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.
Final fall DC value from previous year
Winter precipitation (mm)
User selected values accounting for carry-over fraction (view table below)
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.
|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|
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).
wDC returns either a single value or a vector of wDC values.
Xianli Wang, Mike Wotton, Alan Cantin, and Mike Flannigan
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
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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
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  109.4657  16.35315  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  385.0668  141.383
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