Spatio-Temporal Mapping and Analysis of Surface-Water Dynamics in Kopa Dam, Niger State, Nigeria, Using Sentinel-2 Level-2A and GIS Techniques

Abstract

Surface-water extent provides a spatial measure of reservoir condition and supports water-resource monitoring where field observations are limited. This study assessed the spatio-temporal dynamics of surface water in Kopa Dam, Niger State, Nigeria, from January 2022 to January 2026 using Sentinel-2 Level-2A imagery and the Modified Normalized Difference Water Index (MNDWI). One image was selected for January of each year to maintain seasonal consistency. MNDWI was calculated using the green band (B03) and shortwave infrared band (B11), with B11 resampled to 10 m. Water and non-water reference points were used to determine image-specific thresholds of −0.01, −0.01, −0.18, −0.01, and −0.25 for 2022, 2023, 2024, 2025, and 2026, respectively. Binary water masks were generated and refined using the Majority Filter. Mapped water areas were 3.3097, 3.2929, 4.4854, 3.7591, and 4.2103 km² for 2022–2026, respectively. The largest increase occurred between 2023 and 2024 (+36.21%), while the largest decline occurred between 2024 and 2025 (−16.19%). Overall, water area increased by 0.9006 km² (27.21%) between 2022 and 2026. The linear time-series model indicated a positive tendency (R² = 0.4523). Spatial transition analysis identified 95.46 ha of water gain, 5.64 ha of water loss, 325.31 ha of persistent water, and 85.63 ha of stable non-water. The findings indicate substantial annual fluctuations rather than a uniform directional change and demonstrate the usefulness of Sentinel-2 and MNDWI for reproducible reservoir monitoring. Continued application with field water-level observations and bathymetric surveys is recommended for improved reservoir management.