Drought effect on spectral response and burn assessments in tropical environments

Vasquez Hernandez M.R.; Garcia Leite H.; Costa dos Santos J.F.; Ribeiro Júnior J.I.; Cosenza D.N.; Marinaldo Gleriani J. · International Journal of Applied Earth Observation and Geoinformation (2026) · DOI: 10.1016/j.jag.2026.105376

Resumen

Fires are a recurring concern in non-adapted or fire-intolerant ecosystems, harming biodiversity, the economy, human health, and safety. In tropical environments, fire events are concentrated during the dry seasons and are routinely monitored using orbital sensors. A standard method for assessing burn scars and burn severity is the Normalized Burn Ratio (NBR) and its differenced form (dNBR), initially developed for temperate forests. However, their applicability and accuracy in tropical ecosystems remain underexplored. This study evaluated the spectral reflectance of vegetation in green, dry, and burned conditions across five tropical environments (Amazon rainforest, woodland, grassland, pasture, and sugarcane). Using Sentinel-2 surface reflectance data, we calculated the spectral similarity, assessed the separability of burned areas, and classified the burn severity using the standard dNBR thresholds. Spectral angle and graphical analyses revealed that the spectral reflectance of dry vegetation differs significantly from the green vegetation for which the NBR was originally proposed. Because dry vegetation exhibits naturally lower reflectance in the near-infrared (NIR) region, the pre-fire NBR baseline is reduced. Consequently, the evaluated environments were classified withing the lowest severity ranges or even as unburned, despite showing significant separability values for distinguishing burned areas. These findings highlight that while the NBR is an effective tool for fire detection, standard dNBR thresholds may severely underestimate burn severity in tropical ecosystems due to the unique spectral characteristics of dry vegetation prior to fire events.

Instituciones: Universidad Nacional Toribio Rodríguez de Mendoza de Amazonas