1. Introduction
Land use and land cover (LULC) are constantly being modified, especially concerning vegetation cover. When vegetation suffers from any change, there is also a modification in ecosystems, which can, within the context of environmental devastation, reduce resources in soil, fauna, and flora [
1]. One factor contributing to modifying vegetation cover is forest fires, which can occur through natural or human causes [
2,
3]. Forest fires frequently occur in regions with a dry climate, wind, available dry vegetation, and favorable topographical conditions [
4,
5,
6,
7].
With ever-changing land cover, especially in the context of climate change, forest fires destroy thousands of hectares yearly, threatening the ecological environment and human life [
8]. A worrying issue presented by forest fires is how quickly they spread, making them difficult to contain [
9]. Detecting forest fires and wildfires is gaining more attention in research worldwide [
10,
11].
In the Brazilian context, the Cerrado biome presented the highest number of hotspots related to fires and wildfires in 2021 (n = 137,631) [
12]. Some research has linked these fires to uncontrolled or unsustainable agricultural practices [
13,
14]. Fire can cause several adverse effects, such as problems related to human health [
15] and climate change at regional and global levels [
16]. Thus, the study of fires is critical since there is an ever-increasing need to preserve and recover natural resources such as vegetation [
17,
18].
The Cerrado biome is the savannah that houses the most extensive biodiversity in the world [
19,
20], occupying 2,036,448 km
2, equivalent to approximately 23.9% of the Brazilian territory [
20]. According to MMA, 2021, the Cerrado biome has about 11,627 cataloged native plant species [
21]. The Cerrado biome combines characteristics conducive to fires and wildfires. The dry period occurs from May to September [
22], and the prevailing climate is seasonal tropical [
23]. The vegetation predominantly comprises forest, savannah, grassland formations, and terrain with topography ranging from flat to smooth and rounded [
24]. Remarkably, physiographic characteristics favor agricultural production in this region [
25]. Here, the focal point is that the Cerrado biome brings together a set of characteristics conducive to spreading fire [
26].
Specifically, with the growing demand for agricultural land in the Cerrado, management practices occur that, when incorrect, increase forest fire risk [
27]. For example, a common practice is using fire to clean and prepare pastures for planting, which can result in uncontrolled fire, a significant threat to biodiversity conservation [
2].
Considering the importance of preserving the environment and avoiding continuous degradation, it becomes necessary to identify the regions that suffer from fires more frequently [
28]. For this, information should be collected regarding the locality of fires, aiming to implement strategies to minimize their occurrence [
29]. The research investigations examining forest fires usually observe the precise location of occurrence to obtain accurate information about the ignition point [
30]. In contrast, field research demands time and resources, making it challenging to obtain data on the historical basis of the event, limiting fire prediction [
31,
32].
One solution to overcome such limitations is using remote sensing methods, making it possible to remotely study the events occurring in each region with available time series data [
33]. In addition, in Brazil, records of hot spots are accessible on online platforms, such as the burning program of the National Institute for Space Research—INPE—which makes the data available for free.
Spectral indexes are one of the tools used to distinguish fire-related features in vegetation and regions with bared soil [
34]. Spectral indexes are mathematical expressions that combine spectral bands to highlight information on the surface [
35]. Each spectral index can highlight specific information while discriminating it from other objects on the surface [
36]. The Normalized Burn Ratio (NBR) is among the most widely used indexes for mapping the severity of forest fires and fire scars [
37]. The NBR index focuses on information regarding ash, coal, soil, and surface moisture [
38]. The NDVI is most often used to analyze vegetative vigor on the earth’s surface [
39]. The SAVI index was created to minimize soil’s effects on vegetation detection, and the factor L varies from 0 to 1, according to the type of vegetation cover [
40]. Regions with high vegetation cover receive L = 0, with moderate vegetation L = 0.5 and low vegetation L = 1 [
40]. The CSI index detects recently or severely burned areas [
41].
Agriculture, especially in regions with a predominance of temporary crops, can hide the detection of fire scars since these crops have phenological stages [
42] that modify the spectral response [
43,
44]. Thus, a given temporary crop region, for example, has different spectral responses during the crop cycle since there will be modifications in the physiognomic and physiological structure of the plant.
Because spectral indexes are mathematical expressions that work with reflectance values, it is natural that the detection of phenomena such as fire scars encounters barriers. For example, ref. [
45] indicate that the most significant difficulty lies in confusion with the class “bared soil.” Ref. [
46] explain that in regions with urban areas or bared soil, there may be confusion with the burning class due to the sensitivity of the indexes in correctly discriminating land cover information. Ref. [
47] points out that the confusion occurs due to highly dark pixels, which can be confused with areas of bared soil or natural/artificial dark spots. Therefore, it is necessary to fill this knowledge gap using robust tools such as machine learning. According to [
48], ML algorithms can work with various data without making previous assumptions regarding their distribution. Machine learning (ML) seeks to extend computational functions to improve performance automatically through its own experience [
49]. ML is increasingly being used in remote sensing [
50].
