Monitoring and Estimating the Burn Period
The NWCG Fire Glossary defines the burn period as that part of each 24-hour period when fires spread most rapidly; typically from 1000 to sundown.
In most cases, the burn period refers to the period when fire is actively spreading at the head of the fire. If the 6 categories of visual fire behavior are considered, the 24-hour day includes all of them. Field Observers should be careful to report/describe their estimate of burn period accurately and purposefully:
- Smoldering fire behavior continues around the clock for most active fires. It does not represent any part of an active burn period if reported at the head.
- Creeping fire behavior may continue through the night, but is generally transitional between smoldering and running fire behavior. Generally, it produces little overall fire spread and is not considered part of the burn period if observed at the head of the fire.
- Running fire behavior describes what is encountered during the burn period on most days when fire spread and overall growth is low to moderate. However, it may represent transitional fire behavior when more significant Torching/Spotting or Crowning fire behavior occurs during peak hours.
- On days when Torching/Spotting and Crowning fire behavior is observed, the burn period should probably exclude much of the time the fire behavior is Running at the beginning and the end of the most active period.
Burn period can vary from day to day for a variety of reasons:
- Solar Radiation heats fuels as well as warming the air and lowering relative humidity. These influences lower fuel moisture, creating conditions favorable for active burning. Affected by the sun angle based on the time of year and latitude. Cloud cover and canopy shading can further reduce solar radiation.
- Fuelbed characteristics can influence burn period as well. Moisture content of light fuels, such as grasses, respond more quickly to changes in temperature and humidity.
- Diurnal fuel moisture trends are affected by the quality of night time humidity recovery and inversions. Slope/aspect and recent precipitation all affect the length of the burning period for a given situation.
- Drought can influence the length of the burn period through the heat produced in the burning of heavy fuels.
- Direction of Spread can be an important factor as well. Backing spread can start later and end earlier in the day for a given situation.
In the validation of your estimate, there are tools and criteria that can help identify when the burn period starts and ends.
- Fireline Observations are probably the first and most important source of information for determining the burn period. Try and get answers to specific questions as you pursue a reasonable estimate. When and where did fire begin to move and when did it slow down on previous days? Was there signficant spread during the night? What were observed spread rates and when?
Sometimes these reports are incomplete and need to be correlated to other information as suggested below. FSPro seeks burn period information for different types of days. These factors suggest that fireline observations should be reinforced with these other information sources where possible
- Sunrise-Sunset Tables (time of year and latitude) from BehavePlus and solar radiation sensors can show periodicity and suggest timing of beginning and end of active spread.
- Diurnal Wind, Weather and Fuel Moisture Trends can similarly show a periodicity that can suggest timing of active spread. Graphs displaying these trends are readily available at http://mesowest.utah.edu.
- Fire Progression Maps suggest the overall daily spread around the fire, and with knowledge of weather conditions, fuels, slope and spread direction, can be compared to modeled growth. A new resource called the Wildland Fire Library, http://firelibrary.org, provides a variety of historical references including fire progression maps
WFDSS Help suggests that “The default burn period in NTFB is 24 hours; however, modeling a fire overnight is generally not advised. NTFB, like FARSITE, has a tendency to over-predict overnight fire spread. For this reason, most analysts shorten the duration that the modeled fire is allowed to burn each day.”
Each fire growth projection, whether using non-spatial tools (BehavePlus) or spatial tools (WFDSS analyses NTFB and FSPro) specify a duration as the number of hours or minutes to obtain a resulting fire size and/or perimeter. Characterizing the duration as the number of hours or minutes in a day (burn period) for a projection allows the user to model growth for multiple days.
Evaluating Burn Day Frequency
Most of the area burned across any management unit, geographic area, or ecological region occur on relatively few days over the life of the fires that are included in them. Burn Day can help identify which days have conditions receptive to significant fire spread. The concept recognizes that there are thresholds in environmental conditions below which significant spread is substantially limited, even when fire spread models estimate that spread will continue. A simple classification of days into [Burn Days] and ‘non [Burn Days]’ can greatly improve estimates of area burned. (Podur and Wotton, 2011).
- WFDSS FSPro analysis defines a threshold ERC for each analysis, below which no fire spread is modeled for the entire day. Among its outputs, a distribution of “burn days” spread across the ERC classes is provided. It is possible to define daily environmental conditions favorable to significant growth and use those to estimate the frequency with which they occur to adjust the default 60th percentile ERC as that threshold (Ziel, 2015).
- WFDSS NTFB analysis requires input of a burn period for each of the included days. Specifying same start and end hour for a given day essentially defines it as a non-burn day. The same daily conditions mentioned for FSPro can be used as criteria to exclude specific days in NTFB analyses.