Modeling streamflow in the Brahmaputra River Catchment: Assessing hydrological dynamics in a transboundary river system

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The Brahmaputra River Basin (BRB), spanning multiple countries, presents major challenges for hydrological modeling due to its transboundary complexity and climate change vulnerability. Accurate streamflow assessment is critical for flood management and sustainable water resource planning. This study applied the Soil and Water Assessment Tool (SWAT) to simulate basin hydrology from 1981 to 2000 using data from 72 weather stations. The model estimated mean annual precipitation of 1806.9 mm, generating 354.79 mm of surface runoff, 531.2 mm of actual evapotranspiration, and substantial shallow aquifer recharge and return flow (877.22 mm and 869.11 mm, respectively), while lateral flow and deep percolation were minor. Calibration and validation at Bahadurabad (Bangladesh) and Pandu Ghat (India) showed strong agreement between observed and simulated discharge, with Nash–Sutcliffe Efficiency (0.79–0.90), Coefficient of Determination (0.75–0.85), and Percent Bias (3.6–8.5%) confirming model robustness. Unlike previous studies limited to sub-catchments or statistical approaches, this research develops a basin-wide, physically based SWAT model integrating multi-source meteorological data from four countries. The framework provides a reliable tool for transboundary flood forecasting, climate adaptation, and cooperative water management, highlighting the role of monsoon rainfall and groundwater–surface water interactions in shaping seasonal water balance.