Stage–dependent impacts of altered downstream stage–discharge relationships on the hydrological regime of Dongting Lake

Study region Dongting Lake, China. Study focus This study examines how changes in the downstream stage–discharge relationship affect the lake water levels, outlet discharge, spatial water-level responses, and duration of hydrological states in Dongting Lake. A two-dimensional MIKE 21 FM model was used to simulate the observed 2020 flood-year, 2018 normal-year, and 2023 dry-year hydrographs. Upstream inflows, bathymetry, mesh configuration, and model parameters were held constant, while only the downstream stage–discharge relationship differed between pre- and post-Three Gorges Dam (TGD) conditions. New hydrological insights for the region The post-TGD stage–discharge relationship produced lower downstream water levels at low discharge but higher levels at high discharge. Under the 2020 flood condition, the post-TGD stage–discharge relationship increased the peak water level at Chenglingji, the lake outlet, by 0.54 m and prolonged high-water duration by 17.92 days, while reducing peak outlet discharge by 660.70 m³ s⁻¹ (2.19%) with little change in mean discharge. Under the 2023 dry-year condition, mean water levels decreased by 0.40 m at Chenglingji and 0.20 m at Yingtian in East Dongting Lake. The 2018 normal-year simulation exhibited the same stage-dependent pattern, with lower mean water levels near the outlet but a 0.28 m increase in the annual peak at Chenglingji. Low-water duration at Chenglingji increased by 19.08, 9.88, and 25.75 days under the flood-, dry-, and normal-year conditions, respectively. The response was concentrated near the outlet and East Dongting Lake and attenuated toward the inner lake. These results demonstrate that a changing downstream stage–discharge relationship modifies lake water levels, outlet-discharge responses, and the duration and spatial extent of high- and low-water conditions.