Investigating the present status, spatial change, and emerging issues related to riparian wetlands of Bhagirathi–Jalangi Floodplain (BJF) in lower deltaic West Bengal, India

Floodplain wetlands have considerable ecological and economic significance. The study area, consisting of the Bhagirathi–Jalangi Floodplain (BJF), is endowed with numerous wetlands. Based on the tentative time of origin and wetland index map, the investigation selects eight major riparian wetlands and their surroundings from the meander belt of the rivers Bhagirathi and Jalangi. The study primarily emphasizes on periodic mapping for areal change detection and identification of emerging issues between 1955 and 2018. The data used for the present research includes Topographical maps, Landsat imagery, Google maps, Sentinel 2B imagery of different time periods and some ancient maps which depict the old courses of river Bhagirathi and Jalangi. By using geospatial techniques and field investigation, the study reveals that from 1955 to 2018, wetlands have been shrinking rapidly. It is found from the investigation that total wetland area has reduced by about 39.60% due to land reclamation and wetland transformation. The land use change detection depicts that agricultural expansion, which has increased up to 35%, is the dominant control factor for wetland decay. More than 87% of the selected wetlands have experienced area loss. Among the selected wetlands, 25% have lost more than 90% of their areal extent during the stipulated study period. Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) are the two indices used for measuring the concentration of aquatic plants. Geospatial analysis suggests that the NDVI value is higher in pre-monsoon season in comparison with post-monsoon. High NDVI value which is ≥ 0.51 covers more than 81% of the wetland surface area. In contrast, low NDWI value (≤ 0.20), covering more than 60% of the wetland surface area, is found in 75% of the selected wetlands. The study finally concludes that due to accelerated anthropogenic intervention floodplain wetlands in the BJF require special attention to ensure future sustainability.