| hffmc {cffdrs} | R Documentation |
Hourly Fine Fuel Moisture Code
Description
hffmc is used to calculate hourly Fine Fuel Moisture Code
(FFMC) and is based on a calculation routine first described in detail by Van
Wagner (1977) and which has been updated in minor ways by the Canadian Forest
Service to have it agree with the calculation methodology for the daily FFMC
(see fwi). In its simplest typical use this current routine
calculates a value of FFMC based on a series of uninterrupted 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 time.step 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 time.step's value of FFMC in its calculation as well. This
function could be used for either one weather station or for multiple
weather stations.
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 time step; 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 time
step much greater than one hour. An important and implicit assumption in
this calculation is that the input weather is constant over the time step 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 time step, 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.
Typically this dataframe also contains date and hour fields so outputs can be associated with a specific day and time, however these fields are not used in the calculations. If multiple weather stations are being used, a weather station ID field is typically included as well, though this is simply for bookkeeping purposes and does not affect the calculation.
Usage
hffmc(
input,
ffmc_old = 85,
time.step = 1,
calc.step = FALSE,
batch = TRUE,
hourlyFWI = FALSE
)
Arguments
input |
A dataframe containing input variables of hourly weather observations. It is important that 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 important.
| ||||||||||||||||||||||
ffmc_old |
Initial FFMC. At the start of calculations at a particular
station there is a need to provide an estimate of the FFMC in the previous
timestep; this is because the FFMC is, in essence, a bookkeeping system for
moisture. If no estimate of previous hour's FFMC is available the function
will use default value, | ||||||||||||||||||||||
time.step |
Is the time (in hours) between the previous value of FFMC
and the current time at which we want to calculate a new value of the FFMC.
When not specified it will take on a default value of | ||||||||||||||||||||||
calc.step |
Optional for whether time step between two observations is calculated. Default is FALSE, no calculations. This is used when time intervals are not uniform in the input. | ||||||||||||||||||||||
batch |
Whether the computation is iterative or single step, default is
TRUE. When | ||||||||||||||||||||||
hourlyFWI |
Optional for the computation of hourly ISI, FWI, and DSR.
Default is FALSE. While |
Value
hffmc 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
Examples
library(cffdrs)
data("test_hffmc")
# show the data format:
head(test_hffmc)
# (1)hffmc default:
# Re-order the data by year, month, day, and hour:
test_hffmc <- test_hffmc[with(test_hffmc, order(yr, mon, day, hr)), ]
# Because the test data has 24 hours input variables
# it is possible to calculate the hourly FFMC chronically
# through multiple days(with the default initial ffmc_old=85):
test_hffmc$ffmc_default <- hffmc(test_hffmc)
# (2) Calculate FFMC for multiple stations:
# Calculate hourly FFMC with only one initial
# value (ffmc_old=85), but multiple weather stations.
# Sort the input by date/time and the station id:
test_hffmc <- test_hffmc[with(test_hffmc, order(yr, mon, hr)), ]
# Add weather station id:
test_hffmc$id <- rep(1:10, nrow(test_hffmc) / 10)
# check the data:
head(test_hffmc)
test_hffmc$ffmc01 <- hffmc(test_hffmc, batch = TRUE)
# With multiple initial FFMC (ffmc_old) as a vector:
test_hffmc$ffmc02 <- hffmc(
test_hffmc,
ffmc_old = sample(70:100, 10, replace = TRUE),
batch = TRUE
)
# One time step assuming all records are from different
# weather stations:
foo <- hffmc(test_hffmc, batch = FALSE)
# (3) output all hourly FWI System variables:
test_hffmc$id <- NULL
test_hffmc <- test_hffmc[with(test_hffmc, order(yr, mon, day, hr)), ]
foo <- hffmc(test_hffmc, hourlyFWI = TRUE)
# this will not run: warning message requesting for daily BUI
test_hffmc$bui <- 100
foo <- hffmc(test_hffmc, hourlyFWI = TRUE)
# (4) Calculate time steps in case the time intervals are
# not uniform:
dat0 <- test_hffmc[sample(1:30, 20), ]
dat0 <- dat0[with(dat0, order(yr, mon, day, hr)), ]
# with or without calc.step, hffmc is going to generate
# different FFMC values.
# without calculating time step (default):
hffmc(dat0, time.step = 1)
# with calc.step=TRUE, time.step=1 is applied to
# only the first record, the rests would be calculated:
hffmc(dat0, time.step = 1, calc.step = TRUE)