Spatiotemporal response of depth‑to‑water table (ZWT) to the Three Gorges Reservoir impoundment across hydrological year types

The Jianghan Plain (middle–lower Yangtze River, China) is a low-lying lake–river plain where shallow groundwater is strongly coupled to river stages and human water use, making it sensitive to large-reservoir regulation. Within this plain, our focus is the downstream acute-angle interfluve sector between the Yangtze River (mainstem regulated by the TGR) and its largest tributary, the Hanjiang River, where surface water, soil water, and shallow groundwater are highly coupled and therefore expected to show the most pronounced response to regulated river stages. Our analyses are extracted for this Yangtze–Han interfluve focus region, which defines the fixed study-area mask used throughout the diagnostics. We quantify the spatiotemporal response of ZWT to the Three Gorges Reservoir (TGR) impoundment and assess how this response varies across hydrological year types (wet, normal, and dry). We drive a 0.05° regional configuration of the Community Earth System Model version 2 (CESM2) coupled with the Community Land Model version 5 (CLM5) and the Model for Scale Adaptive River Transport (MOSART), using high-resolution meteorological forcing and China-specific crop/land-use/soil–bedrock datasets to simulate ZWT during 1990–2018. Hydrological year types are classified using a standardized runoff-anomaly approach (thresholds explicitly defined for reproducibility). We compare pre-impoundment (1990–June 2003) and post-impoundment (July 2003–2018) periods using trend, changepoint, and spatial diagnostics. The simulations reproduce the observed seasonal phase and spatial pattern of ZWT. Regionally, the long-term ZWT decline weakens after impoundment (about 8.78% at the regional mean) and a statistically significant changepoint is detected in April 2005, consistent with a lagged adjustment of the river–lake–aquifer system. Under a linear approximation, corresponding to an approximately 0.84 m cumulative water-table rise along the near-Yangtze corridor over 2004–2018. Hydrological year type strongly modulates the response: wet years show the largest affected area and the strongest seasonal-amplitude contraction, with pronounced deepening in summer–autumn. Spatially, ZWT deepening organizes into a narrow corridor near the Yangtze (112.78°–113.12°E / 29.48°–29.62°N), while localized shallowing clusters occur in the northern/northwestern plain (111.88°–112.03°E / 30.38°–30.88°N). These results highlight that regulated-flow impacts on groundwater are not uniform in space or climate state, supporting targeted monitoring and groundwater-risk zoning for the Jianghan Plain and other downstream alluvial plains. • Maps ZWT spatiotemporal patterns pre/post-impoundment across wet, normal, dry years. • Following impoundment, wet years exhibit the sharpest decline in groundwater level. • Near-shore Yangtze groundwater levels rose cumulatively by 0.84 m from 2004 to 2018.