Assimilation of SWOT Water Surface Elevation Enhances Simulations of River Dynamics in the Ohio River Basin

Abstract Surface Water and Ocean Topography (SWOT) provides water surface elevation (WSE) that can constrain river hydraulics through data assimilation (DA), but basin‐scale impacts of assimilating real observations remain uncertain under heterogeneous availability and regulation. We assimilate node‐level SWOT WSE into a large‐scale hydrodynamic model over the Ohio River Basin using an ensemble Kalman filter and test river‐network‐aware localization. Assimilation improves discharge skill at most of the 66 USGS gages, including sites without direct SWOT overpasses, with ∼60% showing higher KGE and ~3% degrading. These improvements suggest that SWOT WSE assimilation extends beyond direct observations, with localization adding benefit. Impacts are governed by transferable constraints, including quality control (QC), residual observation uncertainty, and river‐network context. Comparison with SWOT‐derived discharge at 27 overlap sites provides complementary context and supports using WSE as a direct hydrodynamic constraint. These results show that SWOT WSE assimilation improves discharge estimates under real‐world constraints, supporting water management in data‐sparse and transboundary basins globally.