2. STUDY AREA, DATA AND METHODOLOGY 2.1. Study area
Batak Reservoir is located in the catchment area of the Matnitsa River( the right tributary of the Chepinska River)( Fig. 1).
Fig. 1. Study area
It is part of the Batashki Hydroelectric System, known as the " Batak Hydroelectric Cascade," which includes the associated intake structures " Bistritsa," " Ravnogor," " Nova Mahala," " St. Konstantin," " Sokola," and " Peshtera " hydroelectric plants. Its owner is the Natsionalna Elektricheska Kompania EAD( NEK EAD). The reservoir was built in 1959. It is currently used year-round for a complex purpose for tourism and fishing. In its southwestern part is located an island " Golaka " [ 28 ]. The climate is temperate continental and is characterized by mild winters, small annual amplitudes of air temperature, two minima( August and September), and two maxima( July and November) of precipitation [ 24 ].
2.2. Data and methodology
The following Sentinel 2 Level 1C optical satellite data from the European Union ' s Earth observation Copernicus program was used in this study: Copernicus Sentinel-2 Collection 1 MSI Level-1C( L1C) from 11 / 1 / 2022, cloud cover: 0.0, and from 07 / 28 / 2024, cloud cover: 0.0: S2B _ MSIL1C _ 20221101T091029 _ N0400 _ R050 _ T35TKG _ 20221101T100434. SAFE and
S2A _ MSIL1C _ 20240728T090601 _ N0511 _ R050 _ T35TKG _ 20240728T125237. SAFE.
Free data from the Ministry of Agriculture and Food for the geographical scope of the specialized layers of the Agricultural Parcel Identification System for physical blocks( land parcels) for 2022 and 2023 [ 27 ], digital terrain model with grid size 30 m [ 6 ], and data from Google Earth Engine were used for validation of the results. The dates of these images are close to the Sentinel 2 images used in this study.
An atmospheric correction of the Sentinel 2 Level 1C product was performed by calculating the Bottom of Atmosphere( BOA) reflectance and using the Sentinel Applications Platform( SNAP, http:// step. esa. int /). In a next step, all frequency bands with a pixel size of 20 and 60 m were transformed to a spatial resolution of 10x10 m. The following water indices were calculated in the QGIS environment to identify and extract water bodies: Normalized Difference Water Index( NDWI), Sentinel Water Mask( SWM), and Water Ratio Index( WRI). From the results obtained, the water area of the " Batak " Reservoir was extracted by selecting appropriate threshold values from their histograms applying the Threshold to ROI tool of QGIS [ 29 ].
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