Quantification of Key Ecological Water Demand Processes and Adaptive Regulation Strategies in the Xiajiang Section of Ganjiang River After Reservoir Operation

Objective The Xiajiang section of the Ganjiang River is an important reserve and spawning ground for the four major Chinese carps. Since the operation of the Xiajiang Water Control Project, the natural hydrological regime of this river section has been significantly altered, posing sustained stress on the river ecosystem. This study aims to reveal the key ecological water demand processes for the natural reproduction of the four major Chinese carps, and propose targeted adaptive regulation strategies, thereby providing experiences and methods for restoring the natural hydrological rhythm of the river channel to meet ecological needs. Method Taking the reserve of the four major Chinese carps in the Xiajiang section of the Ganjiang River as the research area, long-series hydrological data from the Xiajiang Hydrological Station were used, and the Indicators of Hydrologic Alteration-Range of Variability Approach (IHA-RVA) was adopted to quantitatively evaluate the degree of hydrological regime alteration. Four key hydrological components, namely water level variation range, flood rise duration, flow velocity, and water temperature, were selected to establish hydrological-ecological response relationship curves. By integrating hydrological analysis and ecological response mechanism interpretation, the ecological flow hydrograph for the spawning of the four major Chinese carps was constructed, and an ecological flow regulation strategy oriented by ecological goals was proposed. Result (1) After the operation of the Xiajiang Water Control Project, the eco-hydrological regime of the downstream river section has changed to a certain extent. Indicators such as the maximum 3-day, 7-day, and 90-day flows, as well as the frequency and duration of high-flow pulses, have undergone moderate changes, with the RVA alteration degree ranging from 45.5% to 63.6%. In 2025, the evaluation of the eco-hydrological response to the natural reproduction of the four major Chinese carps in the Xiajiang section of the Ganjiang River was "non-compliant". (2) The ecological flow hydrograph for the natural reproduction of the four major Chinese carps was quantified, and the thresholds of its main hydrological indicators are as follows: minimum flow of 1890 m³/s, suitable flow of 2900–4200 m³/s, daily flow rise rate of 416 m³/(s·d), total flow rise of more than 1248 m³/s, and peak flow of 3138–4200 m³/s. (3) The key ecological water demand processes identified in this study provide precise quantitative targets for the ecological regulation of the Xiajiang Water Control Project. The research shows that ecological flow regulation can significantly improve the eco-hydrological compliance of the spawning of the four major Chinese carps in the Xiajiang section of the Ganjiang River.