Assessment of Long-Term Dynamics and Trend Extrapolation of Vegetation-Cover Changes in the River Basins of the Southwestern Caspian Region Using NDVI, EVI, and NDMI Data Derived from MODIS Satellites

The vegetation cover in the basins of small and medium-sized rivers of the southwestern Caspian region performs crucial ecosystem functions; however, arid and semi-arid conditions, dissected topography, and intensive anthropogenic pressure make these landscapes highly vulnerable. Conventional ground-based monitoring is hampered by limited accessibility and a sparse meteorological network, while comprehensive predictive studies using a basin-wide approach have not been previously conducted. This study assesses long-term dynamics (2001–2024) and forecasts vegetation-cover changes in nine river basins (Sunzha, Sulak, Shuraozen, Ulluchay, Samur, Karachay, Atachay, Kheraz, and Gorgan) using NDVI, EVI, and NDMI derived from MODIS data (Google Earth Engine, MOD09A1 and MOD13Q1). Statistical characteristics, Theil–Sen trends with Mann–Kendall testing, and Hurst exponent-based persistence forecasts were calculated for each basin. Spatial distribution of all indices was highly heterogeneous. Statistically significant positive NDVI and EVI trends (indicating increasing green phytomass) and a significant negative NDMI trend (declining moisture availability) were found across the entire territory. The lowest multi-year mean NDVI was recorded for the Kheraz basin (0.18), and the highest for the Ulluchay basin (0.44). The positive NDVI trend was significant (p < 0.05) for all basins except Shuraozen, Karachay, and Atachay. Predictive mapping indicates continuous degradation in the estuarine and most intensively developed parts of the Sunzha, Sulak, and Shuraozen basins, particularly reflected by NDMI moisture deficit, whereas sustained improvement is projected for the lower Samur basin. For most of the territory, the change tendency remains uncertain.