Transboundary perspective to ensure salinity-based environmental flow for the Gorai river system

Abstract

Salinity intrusion poses a significant threat to the ecological and socioeconomic stability of coastal regions in Bangladesh, particularly within the Gorai River system. This study investigates the spatiotemporal variations in salinity and proposes a water allocation mechanism to mitigate its impacts. Spatial analysis identifies the Sibsha River as having the highest salinity levels, while temporal analysis over three critical years (2011, 2017, and 2021) reveals that 2021 was the most saline year, with the majority of monitoring stations recording elevated salinity levels compared to the other years. Notably, the Chardoani, Patgati, and Batiaghata points experienced salinity increases of 51.40%, 171.34%, and 76.58%, respectively, between 2011 and 2021. The sharp rise in salinity at Patgati station is likely due to a significant reduction in upstream freshwater flow, compounded by transboundary water diversion at the Farakka Barrage, sedimentation-induced flow constraints within the Gorai-Modhumoti channel, and increased tidal influence from downstream. Additionally, inadequate dry-season flow caused the salinity front to advance further upstream, exacerbating intrusion into this critical midstream zone. Using a combination of numerical modeling (HEC-RAS) and GIS-based spatial analysis, the study determines the minimum freshwater flow requirements necessary to maintain salinity levels within acceptable limits. These thresholds are critical for the survival of keystone species such as Heritiera fomes (Sundari) and the Gangetic dolphin (Platanista gangetica). The estimated salinity-based environmental flow for the dry season (January–May) in 2017 was calculated to be 450 m³/s. However, the study also revealed a critical water deficit of 396.81 m³/s, which is required to keep salinity below 10 ppt during this period. The findings indicate that reduced freshwater inflows, exacerbated by upstream water diversions and climate variability, have led to significant salinity intrusion, threatening the biodiversity of the Sundarbans and the livelihoods of dependent communities. The core focus of this study lies in adopting a transboundary perspective to ensure environmental flow in the Gorai River system, recognizing the need for equitable water sharing across national boundaries. The proposed water allocation mechanism integrates ecological, hydrological, and transboundary governance considerations, offering a pathway to sustainable water resource management and alignment with Sustainable Development Goal 6. This study underscores the urgent need for collaborative policymaking and integrated management strategies to address the dual challenges of environmental conservation and water security in transboundary river systems.