Quantifying fire severity: a brief review and recommendations for improvement

Figures & data

Figure 1. Examples of 2001 Hoover fire severity classes (van Wagtendonk, Root, and Key 2004), a, unburned; b, high; c, moderate; d, light

Table 1. Canopy-severity index and their characteristics descriptions

Fire severity	Description
0 – Unburned	Tree canopy is green and unaltered; there is no direct effect from fire
1 – low	Most needles remaining
2 – low to moderate	With few needles but most small twigs remaining
3 – moderate	With few small twigs remaining but many branches
4 – moderate to high	Many lower canopy branches consumed
5 – high	Most branches and cones consumed

Table 2. General forest fire severity classes and their characteristics descriptions, summarized and modified from (Keeley 2009; Ryan 2002; Ryan and Noste 1985; Turner et al. 1994)

Fire severity	Description
Unburned	Plant parts are green and unaltered; there is no direct effect from heat.
Scorched	Unburned, but plants exhibit leaf loss from radiated or convected heat. Mosses and leaves are brown or yellow, but species characteristics are still identifiable.
Light	Trees with some canopy cover killed; canopy trees with green needles (foliage) although stems scorched. Fine dead twigs, mosses, and herbs charred or consumed, but soil organic layer is unaltered.
Moderate	Canopy trees killed and all understory plants charred or consumed, but needles (foliage) not consumed. Fine dead twigs on soil surface consumed and logs charred. Surface litter, mosses, and herbaceous plants are consumed. Surface litter of all sizes and soil organic layer are largely consumed.
Heavy	Soil organic layer, mosses, herbaceous plants, shrubs, twigs, and small branches are almost entirely consumed. Rotten logs and stumps are consumed. The top layer of the mineral soil is visibly oxidized, reddish to yellow. Foliage on canopy trees consumed or not, but are almost dead, organic matter charred to several centimeters in depth.

