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Wildland Fire Potential Index (WFPI)

By Fire Danger Forecast

WFPI (Figure 1) is developed using an algorithm that describes the ratio of live to dead fuel and includes variables for wind speed, dry bulb temperature, and rainfall. WFPI is a unit-less number that ranges from 0 to 150 and relates to vegetation flammability. It is an enhancement of the legacy Fire Danger Forecasting Fire Potential Index (FPI) dataset.

Wildland Fire Potential Index (WFPI)
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Figure 1. Example WFPI Map

WFPI Model inputs:

  1. Maximum Live Ratio: Live/Dead ratio for a given pixel when the vegetation is at maximum greenness, scaled to range from 25% to 75%.
  2. Dead Fuel Extinction Moisture: The dead fuel moisture content at which fire will no longer spread.
  3. Fuel Model: A set of numbers that assign values to unique vegetation types represented by the National Fire Danger Rating System. 
  4. Wind Reduction Factor: A number that accounts for slowing of the wind from anemometer height to mid-flame height, due to the resistance of vegetation to airflow.
  5. Normalized Difference Vegetation Index (NDVI): Quantifies vegetation by measuring the difference between near-infrared (which vegetation strongly reflects) and red light (which vegetation absorbs).
  6. Relative Greenness (RG): Indicates how green each pixel currently is in relation to the range of historical NDVI observations, scaled from 0 to 100.
  7. 10-Hour Dead Fuel Moisture: The moisture content of small (0.25 - 1.0-inch diameter) dead vegetation.
  8. Wind Speed: Measured in knots per hour.
  9. Rain: Measured in mm.
  10. Dry Bulb Temperature: Measured in degrees Fahrenheit.

WFPI Methodology:

The following figure illustrates the process flow of WFPI.

Fire Potential Index Data Flow
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Figure 2. FPI and WFPI Flow chart.

WFPI Results:

Historically, higher number of large fires (>500 acres) have occurred at the highest levels of WFPI (Table 1).

Additionally, higher percentages of existing one-acre fires have spread to burn more than 500 acres (Table 1). 

Table 1: Observed proportion of: large fires (>500 acres) per voxel (km2-day); and one-acre fires that spread to become a large fire, for each WFPI group.  

WFPI

 

Observed LFP1

(per million voxel3)

Observed SP2

(percent)
0-10    0.04 1.24
10-20 0.08

1.59
20-30    0.20 1.48
30-40     0.28 1.74
40-50     0.47 2.12
50-60 0.61 2.48
60-70 0.82 2.50
70-80 1.12 3.22
80-90 1.57 4.95
90-100 2.12 8.32
100-120 2.88 14.47
120-147 8.26 32.70

1 Observed proportion based on historic period 2001-2017

2 Observed proportion based on historic period 2001-2015

3 voxel = km2-day

The accuracy of the predictions is demonstrated by the reliability diagram (Figure 3), where observed values are plotted against grouped WFPI levels. 

Reliability diagrams of the WFPI model
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Figure 3. Reliability diagrams of the WFPI model. (Left panel) Historic observed yearly fractions of 500+ acre fires in each of the grouped WFPI levels. (Right panel) historic observed yearly fraction of existing one-acre fires spreading to 500+ acres in each of the grouped WFPI levels.

The sensitivity of the FPI to wind is illustrated in Figure 4, which shows how much wind increases the legacy FPI for several wind speeds. 

Increase in FPI as a Function of Windspeed
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Figure 4. Sensitivity of FPI to wind speed with the wind speed reduction factor equal to 0.6.