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Verification, Calibration, and Validation

Reference Fire History

Wildland Fire Library (https://firelibrary.org/)

Wildland Fire Library

General Analysis Factors

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.

Burning Thresholds

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 evaluation 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).  There can be important within and between season variation in this pattern.

  • 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).
ERC Class Burn Days Summary
  • 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.
NTFB Daily Burn Periods

The Weather Forecast

In most cases, the forecast has difficulty outperforming climatology beyond 48-72 hours. Most important are confidence in wind and precipitation elements. Read the area forecast discussion for insight into the confidence horizon.

Extend STFB and NTFB analyses only to the limits of very high confidence, usually 1-3 days. FSPro Analyses combine forecast and climatology. Extend forecast out beyond 3 days only if confidence for the extended forecast is unusually high. Critically examine the wind speed and direction in the forecast.

Winds

With a sparse network of RAWS stations that provide hourly reports, it may be difficult to find a weather station that represents both windspeeds and directions appropriately for the analysis. Review observations and forecasts carefully for both speeds and direction. Try analyses with alternatives being considered.

Weather Station climatology in FSPro analysis is generally restricted to RAWS installations. For that reason, many analysts choose a combination of 10-minute average and gust windspeeds in those climatological distributions. This results in approximately a 50% increase in windspeeds. ASOS/AWOS climatologies often reflect this higher windspeed. Wind roses for ASOS/AWOS stations are at http://mesonet.agron.iastate.edu/sites/locate.php. This difference is often found in NDFD forecast windspeeds as well. Consider a similar adjustment to 10-min RAWS winds when conducting calibration analyses in STFB and NTFB.

Live Fuel Moistures

Evaluating and Adjusting your Assessment

Verification

a demonstration that the modeling formalism is correct. 

Though much of the verification work was completed in the development of a model, end users have a responsibility to evaluate Large Scale factors to ensure that the range of variability used produces the range in results that generally fit observed outcomes. Default analysis inputs provide a first step

Calibration 

the estimation and adjustment of the model parameters and constants to improve the agreement between model output and a data set

Calibration Factors to Consider

Variability

Large Scale

Medium Scale

Small Scale

Diurnal Changes

Windspeed & Wind Direction,

Cloud Cover

Precipitation

1-hr (when over 24 hours in primarily grass landscapes)

1-hr (when over 24 hours in mixed forest, shrub and grass landscapes)

1-hr (if burn period only includes peak hours)

Day-to-Day Variability

Burn Period length,

Burn Day frequency

1-hr fuel moisture

10-hr, 100-hr fuel moistures

Seasonal Trends

Burn Period Length,

Burn Day frequency,

Herb. Fuel Moisture

Woody Fuel Moisture

 

Analysis Constants

Canopy Cover

Terrain: Slope, Aspect, Elevation

Fuel Model

Crown Fire Method

Spotting Frequency

Canopy Base Height

Canopy Bulk Density

Stand Height

 



Calibration Steps to employ:
  • Consider edits to Large Scale Constants first. 
Fuel Model & canopy layers can require local edits due to disturbance, classification errors, and possibly seasonal trends. These fuel characteristics can have a large impact on outputs. It is CRITICAL to account for things that will check or stop fire spread, so ensure that recent historical fires are reflected in the LCP. Also, make sure that rock and water are adequately mapped. But other fuel layer adjustments should be considered only when the changes you contemplate are appropriate and will result in significant changes in fire growth. Otherwise, edits may mask necessary variability.

Crown fire method and Spotting Frequency are frequently settings applied consistently with experience in particular landscapes. As these become standard, they become “verified.”
  • Consider edits to medium and large scale Day-to-Day and Seasonal factors. 
Burn period length and burning threshold edits should be consistently applied according to seasonally changing day length and according to daily variability when limiting burn period.

Herbaceous and woody fuel moisture trends should reflect reasonable ranges for season and drought.
  • Edits to diurnal weather-related inputs (temp, rh, windspeed/dir, cloud cover) should be limited to standard kinds of adjustments to preserve relevance of observed calibrations to forecast projections. 
Windspeed and direction is most important factor. Ensure that “observed” weather reflects observed fire spread and that winds observations are appropriate for situation. Precipitation estimate may not represent fireline amounts. Consider adjustments carefully.
  • Calibration may require a couple iterations.
Do not over-calibrate to overfit the result.

Validation

A demonstration that a model, within its domain of applicability, possesses a satisfactory range of accuracy consistent with the intended application of the model. 

Validation is used to evaluate forecast projections from applied calibration adjustments. Keep in mind that forecast errors have large impact on calibrations.


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