Revealing inconsistency of the sediment-water relationships in the Brahmaputra River: A comparative analysis of upstream and downstream cross-sections

Understanding the relationship between sediment and discharge is crucial for effective river management and water-sediment modeling, especially in the Brahmaputra River, one of the large transboundary rivers with high sediment and discharge variability in South Asia. Current knowledge of sediment-water relations is constrained by limited data, hindering effective transboundary river management. Using multivariate linear regression, climate elasticity coefficient, and traditional sediment rating curve, this study is designed to compare the sediment-water relations of the upstream (Nuxia) and the downstream (Bahadurabad). The results reveal significant variability between the two stations. In the upstream Nuxia, the simulation strongly correlates with observed suspended sediment load (SSL) and discharge (Q) (Pearson's r of 0.62 and 0.68, respectively). Conversely, at downstream Bahadurabad, weaker correlations (r = 0.31 for sediment and r = 0.51 for discharge simulation) indicate a reduced relation. This contrast reflects the non-linear nature of sediment-discharge coupling along the river continuum, shaped by both climatic and anthropogenic influences. Elasticity ( \varepsilon ) analysis highlights the dominant role of precipitation in shaping sediment-water dynamics (   \mathttε_P-\mathrmS\mathrmS\mathrmL\mathrm=2.53,\mathttε_P-Q=1.01) at Nuxia, while Bahadurabad ( \mathttε_P-\mathrmS\mathrmS\mathrmL\mathrm=0.41,\mathttε_P-Q\mathrm=0.82) reflects a reduced sensitivity, possibly due to sediment retention along the floodplain. Air temperature elasticity ( \mathttε_T-\mathrmS\mathrmS\mathrmL , -0.15 & -3.06 at Nuxia and Bahadurabad, respectively) reveals contrasting impacts, strongly negatively influencing sediment transport at Bahadurabad. These findings highlight the significance of spatial variability and climatic influences on sediment dynamics, underscoring the necessity for site-specific management strategies. The sediment rating curve (SRC) analysis reveals a strong relationship between sediment and discharge (R² = 0.88) at Nuxia and a relatively weaker relationship (R² = 0.14) at Bahadurabad, which demonstrates a lower sediment-discharge coupling that could be affected by downstream factors such as sediment deposition, channel morphology, and anthropogenic activities. This study offers valuable insights into sediment-water dynamics, highlighting the importance of understanding nonlinear relationships in the Brahmaputra River.