Spatio-temporal assessment of hydro-ecological connectivity in Themchi Beel, Northeast India using multi-temporal remote sensing and GIS

Wetlands are dynamic ecosystems whose ecological functioning depends on interactions among surface water dynamics, surrounding land cover, vegetation, and hydrological processes. Hydro-ecological connectivity refers to the functional linkage between wetlands and their surrounding landscape through surface water pathways, vegetation and land-cover interactions, and terrain-controlled hydrological processes that facilitate the movement of water and ecological functions. However, its long-term dynamics are little understood in the Himalayan foreland of Northeast India. This study develops a Hydro-Ecological Connectivity Index (HECI) to evaluate the spatial and temporal dynamics of hydro-ecological connectivity in Themchi Beel, Assam, India. Landsat Surface Reflectance photos (1990, 2000, 2010, 2020, and 2025), Sentinel-2 images (2025), and Shuttle Radar Topography Mission (SRTM) Digital Elevation Model data were processed in Google Earth Engine. This study integrates multi-temporal remote-sensing indicators, including the Modified Normalized Difference Water Index (MNDWI), Normalized Difference Vegetation Index (NDVI), and land use/landcover (LULC), with DEM-derived hydrological parameters, including flow direction and flow accumulation, within a 1-km surrounding landscape. The indicators were normalized using the min-max method, and Shannon entropy weighting was applied to objectively determine the relative contribution of individual indicators.

The results indicate a notable transformation in the wetland landscape between 1990 and 2025. Wetland area decreased by approximately 23.2 ha, accompanied by a decrease of 67.4 ha in agricultural land, whereas built-up land and vegetation increased by 47.08 ha and 43.81 ha, respectively, indicating progressive land-use transformation and increasing anthropogenic pressure within the study area. The results showed temporal variation in mean HECI, with mean values of 0.0297, 0.0294, 0.0327, 0.0333, and 0.0322 for 1990, 2000, 2010, 2020, and 2025, respectively. Overall, the mean HECI increased by approximately 8.42% between 1990 and 2025, despite fluctuations during the study period. The spatial classification of HECI using fixed thresholds of 0-0.25, 0.25-0.50, 0.50-0.75, and 0.75-1.00 revealed that approximately 98.98% - 98.99 % of the assessed area consistently remained within the low-to-moderate connectivity (HECI 0-0.50), while approximately 1% - 1.02% was characterized by high-to-very-high connectivity (HECI 0.50-1.00). The scientific contribution of this study is the development of an integrated, spatially explicit HECI framework that combines ecological indicators derived from satellite observations with terrain-controlled hydrological connectivity. The proposed methodology provides a quantitative and transferable approach for identifying connectivity hotspots, detecting areas of declining ecological connectivity, and supporting evidence-based wetland conservation and management in rapidly changing landscapes.