Attenuated Floods yet Amplified Droughts: Dual-Threshold Attribution in Poyang Lake, China

Conventional single-threshold methods for detecting hydrological extremes conflate absolute water deficits with relative seasonal anomalies in seasonally fluctuating lakes, a distinction that is becoming increasingly critical as these systems undergo unprecedented regime shifts driven by climate change and water infrastructure expansion. This study develops a dual-threshold framework that integrates fixed and variable thresholds to independently quantify these dimensions, enabling robust attribution of regime shifts in seasonally fluctuating lakes. The fixed threshold captures absolute extremes based on historical percentiles, while the variable threshold accounts for seasonal hydrological norms, thereby disentangling water deficits driven by external forcing from those arising from seasonal variability. Applied to Poyang Lake (China’s largest freshwater lake and a Ramsar-listed wetland) using 1960–2020 hydrometeorological records and 1984–2020 Landsat imagery, the framework reveals a fundamental transition from flood-dominated to drought-dominated extremes following the 2003 operation of the Three Gorges Dam upstream: extreme floods decreased from 7 events in 1960–2002 (43 years) to 2 events in 2003–2020 (18 years), with delayed midsummer timing and contracted inundation extents, while extreme droughts increased from 3 to 6 events over the same periods, manifesting as persistent autumn-winter phenomena. SHAP-based attribution demonstrates that absolute extremes are governed by basin-scale hydrodynamic forcing (Yangtze backwater and tributary inflow), whereas relative anomalies reflect enhanced atmosphere–hydrology coupling with potential evaporation as a key driver. By decoupling hydrodynamic and climatic controls on dual scales, this work advances mechanistic understanding of asymmetric extremes and provides a transferable diagnostic tool for adaptive management in seasonally fluctuating lakes under non-stationary conditions worldwide. The framework offers a systematic approach applicable to other large lakes facing similar hydrological alterations.