wDC: Overwintering Drought Code

Description Usage Arguments Details Value Author(s) References See Also Examples

View source: R/winter_DC.r

Description

wDC calculates an initial or season starting Drought Code (DC) value based on a standard method of overwintering the Drought Code (Lawson and Armitage 2008). This method uses the final DC value from previous year, over winter precipitation and estimates of how much over-winter precipitation 'refills' the moisture in this fuel layer. This function could be used for either one weather station or for multiple weather stations.

Usage

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wDC(DCf=100,rw=200,a=0.75,b=0.75)

Arguments

DCf

Final fall DC value from previous year

rw

Winter precipitation (mm)

a

User selected values accounting for carry-over fraction (view table below)

b

User selected values accountain for wetting efficiency fraction (view table below)

User-selected values and criteria for constants a and b, overwintering the Drought Code. Source: Lawson and Armitage (2008) - Table 9.

Constant Value Criteria
Carry-over fraction of last fall's moisture (a) 1.0 Daily DC calculated up to 1 November; continuous snow cover, or freeze-up, whichever comes first
0.75 Daily DC calculations stopped before any of the above conditions met or the area is subject to occasional winter chinook conditions, leaving the ground bare and subject to moisture depletion
0.5 Forested areas subject to long periods in fall or winter that favor depletion of soil moisture
Effectiveness of winter precipitation in recharging moisture reserves in spring (b) 0.9 Poorly drained, boggy sites with deep organic layers
0.75 Deep ground frost does not occur until late fall, if at all; moderately drained sites that allow infiltration of most of the melting snowpack
0.5 Chinook-prone areas and areas subject to early and deep ground frost; well-drained soils favoring rapid percolation or topography favoring rapid runoff before melting of ground frost

Details

Of the three fuel moisture codes (i.e. FFMC, DMC and DC) making up the FWI System, only the DC needs to be considered in terms of its values carrying over from one fire season to the next. In Canada both the FFMC and the DMC are assumed to reach moisture saturation from overwinter precipitation at or before spring melt; this is a reasonable assumption and any error in these assumed starting conditions quickly disappears. If snowfall (or other overwinter precipitation) is not large enough however, the fuel layer tracked by the Drought Code may not fully reach saturation after spring snow melt; because of the long response time in this fuel layer (53 days in standard conditions) a large error in this spring starting condition can affect the DC for a significant portion of the fire season. In areas where overwinter precipitation is 200 mm or more, full moisture recharge occurs and DC overwintering is usually unnecessary. More discussion of overwintering and fuel drying time lag can be found in Lawson and Armitage (2008) and Van Wagner (1985).

Value

wDC returns either a single value or a vector of wDC values.

Author(s)

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

References

Lawson B.D. and Armitage O.B. 2008. Weather Guide for the Canadian Forest Fire Danger Rating System. Natural Resources Canada, Canadian Forest Service, Northern Forestry Centre, Edmonton, Alberta. 84 p. http://cfs.nrcan.gc.ca/pubwarehouse/pdfs/29152.pdf

Van Wagner, C.E. 1985. Drought, timelag and fire danger rating. Pages 178-185 in L.R. Donoghue and R.E. Martin, eds. Proc. 8th Conf. Fire For. Meteorol., 29 Apr.-3 May 1985, Detroit, MI. Soc. Am. For., Bethesda, MD. http://cfs.nrcan.gc.ca/pubwarehouse/pdfs/23550.pdf

See Also

fwi, fireSeason

Examples

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library(cffdrs)
# The standard test data:
data("test_wDC")
# (1) Simple case previous fall's DC was 300, overwinter 
# rain 110mm
winter_DC <- wDC(DCf=300,rw=110)
winter_DC
#(2) modified a and b parameters. Find table values in listed 
# reference for Lawson and Armitage, 2008.
winter_DC <- wDC(DCf=300,rw=110,a=1.0,b=0.9)
winter_DC
#(3)with multiple inputs:
winter_DC <- wDC(DCf=c(400,300,250), rw=c(99,110,200),
                   a=c(0.75,1.0,0.75), b=c(0.75,0.9,0.75))
winter_DC
#(4) A realistic example:
#precipitation accumulation and date boundaries
input <- test_wDC
#order data by ID and date
input <- with(input,input[order(id,yr,mon,day),])
input$date <- as.Date(as.POSIXlt(paste(input$yr,"-",input$mon,"-",input$day,sep="")))
#select id value 1
input.2 <- input[input$id==2,]
#Explicitly defined fire start and end dates.
data("test_wDC_fs")
print(test_wDC_fs)
#Set date field
test_wDC_fs$date <- as.Date(as.POSIXlt(paste(test_wDC_fs$yr,"-",test_wDC_fs$mon,"-",
                                             test_wDC_fs$day,sep="")))
#match to current id value
input.2.fs <- test_wDC_fs[test_wDC_fs$id==2,]
#assign start of winter date (or end of fire season date)
winterStartDate <- input.2.fs[2,"date"]
#assign end of winter date (or start of new fire season date)
winterEndDate <-  input.2.fs[3,"date"]
#Accumulate overwinter precip based on chosen dates
curYr.prec <- sum(input.2[(input.2$date>winterStartDate & input.2$date < winterEndDate),]$prec)
#Assign a fall DC value
fallDC <- 500
#calculate winter DC
winter_DC <- wDC(DCf=fallDC,rw=curYr.prec)
winter_DC
#Assign a different fall DC value
fallDC <- 250
#calculate winter DC
winter_DC <- wDC(DCf=fallDC,rw=curYr.prec,a=1.0)
winter_DC

Example output

Loading required package: rgdal
Loading required package: sp
rgdal: version: 1.5-18, (SVN revision 1082)
Geospatial Data Abstraction Library extensions to R successfully loaded
Loaded GDAL runtime: GDAL 3.0.4, released 2020/01/28
Path to GDAL shared files: /usr/share/gdal
GDAL binary built with GEOS: TRUE 
Loaded PROJ runtime: Rel. 6.3.1, February 10th, 2020, [PJ_VERSION: 631]
Path to PROJ shared files: /usr/share/proj
Linking to sp version:1.4-4
To mute warnings of possible GDAL/OSR exportToProj4() degradation,
use options("rgdal_show_exportToProj4_warnings"="none") before loading rgdal.
Loading required package: raster
Loading required package: foreach
[1] 109.4657
[1] 16.35315
[1] 177.52181  16.35315  15.00000
  id lat long   yr mon day fsdatetype
1  1  50  -85 1999   5   1      start
2  1  50  -85 1999   9  30        end
3  1  50  -85 2000   6   5      start
4  1  50  -85 2000  10   1        end
5  2  55 -110 1980   4  20      start
6  2  55 -110 1980  10   6        end
7  2  55 -110 1981   5  28      start
8  2  55 -110 1981   9  30        end
[1] 385.0668
[1] 141.383

cffdrs documentation built on July 1, 2020, 6:04 p.m.