Verbyla, D., Kasischke, E. S., and Hoy, E. E. 2008.
Seasonal and topographic effects on estimating fire severity from Landsat TM/ETM+ data.
International Journal of Wildland Fire. (in review)
The maximum solar elevation is typically less than 50 degrees and varies substantially during the growing season in the Alaskan boreal region. Because of the relatively low solar elevation, the effect of topography on spectral reflectance can influence fire severity indices derived from remotely sensed data. We used Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper (ETM) data to test the effect of changing solar elevation and topography on the normalized burn ratio (NBR) and the differenced normalized burn ratio (dNBR). When a time series of unburned pixels from black spruce forests examined, we found that NBR values consistently decreased from June through September. At the stand level, dNBR derived values from similar unburned and burned black spruce stands were substantially higher from September imagery relative to July/August imagery. Within the Boundary Burn, we found mean post-fire NBR to consistently vary due to topographic control of potential solar radiation. To minimize spectral response due to topographic control of vegetation and fire severity, we computed a dNBR using two post-fire images. There was a negative bias in remotely sensed fire severity estimates as potential solar radiation decreased due to topography. Thus fire severity would be underestimated for stands in valley bottoms dominated by topographic shading or on steep north-facing slopes oriented away from incoming solar radiation. This is especially important since highly flammable black spruce stands typically occur on such sites. We tested the effect of image date on fire severity estimates and found a significantly greater mean dNBR from burned pixels from using a July pre-fire image relative to a June pre-fire image. Because NBR values vary substantially as solar elevation and plant phenology changes, the use of NBR to assess burn severity trends across regions or years may be difficult in high latitude areas such as the Alaskan boreal forest.
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