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South America
DATES: AUGUST-SEPTEMBER 2019 | SATELLITE: SENTINEL 2 | CATEGORY: FIRES
The fires which occurred simultaneously in various points of the Amazon rainforest in the months of August and September 2019 came under the scrutiny of the entire planet for weeks. The rising temperatures across the globe is fostering the proliferation of forest fires, but the main threat in this case was the intentional burning of the jungle. The policies of Brazilian president Jair Bolsonaro have been criticised the world over as they promote large extensions of single-crop plantations at the cost of burning hundreds of thousands of hectares of tropical rainforest and savannah, which includes the Amazon jungle.
Forest fires in the rainforest and tropical savannah simultaneously spread in several locations and were practically unstoppable, as neither aircraft nor firefighting teams could stop their spread. In the summer of 2020, far from improving, deforestation reached record numbers not seen since the year 2008. The crisis brought about by Covid-19 shifted the media spotlight elsewhere, but the truth is that according to the INPE, the Brazilian Institute of Space Research, more than one million hectares of Amazon rainforest was deforested in the summer of 2020.
At first glance, you cannot differentiate the burnt terrain, which is why we need the band combination and the application of certain algorithms between bands to bring out certain aspects in the area, such as the vegetation, humidity, or fire. This is the task the INPE is carrying out, which is using satellite images to quantify the deforestation of a good part of the tropical forest of South America and question the empirical data of official sources.
Certain band combinations clearly visualise the extent of the burning from the rest, while others even show the encroachment of the fire itself. Some examples given below clearly show the burnt areas of the jungle from the untouched areas:
On the left, Brazilian Amazon fire (in the north of the Goiás state) on September 15th 2019. Band combination in natural color (4-3-2). CC BY 4.0 Sentinel Hub EO Browser.
On the right, Brazilian Amazon fire (in the north of the Goiás state) on September 15th 2019. NDVI o Normalized Difference Vegetation Index. CC BY 4.0 Sentinel Hub EO Browser.
The NDVI index (Normalized Difference Vegetation Index) combines bands related to the colours red and near infrared, which in the case of the Sentinel-2 satellite are bands 4 and 8, respectively. When the vegetation is flourishing, it reflects a great deal of the sun’s radiation in near infrared and very little in red. On the other hand, when the vegetation is diseased, the opposite occurs. The NDVI is calculated using the formula below; (B8-B4) / (B8+B4)
The result is an index with values ranging between -1 and +1, where the negative values correspond to bodies of water and artificial ground coverings (brown tones), values between 0 and 0.3 are bare ground (light green tones), while higher values indicate vegetation, with the values closest to 1 more flourishing vegetation (dark green tones).
On the left, Brazilian Amazon fire (in the north of the Goiás state) on September 15th 2019. Band combination in natural color (4-3-2). CC BY 4.0 Sentinel Hub EO Browser.
On the right, Brazilian Amazon fire (in the north of the Goiás state) on September 15th 2019. Band combination in false color (12-11-4). CC BY 4.0 Sentinel Hub EO Browser.
To detect flames or the advancing line of fire, we opt for waves able to capture temperature, which are infrared rays. The combination of short-wave infrared or SWIR bands (SWIR o short-wave infrared), that is, bands 12 and 11, with a red light (band 4) provide us images that can identify the presence of fire at the very instant the image was captured.
On the left, Brazilian Amazon fire (in the north of the Goiás state) on September 15th 2019. Band combination in natural color (4-3-2). CC BY 4.0 Sentinel Hub EO Browser.
On the right, Brazilian Amazon fire (in the north of the Goiás state) on September 15th 2019. Band combination in false color (12-8-4). CC BY 4.0 Sentinel Hub EO Browser.
On the other hand, if we want to identify burnt areas and the line of fire in a single image, we can combine the short-wave infrared (band 12) which denote the fire, with the NIR or near infrared (NIR o near infrared, band 8) and red light (band 4), which indicate vegetation.
The GIS and Remote Sensing Service (SIGTE) of the University of Girona is part of the Copernicus Academy, a network of universities, research centers and organizations with the goal to exchange knowledge and experiences on the Copernicus, Earth observation program.
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