3D flow structure responses to water diversion in China’s fifth largest freshwater lake: Lagrangian observations and modeling

Large water diversion projects have been constructed worldwide to optimize water resource allocation and mitigate eutrophication, how the operations of these projects impact current structure in large lakes remains unclear. This study combines 3D Lagrangian drifter platforms and Acoustic Doppler Current Profiler systems to track transport trajectories of diverted water and subsequent changes in current structure in Lake Chaohu during a large-scale water diversion practice. Hydrodynamic model was setup to investigate the spatiotemporal evolution of current structure under six regulation scenarios. Monitoring results revealed significant three-dimensional heterogeneity; Lagrangian drifter observations showed a distinct vertical velocity stratification pattern, with vertical velocity generally following a pattern of upper (4.9 cm/s) > middle (4.1 cm/s) > bottom (3.5 cm/s). Meanwhile, ADCP measurements indicated that the multi-station average current velocity increased from a background value of 0.59 cm/s to 0.82 cm/s during the diversion period. The Western Lake region exhibited localized circulations due to multiple inflows, while lake-wide currents stabilized post-diversion. Simulations identified diversion pathways and discharge allocation as primary regulators of current patterns. The Pai River route effectively activated water movement in the western lake, the average current velocity across the entire western lake region increased from 0.05 cm/s to a maximum of 0.3 cm/s, with peak velocities near the inflow points exceeding 10 cm/s, whereas the Baishitian River route enhanced connectivity between the Eastern and Western regions. In dual-line diversion schemes, increasing the flow ratio to the Pai River improved western lake exchange but also intensified backwater effects on northwestern tributaries. This study elucidates the hydrodynamic mechanisms by which large-scale diversions alter current structures in shallow lakes. The proposed multi-path optimization strategy provides a scientific basis for the eco-oriented operation of water diversion project and the integrated water environment management of Lake Chaohu.