Non-stationarity and spatial heterogeneity of extreme discharge and flood return levels in the Brahmaputra Valley, India

Understanding spatial variability in extreme river discharge is crucial for accurately assessing flood risk in extensive, monsoon-dominated river basins. This study highlights the characteristics of extreme discharge and its correlation with flood events along the Brahmaputra River. Extreme hydrological events were identified through a threshold-based methodology, with the 99th percentile of daily discharge established as a benchmark for high-impact flood conditions. To ensure the independence of events, exceedances were declustered using a three-day separation window, retaining only peak discharge values from individual events. The results indicate a pronounced downstream amplification in the magnitude of extreme discharges, characterized by increasing event peaks from upper to lower Assam. This phenomenon reflects the cumulative integration of hydrological processes across the basin, underscoring the influence of basin-wide hydro-climatic factors.  Seasonal analysis reveals that extreme discharge events are predominantly concentrated during the monsoon season, underscoring the critical role of monsoon rainfall and the upstream catchment's response in the genesis of flood events. Furthermore, Flood return periods, computed using declustered peak discharges, yield reach-specific flood estimates, indicating significantly higher 50-year return levels in the downstream sections. A comparative analysis of extreme event characteristics between pre- and post-2000 periods reveals an increase in both mean and maximum flood magnitudes in recent years, suggesting potential non-stationarity in the flood regime. The frequency of smaller flood return periods has increased in recent times, primarily due to shifting precipitation patterns within the basin. Overall, this study demonstrates the longitudinal coherence in the behaviour of extreme discharge along the Brahmaputra River, characterized by the downstream amplification of flood magnitude and the persistence of basin-scale drivers influencing extreme event frequency. These findings underscore the importance of conducting spatially distributed assessments of extreme flows for effective flood risk management in extensive Himalayan river systems under changing hydroclimatic conditions.