A New Metric for Quantifying Burn Severity: The Relativized Burn Ratio

Sean Parks,Gregory Dillon,Carol Miller

doi:10.3390/rs6031827

Sean Parks, Gregory Dillon + Show 1 more

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https://doi.org/10.3390/rs6031827

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Abstract

Satellite-inferred burn severity data have become increasingly popular over the last decade for management and research purposes. These data typically quantify spectral change between pre-and post-fire satellite images (usually Landsat). There is an active debate regarding which of the two main equations, the delta normalized burn ratio (dNBR) and its relativized form (RdNBR), is most suitable for quantifying burn severity; each has its critics. In this study, we propose and evaluate a new Landsat-based burn severity metric, the relativized burn ratio (RBR), that provides an alternative to dNBR and RdNBR. For 18 fires in the western US, we compared the performance of RBR to both dNBR and RdNBR by evaluating the agreement of these metrics with field-based burn severity measurements. Specifically, we evaluated (1) the correspondence between each metric and a continuous measure of burn severity (the composite burn index) and (2) the overall accuracy of each metric when classifying into discrete burn severity classes (i.e., unchanged, low, moderate, and high). Results indicate that RBR corresponds better to field-based measurements (average R2 among 18 fires = 0.786) than both dNBR (R2 = 0.761) and RdNBR (R2 = 0.766). Furthermore, the overall classification accuracy achieved with RBR (average among 18 fires = 70.5%) was higher than both dNBR (68.4%) and RdNBR (69.2%). Consequently, we recommend RBR as a robust alternative to both dNBR and RdNBR for measuring and classifying burn severity.

Highlights

Over the last decade, substantial time, effort, and money have been invested in developing satellite-inferred wildfire burn severity maps
The goal of this study was to propose and evaluate an alternative relativized burn severity metric that is sensitive to changes where pre-fire vegetation cover is low but avoids the difficulties associated with the RdNBR equation; we call this new metric the relativized burn ratio (RBR)
The RBR is an improvement upon RdNBR in terms of correspondence to field measures of burn severity and overall classification accuracy. This improvement may appear marginal, one of the key strengths of the RBR equation is that it avoids some of the mathematical difficulties associated with the RdNBR equation

Summary

Introduction

Substantial time, effort, and money have been invested in developing satellite-inferred wildfire burn severity maps. Consistent with major burn severity mapping efforts [1], we define burn severity as the degree of fire-induced change to vegetation and soils, as measured with Landsat-based metrics. The two most commonly used Landsat-based metrics of burn severity are the delta normalized burn ratio (dNBR) [9]. The equations for dNBR (Equation (2)) and RdNBR (Equation (3)) use NBR derived from pre- and post-fire satellite images to quantify spectral change. Both metrics are sensitive to changes commonly caused by fire [11,12,13] and are often strongly correlated to field-based measures of burn severity [14,15,16]. Maps of dNBR and RdNBR provide depictions of landscape change on a continuous scale, researchers and practitioners commonly classify these continuous metrics into categorical maps representing unchanged, low, moderate, and high burn severity (e.g., [10])

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