Considering these problems, we answered the following questions: What aspects hinder the detection of burned areas when applying spectral indexes to map the occurrence of fire scars? Have agricultural practices in the Cerrado favored the occurrence of fires? Thus, here we developed an accurate and low-cost solution to detect fire scars based on artificial neural networks and satellite images, overcoming the aspects that hinder those scars from detection; we also analyzed the association between the areas with fire scars and the types of crops within a representative region inside the Cerrado.
2. Material and Methods
2.1. Study Area
The microregion of Patrocínio is in the southeast region of Brazil, in the state of Minas Gerais, at the intersection of latitudes ranging from 18°40′44.004″ S to 19°03′58.674″ S, and longitudes 47°09′51.546″ W to 46°32′46.413″ W (
Figure 1). The study region has an area of approximately 11,986.16 km
2, which covers eleven municipalities, namely: Abadia dos Dourados, Coromandel, Cruzeiro da Fortaleza, Douradoquara, Estrela do Sul, Grupiara, Iraí de Minas, Monte Carmelo, Patrocínio, Romaria, and Serra do Salitre.
The microregion of Patrocínio is known for its agro-industrial activities [
51]. The main crops are corn, sugarcane, soybeans, coffee, and animal husbandry [
52]. According to [
53], agriculture is the main economic activity in this geographic region because of the uneven relief, favorable hydroclimatic conditions, and fertile soils. These conditions save expenses on soil correction inputs and enable agricultural machinery to maximize productive potential at lower costs [
54,
55].
The study area was chosen because it is in the Cerrado of Minas Gerais, which has several characteristics that make it prone to the occurrence of fires, supporting the hypothesis tested in this article. In addition, its economic importance and potential for supplying natural resources make this area a study priority in the region.
According to data from the National Institute of Meteorology, NMET, the period of scarce rainfall in the region starts around April/May and continues until September/October.
Figure 2 shows the rainfall record in the years of study, data which helped us define the period of interest and data collection.
Figure 3 presents the graph of the fire spots detected monthly each year.
Figure 4 shows the distribution of fire spots, and
Figure 5 illustrates the occurrence of fires per km
2 in the municipalities of the Patrocínio microregion from 2018 to 2021 (in dry months). On average, the dry months in the region range from April to October.
We divided the research into two experiments. The objective of experiment 1 was to evaluate the band’s relationship configuration to allow the best detection of burned areas in the image classification process. The aim of experiment 2 was, based on experiment 1, to analyze the association of agricultural practices with the occurrence of fire spots. Thus, in the following items, we describe each procedure in detail.
2.2. Data Acquisition and Pre-Processing
Data on hotspots in the study area were obtained from the INPE website. We searched for the dry season interval for each year of interest. The dry season was established by rainfall data from stations located in the region, ranging from April to October in the years of study. Pre-fire images are used to verify what was in the region before the dry season. Thus, based on the definition of dry spells, images from 2018 to 2021 were acquired, as shown in
Table 1. Sentinel-2 sensor MSI images were acquired through the official ESA website (
https://dataspace.copernicus.eu/explore-data/data-collections/sentinel-data/sentinel-2, accessed on 15 March 2022), all six referring to the same data.
Considering the presence of particles in the atmosphere, such as water, nitrogen, oxygen, and aerosols, it was necessary to perform atmospheric correction to reduce the particles’ influence on the image [
56]. The atmospheric correction of the images was performed using the Semi-Automatic Classification Plugin (SCP) in the QGIS 3.16.4 software. The SCP plugin performs atmospheric correction using the Dark Object Subtraction (DOS) method [
57]. The DOS method estimates atmospheric intervention based on the satellite image’s digital numbers (DNs). According to [
58] shaded regions should present low DNs; however, due to atmospheric scattering, they have DNs values, and thus these pixels are used as a correction parameter. Ref. [
59] states that the analysis is conducted by choosing the shortest wavelength band. From the study of the frequency histogram of the DNs, the value of the darkest pixel is identified. Thus, the correction is made uniformly, subtracting the scattering value of the entire scene [
59].
In addition, all the Sentinel-2 satellite bands were resampled to the spatial resolution of 10 m using SNAP 6.0 software. The nearest neighbor method (NNM) was used to perform the resampling, which uses the gray level values closest to the inverse mapping [
59].
2.3. Mosaic Generation
Due to the size of the study area, it was necessary to use six scenes to compose its limits, the scenes acquired being KHE, KKA, KKU, KKV, KLU, and KLV. The mosaic was made in the pre- and post-fire periods for each year. We selected only images with 30% or less cloud cover.
2.4. Mapping Fire Scars
For the mapping of the fire scars, the indexes present in
Table 2 were used, using NIR (near infrared), SWIR2 (shortwave infrared), and R (red) bands.
