A Framework for Decoupling the Benefits and Challenges of Reservoir Wetlands to Manage the Ecosystem Services in the Eastern Himalayan Transboundary River Basin

The reservoirs of the dams and barrages as instream storage have been affecting the upstream and downstream ecology and simultaneously delivering many services. Therefore, the benefits and challenges of these reservoir wetlands (RWs) are complex and overlapping; as a result, the RWs are often ignored as sources of multiple ecosystem services (ES) in the scientific literature compared to the natural wetlands. The study proposed a framework based on four indicators (hydrological, ecological, economic, and political) to disentangle the benefits and challenges of ES provided by the RWs formed by the dams/barrage on the transboundary Tista River basin in the Eastern Himalayas. These indicators are further divided into various categories and sub-categories to assess the ES for further management. The operation of 13 reservoirs in the mountain basin and a reservoir formed by the diversion barrage in the foothill alluvial plains with a total 89 million M3 storage capacity create many opportunities and challenges for the riparian communities. Hence, the Focus Group Discussions (FGDs) among riparian communities documented variable perceptions regarding the ES of RWs. Further, the hydrological data were analysed using the Mann-Kendall test and Sens slope estimator to understand the role of regulation services. Collectively, the RWs in the basin reduce 5% of the peak flood flow, generate 3295 MW electricity, support flora and fauna, supply 40 million M3/year irrigation water to 0.92-million-hectare targeted cropland, deliver five cumecs drinking water to the cities and create livelihood opportunities through tourism, fishing and boating. Conversely, more than 70% of the dam-released flow fluctuations affected the species richness, about 19 million-metric-ton sediment deposited downstream that reduced the capacity of the water infrastructures by 7%, reservoir construction activities increased >300% sediment, 10% mean-annual flow and 50% of the lean-season flow was regulated. The increasing water demand and the proposed 50-50 proportion of water-sharing geopolitics between India and Bangladesh may further complicate the issues of reservoir ES. Thus, such a framework is significant for future policies and management of the river ecosystem to ensure services for the riparian communities across the basin.