# **2. Data**

The Indian Sundarbans Biosphere Reserve comprises a 9,630 km<sup>2</sup> area, with a core zone of 1,700 km<sup>2</sup> . The remainder is subdivided into development (5,300 km<sup>2</sup> ), managed (2,400 km<sup>2</sup> ), and restoration (230 km<sup>2</sup> ) zones (Nandy and Kuswaha 2011). Its international border with Bangladesh is demarcated by the Harinbhanga River (known as the Raymangal, Kalindi, and Ichhamati in its north). Blasco, Saenger, and Janodet (1996) reported that the Indian mangroves consist of 58 species; Rao (1986) reported 60 species, while Naskar (1988) reported 35 true mangroves, mentioning that the Indian mangroves are richer than any other tropical mangrove formations in the world. Although ecologically resilient, the mangrove species of the Indian Sundarbans are highly sensitive to hydrological changes (Blasco, Saenger, and Janodet 1996), particularly to the salinity profile of the adjacent water column or soils. Climate‐ and/or subsidence‐driven, sea‐level rise is perhaps the single most important factor that threatens the health of the mangroves (McLeod and Salm 2006). Currently, the Indian Sundarbans is already facing a serious freshwater scarcity since most of the rivers in the region have lost connection with their parent river (i.e., glacier‐melt perennial sources of freshwater) because of siltation at off‐take and have turned into tidal‐fed rivers. Their estuarine character is now maintained by the monsoonal runoff alone (Bhadra et al. 2017; Cole and Vaidyaraman 1966; Gopal and Chauhan 2006). Freshwater flow in the region has also been affected by human‐induced hydrologic alterations in the upper reaches of the Bhagirathi Hooghly River and the Ganges‐Brahmaputra‐ Meghna system.<sup>3</sup> The region is likely to experience further changes in its salinity profile due to saltwater intrusion from sea‐level rise in the future.<sup>4</sup>

## _<u>Spatial Distribution of Mangrove Species</u>_

To understand the current spatial distribution of mangrove species in the Indian Sundarbans, Landsat 8 Operational Land Imager (OLI), sentinel, and hyperspectral data from Hyperion were used as the baseline data for this analysis. The acquisition date of the Landsat 8 OLI data is March 18, 2015 and the path/row is 138/45. Landsat 8 (OLI) has 9 spectral channels, ranging from visible to shortwave infrared bands. The spatial resolution is comparable to the ETM+. Temporal resolution of Landsat 8 is 16 days. Hyperspectral data from Hyperion were processed and used for the spectral signature generation of various mangrove species. The acquisition dates of the Hyperion data used are September 10, 2011, November 23, 2014, and November 13, 2016, respectively, and the path/row is 138/45. Hyperion images have 242 bands that include both the Visible and Near Infrared (VNIR) and Shortwave Infrared (SWIR), having a spectral range of 357 to 2,576 nm with a spectral interval of 10 nm (Figure 1).

> 3 <mark>Freshwater flow has become increasingly restricted since the 1975 construction of the Farakka Barrage Township; between 1962 and 2006, water discharge of the Ganges was reduced from 3,700 m</mark><sup>3</sup> <mark>per second to 364 m</mark><sup>3</sup> <mark>per second, strangling an already parched ecosystem and thus making the distributary networks more dependent on tidal flow bringing in sea water from the Bay of Bengal (Islam and Gnauck 2008).</mark>

> 4 The groundwater is also saline, except for a few meter‐thick, confined aquifers.

3