2.5. First Experiment—Bands and Indexes Evaluation
In the first experiment, we evaluated the band configuration that allows better detection of burned areas in the image classification process. Thus, six tests were used each year (2018 to 2021), divided into two groups: post-fire period and variation between pre-fire and post-fire period. In the analysis of the methods used for the post-fire period and the variation between the pre-fire and post-fire period, classification performance was tested: (I) with the use of all bands of the Sentinel-2 sensor and the four indexes; (II) only with the NBR index, and (III) only with the CSI index. The pre- and post-fire period variation refers to the modifications in land cover during the dry season, calculated by a mathematical subtraction between pre- and post-fire data.
For this experiment, Region of Interest (ROI) samples were collected for fire scars, bare soil, native vegetation, reforestation, water, and crops. The other classes were acquired only by visual analysis. The fire samples were identified using INPE fire spot data and visual analysis on the Sentinel-2 satellite’s true color composition (RGB). After collection, the vector files were used in the Envi 5.1 software, which extracted the surface reflectance values for each pixel, band, and index.
The samples were divided into two groups, 80% for training and 20% for validation, as shown in
Figure 6.
The classes used in this study were as follows: 1: fire scars, 2: exposed soil, 3: native vegetation, 4: reforestation, 5: water, and 6: crops. We used WEKA 3.8.5 software in this study, which produces metrics capable of analyzing the available dataset and estimating the subset with the highest possibility of predicting to which class each pixel belongs. The multilayer perceptron (MLP) neural network was used as a classifier, and the defined hyperparameters were learning rate 0.3, momentum 0.2, and training time 500; the number of neurons in the hidden layers was defined by the arithmetic mean between the number of input variables and the number of output classes. As a result of the processing, the overall accuracy was obtained by computing the kappa index, which analyzes the classification according to
Table 3, proposed by [
62]. We also used the confusion matrix, which represents the classification quality based on the correlation between the reference and classified information.
In addition to these evaluation parameters, omission error analysis was performed, through which it is possible to verify the percentage of pixels omitted from the classification, as presented in Equation (1).
where
OE: omission error;
V: the amount of correctly classified pixels; n: class to be calculated; and r: distribution of reference data. In addition, the inclusion error was analyzed, which analyzes and reports the number of pixels that were wrongly included in each class, as presented in Equation (2).
where
IE: inclusion error;
V: the amount of correctly classified pixels; n: class to be calculated; and c: distribution of classified data.
2.6. Second Experiment—Scars Classification
According to the analysis performed in the first experiment, it was possible to determine the best input configuration of the ANN algorithm for classifying burned areas. Thus, the classification was performed using only the images in the post-fire period, together with the use of all bands of the Sentinel-2 satellite and the four indexes we studied.
In this experiment, we used the fire data and the NBR index to refine the samples collected for classification, aiming to improve computational and classifier performance. Thus, we obtained the pixels and their respective surface reflectance values for the burned areas using the ENVI 5.1 software.
Then, WEKA 3.8.5 indicates the classes classified based on the training data as output. In addition, it presents its prediction error, which means the probability that the pixel has been correctly classified. Then, only the areas understood as burned were extracted for each of the years under study. The burning classes were refined using hot spot point data and the NBR index, which allowed the elimination of areas incorrectly classified by WEKA 3.8.5.
To analyze the influence and association of burned areas with the agricultural practices present in the study area, we used raster data from MapBiomas [
63], through which we identified information on land use and cover, emphasizing temporary and perennial crops. Thus, the land use and land cover maps were made, allowing an analysis of the presence of fire scars and the type of crops and plantations present near these burned areas.
4. Conclusions
Here, we could accurately observe the fixed presence of fire scars across the studied geographic region and time period. This reinforces the need to monitor the agricultural practices in the Cerrado, as burning occurs with undesirable frequency, posing several risks to the environment and human health.
This study also presented an accurate, low-cost and innovative method for the ANN classification of burned areas, with the best performances found when all bands of the Sentinel-2 satellite and the four indexes (NDVI, SAVI, NBR, and CSI) were used. The first experiment was necessary because it showed results that aimed to facilitate future research, such as using images only in the post-fire period. Furthermore, classifications were obtained by the abovementioned method with a global accuracy higher than 93.7% and a kappa index higher than 0.92.
Based on the attributes that presented the best performance in the ANN classification, land use and land cover maps were generated, making it possible to analyze the higher incidence of fires near temporary crop areas, such as soybeans, corn, and beans. Although burning is legalized, the agencies responsible and committed to the surveillance of this phenomenon must be attentive to monitoring and investigating the facts.
Based on the land use and land cover maps, it was possible to observe the fixed presence of fire scars in the four years we studied, highlighting the importance of monitoring these areas, because burning, when occurring frequently, can pose several risks to the environment and human health. The methodology used in this study can be integrated into a system for monitoring burned areas to help the responsible agencies monitor burning activities.