep's value of FFMC in its calculation. hffmcRaster takes rasterized inputs and generates raster maps as outputs.

The hourly FFMC is very similar in its structure and calculation to the Canadian Forest Fire Weather Index System's daily FFMC (fwi) but has an altered drying and wetting rate which more realistically reflects the drying and wetting of a pine needle litter layer sitting on a decaying organic layer. This particular implementation of the Canadian Forest Fire Danger Rating System's hourly FFMC provides for a flexible timestep; that is, the data need not necessarily be in time increments of one hour. This flexibility has been added for some users who use this method with data sampled more frequently that one hour. We do not recommend using a timestep much greater than one hour. An important and implicit assumption in this calculation is that the input weather is constant over the timestep of each calculation (e.g., typically over the previous hour). This is a reasonable assumption for an hour; however it can become problematic for longer periods. For brevity we have referred to this routine throughout this description as the hourly FFMC.

Because of the shortened timestep, which can lead to more frequent calculations and conversion between moisture content and the code value itself, we have increased the precision of one of the constants in the simple formula that converts litter moisture content to the 'Code' value. This is necessary to avoid a potential bias that gets introduced during extremely dry conditions. This is simply a change in the precision at which this constant is used in the equation and is not a change to the standard FFMC conversion between moisture and code value (which is referred to as the FF-scale).

The calculation requires the previous hour's FFMC as an input to the calculation of the current hour's FFMC; this is because the routine can be thought of as a bookkeeping system and needs to know the amount of moisture being held in the fuel prior to any drying or wetting in the current period. After each hour's calculation that newly calculated FFMC simply becomes the starting FFMC in the next hour's calculation. At the beginning of the calculations at a station this previous hours FFMC must be estimated. It is typical to use a value of 85 when this value cannot be estimated more accurately; this code value corresponds to a moisture content of about 16% in typical pine litter fuels.

Usage

hffmcRaster(weatherstream, ffmc_old = 85, time.step = 1, hourlyFWI = FALSE)

Arguments

weatherstream

A raster stack or brick containing hourly weather observations. Variable names have to be the same as in the following list, but they are case insensitive. The order in which the input variables are entered is not required.

temp	(required)	Temperature (centigrade)
rh	(required)	Relative humidity (%)
ws	(required)	10-m height wind speed (km/h)
prec	(required)	1-hour rainfall (mm)
bui	(optional)	Daily BUI value for the computation of hourly FWI.
		It is required when hourlyFWI=TRUE.

ffmc_old

A single value of FFMC or a raster of FFMC for the previous hour which will be used for the current hour's calculation. In some situations, there are no previous-hourly FFMC values to calculate the current hourly FFMC, the function will use a default value, ffmc_old=84.

time.step

timestep in hours. Default is 1 hour, set for standard hourly FFMC calculation. While time.step is set to values with decimal places, sub-hourly FFMC would be calculated.

hourlyFWI

Optional for the computation of hourly ISI, FWI, and DSR. Default is FALSE. While hourlyFWI=TRUE, daily BUI is required for the computation of FWI.

Value

hffmcRaster returns a vector of hourly or sub-hourly FFMC values, which may contain 1 or multiple elements. Optionally when hourlyFWI=TRUE, the function also output a data.frame contains input weatherstream as well as the hourly or sub-hourly FFMC, ISI, FWI, and DSR.

Author(s)

Xianli Wang, Mike Wotton, Alan Cantin, Brett Moore, and Mike Flannigan

References

Van Wagner, C.E. 1977. A method of computing fine fuel moisture content throughout the diurnal cycle. Environment Canada, Canadian Forestry Service, Petawawa Forest Experiment Station, Chalk River, Ontario. Information Report PS-X-69. https://cfs.nrcan.gc.ca/pubwarehouse/pdfs/25591.pdf

See Also

fbp, fwi, hffmc

Examples

library(cffdrs)
require(terra)
## load the test data for the first hour, namely hour01:
hour01src <- system.file(
  "extdata",
  "test_rast_hour01.tif",
  package = "cffdrs"
)
hour01 <- terra::rast(hour01src)
# Assign names to the layers:
names(hour01) <- c("temp", "rh", "ws", "prec")
# (1) Default, based on the initial value:
foo <- hffmcRaster(hour01)
plot(foo)
### Additional, longer running examples ###
# (2) Based on previous day's hffmc:
# load the test data for the second hour, namely hour02:
hour02src <- system.file(
  "extdata",
  "test_rast_hour02.tif",
  package = "cffdrs"
)
hour02 <- terra::rast(hour02src)
# Assign variable names to the layers:
names(hour02) <- c("temp", "rh", "ws", "prec")

foo1 <- hffmcRaster(hour02, ffmc_old = foo)


plot(foo1)

# (3) Calculate other hourly FWI components (ISI, FWI, and DSR):
# Need BUI layer,
bui <- hour02$temp
values(bui) <- 50
hour02 <- c(hour02, bui)
# Re-assign variable names to the layers:
names(hour02) <- c("temp", "rh", "ws", "prec", "bui")
# Calculate all the variables:

foo2 <- hffmcRaster(hour02, ffmc_old = foo, hourlyFWI = TRUE)

# Visualize the maps:

plot(foo2)