Figure 2. Typical vegetation type in the Tyrma region. (a) wetland (Mari ), (b) forest, (c) grassland. Other photos about the vegetation of wetland (Mari ) are shown in Figure S1.
2.2 Calculation of normalized difference indices
A flowchart of the landcover classification process is shown in Figure 3. Landsat-8 data (Level2, Collection2, Tier1) with a spatial resolution of 30 m was used for landcover classification. Importing and processing of these data were conducted in Google Earth Engine which is a platform for scientific analysis and visualization of geospatial datasets (Gorelick et al., 2017). First, Landsat-8 data from 2013 to 2021 were extracted. Then, the data in the summer months (June, July, and August: JJA) were extracted. Finally, medians of these JJA data were extracted to calculate normalized difference indices as described in the next paragraph.
For classifying landcover by satellite image analysis, it is important to presume several surface conditions such as vegetation, soil, and water. Thus, we calculated three indices using JJA-median Landsat-8 data: normalized difference vegetation index (NDVI), normalized difference soil index (NDSI), and normalized difference water index (NDWI). The formulas to calculate the three indices are as follows:
NDVI = (Band5NIR – Band4Red) / (Band5NIR + Band4Red)     (1)
NDSI = (Band6SWIR1 – Band5NIR) / (Band6SWIR1 + Band5NIR)   (2)
NDWI = (Band5NIR – Band6SWIR1) / (Band5NIR + Band6SWIR1).   (3)
Here, each band of data was JJA-median value. Band names and wavelength of Landsat-8 are shown on the following website (https://www.usgs.gov/faqs/what-are-band-designations-landsat-satellites). NDVI, NDSI, and NDWI show different ranges depending on vegetation activity, soil types, and water content of leaves (Gao, 1996; Takeuchi and Yasuoka, 2004; Tucker et al., 1985). In this paper, NDVI, NDSI, and NDWI are collectively described as NDXI. The code for extracting JJA-median Landsat-8 data and calculating NDXI in Google Earth Engine is available in the Supporting Information (Figure S2).