Hydrological alteration and its effect on the eco-hydrological state in Tangon river traversing Bangladesh and India

The present study highlighted the multi-temporal behavior of hydrological alteration (HA) of a river mainly triggered by damming over the Tangon river of India and Bangladesh in 1989 and its impact on eco-hydrological health. For measuring hydrological alteration, hydrological variability at month scale, diurnal flow change using histogram comparison approach (HCA), degree of alteration using a heat map, and periodicity analysis using wavelet transformation method were used. The present study used hydro-ecological matrices like the range of variability approach (RVA), eco-deficit/eco-surplus, and degree of impact (ImHA) using 33 indicators of HA (IHA) for hydro-ecological assessment. Apart from this, the present study endorsed a new approach (integrated degree of impact due to HA (IImHA)) for accounting for integrated hydro-ecological impact in an altered river. The results following hydrological and eco-hydrological alterations were derived in the post-dam period: (1) monthly water level (WL) was attenuated by 1.5-3.0 m. (2) monthly variability of flow increased by 10%, (3) degree of negative HA ranged from 10 to 23% with high during non-monsoon months, (4) statistically significant periodicity (5% level) in flow spectrum was identified after the dam, (5) HCA revealed that diurnal flow distribution turned form positive to negatively skewed pattern (6) RVA-based monthly failure rate ranges from 13.95 to 25.58%, (7) ImHA of different IHA groups ranged from 0.46 to 0.56 signifying poor to moderate impact, and (8) proposed IImHA value accounted (0.406) moderate degree of ecological impact. The study recommends to apply IImHA in such similar works for making the study effective and instrumental. The findings of this study would be effective for the policymakers specially for the restoration of flow and ecological health.