Assessing forest burnt severity in Nepal

Forest fire is one of the major disasters that cause severe damage to the forests, people, and property and the global environmental problems. It is essential to obtain quantitative information on forest fires and monitoring of the fires assist in understanding the forest fire phenomenon and its spatiotemporal characteristics (Hua & Shao, 2017). In Nepal, the forest fire damages large forest areas, however there is lack of quantification of forest burnt area. This is a crucial information to the forest department and other decision makers in forestry sector to understand the forest loss due to forest fire, which in turn contributes to carbon emission. Earth observation data can play an important role in providing information on forest burnt area and its severity along with other allied indicators such as forest fire monitoring, risk mapping, identifying potential zones and fire risk prediction. We attempt to generate the forest burnt severity index for forest areas in Nepal with landsat-8 and Sentinel 2 datasets during 2020-2021 for the fire season of 2021 in Google earth engine. The difference between the pre-fire and post-fire normalized burn ratio (NBR) acquired from the images is used to compute the difference normalized burn ratio (dNBR), which is used to estimate the burn severity. Higher the dNBR value more the burn severity, whereas areas with negative dNBR values indicate regrowth following a fire. We selected the duration of 1 July to 30 September, 2020 as pre-fire months and 15 March to 15 May, 2021 as post-fire months to calculate the difference in dNBR. We conducted a first level of validation for the fire burnt severity maps using VIIRS and MODIS fire points of the respective fire season. The burn severity map was classified into five classes with increasing severity. Out of 62560.58 Km2, approximately 46% of forest area was unburnt, followed by 39.03% was burnt with low severity, 11.82% burnt with low-moderate severity, while 1.93 % was burnt with moderate-high severity, and 0.55% was burnt with high severity.