Figure 2. Simplified composite burn index (CBI) field data form

Table 3. Common remote sensing indices for quantifying fire severity

Quantitative model	Equation	Reference
Normalized difference vegetation index, NDVI	$\mathrm{N}\mathrm{D}\mathrm{V}\mathrm{I}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-R}{\mathrm{N}\mathrm{I}\mathrm{R}+R}$	(Tucker 1979)
Normalized difference vegetation index thermal, NDVIT	$\mathrm{N}\mathrm{D}\mathrm{V}\mathrm{I}\mathrm{T}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-R\times TIR}{\mathrm{N}\mathrm{I}\mathrm{R}+R\times TIR}$	(Smith et al. 2007)
Enhanced vegetation index, EVI	$\mathrm{E}\mathrm{V}\mathrm{I}=\frac{2.5\left(\mathrm{N}\mathrm{I}\mathrm{R}-R\right)}{\mathrm{N}\mathrm{I}\mathrm{R}-6\mathrm{R}-7.5\mathrm{B}+1}$	(Huete et al. 2002)
Global environment monitoring index, GEMI	$\gamma =\frac{2\left(\mathrm{N}\mathrm{I}{\mathrm{R}}^2-R^2\right)+1.5\mathrm{N}\mathrm{I}\mathrm{R}+0.5\mathrm{R}}{\mathrm{N}\mathrm{I}\mathrm{R}+R+0.5}$
$\mathrm{G}\mathrm{E}\mathrm{M}\mathrm{I}=\gamma \left(1-0.25\gamma \right)-\frac{R-0.125}{1-R}$	(Verstraete 1992)
Burned area index, BAI	$\mathrm{B}\mathrm{A}\mathrm{I}=\frac{1}{{(0.1+R)}^2+{(0.06+\mathrm{N}\mathrm{I}\mathrm{R})}^2}$	(Chuvieco, Martín, and Palacios 2002)
Char soil index, CSI	$\mathrm{C}\mathrm{S}\mathrm{I}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}}{\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}$	(Smith et al. 2005)
Char soil index thermal, CSIT	$\mathrm{C}\mathrm{S}\mathrm{I}\mathrm{T}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}}{\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}\times TIR}$	(Holden et al. 2005)
Normalized difference SWIR, NDSWIR	$\mathrm{N}\mathrm{D}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-\mathrm{S}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}{\mathrm{N}\mathrm{I}\mathrm{R}+\mathrm{S}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}$	(Gerard et al. 2003)
Mid-infrared burn index, MIRBI	$\mathrm{M}\mathrm{I}\mathrm{R}\mathrm{B}\mathrm{I}=10\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}-9.8\mathrm{S}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}+2$	(Trigg and Flasse 2001)
Differenced MIRBI, dMIRBI	$\mathrm{d}\mathrm{M}\mathrm{I}\mathrm{R}\mathrm{B}\mathrm{I}=\left(\mathrm{M}\mathrm{I}\mathrm{R}\mathrm{B}\mathrm{I}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{e}-\mathrm{M}\mathrm{I}\mathrm{R}\mathrm{B}\mathrm{I}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{e}\right)-\mathrm{M}\mathrm{I}\mathrm{R}\mathrm{B}\mathrm{I}\mathrm{o}\mathrm{f}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{t}$	(McCarley et al. 2017)
Normalized burned ratio, NBR	$\mathrm{N}\mathrm{B}\mathrm{R}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}{\mathrm{N}\mathrm{I}\mathrm{R}+\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}$	(Key and Benson 1999)
NBR thermal, NBRT	$\mathrm{N}\mathrm{B}\mathrm{R}\mathrm{T}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}\times TIR}{\mathrm{N}\mathrm{I}\mathrm{R}+\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}\times TIR}$	(Smith et al. 2007)
Differenced NBR, dNBR	$\mathrm{d}\mathrm{N}\mathrm{B}\mathrm{R}=\hspace{0.17em}\left(\mathrm{N}\mathrm{B}\mathrm{R}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{e}-\mathrm{N}\mathrm{B}\mathrm{R}\mathrm{p}\mathrm{o}\mathrm{s}\mathrm{t}\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{e}\right)-\mathrm{d}\mathrm{N}\mathrm{B}\mathrm{R}\mathrm{o}\mathrm{f}\mathrm{f}\mathrm{s}\mathrm{e}\mathrm{t}$	(Key and Benson 2006)
Relative dNBR, RdNBR	$\mathrm{R}\mathrm{d}\mathrm{N}\mathrm{B}\mathrm{R}=\frac{\mathrm{d}\mathrm{N}\mathrm{B}\mathrm{R}}{{\left|\mathrm{N}\mathrm{B}\mathrm{R}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{e}\right|}^{0.5}}$	(Miller and Thode 2007)
Relativized burn ratio, RBR	$\mathrm{R}\mathrm{B}\mathrm{R}=\frac{\mathrm{d}\mathrm{N}\mathrm{B}\mathrm{R}}{\mathrm{N}\mathrm{B}\mathrm{R}\mathrm{p}\mathrm{r}\mathrm{e}\mathrm{f}\mathrm{i}\mathrm{r}\mathrm{e}+1.001}$	(Parks, Dillon, and Miller 2014)
Normalized difference water index, NDWI	$\mathrm{N}\mathrm{D}\mathrm{W}\mathrm{I}=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-\mathrm{S}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}{\mathrm{N}\mathrm{I}\mathrm{R}+\mathrm{S}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}$	(Gao 1996)
NIR-SWIR-temperature version 1, NSTv1	$\mathrm{N}\mathrm{S}\mathrm{T}\mathrm{v}1=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}{\mathrm{N}\mathrm{I}\mathrm{R}+\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}\times \mathrm{T}\mathrm{s}$	(Veraverbeke et al. 2011)
NIR-SWIR-temperature version 2, NSTv2	$\mathrm{N}\mathrm{S}\mathrm{T}\mathrm{v}2=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-(\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}+\mathrm{T}\mathrm{s})}{\mathrm{N}\mathrm{I}\mathrm{R}+(\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}+\mathrm{T}\mathrm{s})}$	(Veraverbeke et al. 2011)
NIR-SWIR-emissivity version 1, NSEv1	$\mathrm{N}\mathrm{S}\mathrm{E}\mathrm{v}1=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}{\mathrm{N}\mathrm{I}\mathrm{R}+\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}}\times \mathrm{E}$	(Veraverbeke et al. 2011)
NIR-SWIR-emissivity version 2, NSEv2	$\mathrm{N}\mathrm{S}\mathrm{E}\mathrm{v}2=\frac{\mathrm{N}\mathrm{I}\mathrm{R}-(\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}+\mathrm{E})}{\mathrm{N}\mathrm{I}\mathrm{R}+(\mathrm{L}\mathrm{S}\mathrm{W}\mathrm{I}\mathrm{R}+\mathrm{E})}$	(Veraverbeke et al. 2011)

The bands that revealed the highest spectral separability within each spectral range R (red, 0.6 to 0.7 μm), NIR (Near Infrared, 0.7 to 1.3 μm), B (blue, 0.45 to 0.5 μm), SSWIR (shorter short wave infrared, 1.3 to 1.9 μm), LSWIR (longer short wave infrared, 1.9 to 2.5 μm), MIR (mid infrared, 3 to 5.5 μm), TIR (thermal infrared, 8 to 13 μm), E (emissivity) and Ts (surface temperature) were then incorporated in the spectral indices listed in Table 3.

Figure 3. Conceptual model for fire severity measuring. Ellipse indicates latent variable; rectangle indicates manifest variable. Besides, green lines indicate respective relationships that are significant but restricted/ non-unique; purple lines indicate relationships that we expect to recognize; red lines indicate relationships that must be utilized, which are the cornerstones of all fire severity measurement methods; blue lines indicate relationships that are difficult to be observed or exploited

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