Data Sources and Methods
Canadian Forest Fire Weather Index (FWI) System
Data Sources and Methods for Daily Maps
Inputs to the FWI System include elevation and current daily weather data from a variety of sources. Geographic Information Systems (GIS) software is used to interpolate the weather data between stations to produce gridded weather maps. The FWI System components are then calculated on a cell-by-cell basis according to the equations in Van Wagner and Pickett (1985) to produce the FWI maps.
Elevation (view elevation map)
The elevation grid was derived from the US Geological Survey's 1 x 1 km hydrologically correct digital elevation model (DEM) for North America, downloaded from the USGS web site. The DEM is a hydrologically corrected version of GTOPO30, a 30-arcsecond global DEM assembled by the USGS. In Canada, the data for GTOPO30 were taken from two sources, Digital Chart of the World and Digital Terrain Elevation Data, both produced by the US National Imagery and Mapping Agency (formerly the Defense Mapping Agency).
Weather Data (view weather stations map)
The CWFIS currently uses weather data from about 900 stations in Canada and the northern United States. About 700 of these stations are operated by Environment Canada (EC) or other federal departments under contract to EC. Sixteen stations are operated by the National Weather Service (NWS) of the United States, and the remainder of the stations are operated by the provincial governments of Nova Scotia, Quebec, and Saskatchewan. Inclusion, over the next year, of stations operated by other provinces is planned.
The EC and NWS data are obtained via Telesat Canada's Anik satellite. Weather observations are collected from the Canadian stations by the Canadian Meteorological Centre in Montreal and transferred to Telesat Canada in Toronto to be uplinked to the Anik satellite. The data are then downlinked to the Northern Forestry Centre in Edmonton for processing and storage.
The provincial data are collected by fire management agencies and transferred daily to the Northern Forestry Centre by FTP.
Forecast Weather Data
Forecasted weather used in the Canadian Wildland Fire Information System is provided by the Canadian Meteorological Centre, a branch of Environment Canada. These data take the form of spot forecasts for 750 Canadian weather stations or sample points. Weather elements are generated from the regional Global Environmental Multiscale (GEM) model and model output statistics (MOS) for three-hour intervals out to 48 hours. Noon weather is then interpolated from these data and fire danger conditions are calculated. Note that the spot forecasts are straight model outputs or statistically post-processed data and do not include input from regional forecast offices
Extended forecasts are based upon the North American Ensemble Forecast System (NAEFS). Median values of the 40-member ensemble are used to predict temperature, humidity wind speed and 24-hour precipitation at over 250 weather stations for the next 14 days. These values are used to generate the extended forecast maps and the predicted fire weather conditions. Because of the range of these forecasts, accuracy is limited. These forecasts are best used to judge the trend of long-term indices such as the duff moisture code (DMC), the drought code (DC) and the build-up index (BUI).
FWI Calculation Start-up
To determine spring start-up dates and starting fuel moisture code values, the procedures described in Turner and Lawson (1978) are used. There are two methods, depending on snow cover.
For stations that report significant snow cover during the winter, start-up occurrs when the station has been snow-free for three consecutive days. Significant snow cover is defined as a mean snow depth of more than 10 cm, with snow cover present at least 75% of the days in January and February. Start-up values are as follows:
- Fine Fuel Moisture Code (FFMC) set to 85
- Duff Moisture Code (DMC) set to 6
- Drought Code (DC) set to 15*
For stations that do not report significant snow cover during the winter, start-up occurs when the mean daily temperature is 6°C or higher for three consecutive days. This temperature represents the approximate lower limit for plant growth. Start-up values are as follows:
- FFMC set to 85
- DMC set to 2 times the number of days since precipitation
- DC set to 5 times the number of days since precipitation*
*Where overwinter precipitation data are available, the DC is set according to an alternative, more rigorous method, Turner and Lawson (1978), which takes into account the fact that saturation of the deep fuel layers may not occur over the winter. In areas with low overwinter precipitation, the starting value for the DC can be significantly higher than the default.
Daily Weather Processing
Weather observations are received in raw format and must be decoded before being saved in the weather database. The FWI System requires observed temperature, relative humidity, and wind speed at noon local standard time, as well as 24-hour precipitation. Once the noon observations have been received for all the time zones, the 24-hour precipitation is calculated for stations that report every hour, three times per day, four times per day, or irregularly. Various other observations, such as wind direction, dew point, and atmospheric pressure, are also saved in the database to be used for interpolation.
The FWI System requires an unbroken record of daily weather. If a station fails to report or reports missing data, the missing values are estimated from nearby stations by means of inverse distance weighted (IDW) interpolation. For temperature and relative humidity, the IDW interpolated value is corrected for elevation.
Lastly, daily FWI system output values are calculated for each station and saved in the database.
Daily Weather Grids
Daily raster maps (grids) of temperature, relative humidity, wind speed, and precipitation are created by interpolating values between weather stations using IDW interpolation. Values are assigned to each grid cell by calculating a weighted mean of the values of the nearest 12 stations. For each cell, the station values are weighted by the inverse of the square of the distance to the cell.
The weather grids are then used as inputs to the Fire Weather Index (FWI) and Fire Behavior Prediction (FBP) grid calculations.
Temperature and relative humidity grid values are adjusted for elevation using the elevation grid (see above). For temperature, the adjustment is based on the United States Standard Lapse Rate of -6.5°C/km; that is, for every kilometer gain in elevation, the temperature is assumed to drop by 6.5°C. For relative humidity, the mixing ratio (ratio of water vapor to dry air by weight) is assumed to be constant with elevation. The mixing ratio is calculated for each station and interpolated to each grid cell location. The relative humidity is then calculated an a cell-by-cell basis using the elevation-adjusted temperature grid.
Daily Grid Production
The grids for the fuel moisture codes (FFMC, DMC, and DC) are built using both interpolation and calculation. Because the fuel moisture code calculations require the previous day's values as inputs, values for areas where new stations are starting up are interpolated rather than calculated. In areas where the previous day's values are available, the fuel moisture codes are calculated on a cell-by-cell basis using the previous day's grids, together with the current day's weather grids, as inputs. In the output maps, the non-calculating areas are assigned a null value, as are areas above the arctic treeline.
The FWI System fire behavior indices (not to be confused with the Fire Behavior Prediction System outputs) are calculated from the fuel moisture codes. These calculations are done on a cell-by-cell basis to produce the ISI, BUI, and FWI grids. Lastly, the Daily Severity Rating (DSR) grid is calculated from the FWI grid. The DSR gives a relative rating of fire control difficulty, or the amount of work required to suppress a fire.
Turner, J.A.; Lawson, B.D. 1978. Weather in the Canadian Forest Fire Danger Rating System. A user guide to national standards and practices. Environment Canada, Pacific Forest Research Centre, Victoria, BC. BC-X-177.
Van Wagner, C.E.; Pickett, T.L. 1985. Equations and FORTRAN program for the Canadian Forest Fire Weather Index System. Canadian Forest Service, Ottawa, ON. Forestry Technical Report 33.