Contribution identification of climate change and hydro-project operation for dammed-river water level responses: A sixty-year observation study of Hanjiang River.

Understanding the patterns of hydrological responses and identifying the driving factors under the combined influences of hydro-engineering and climate change is crucial for optimizing the utilization of surface water resources. This study focuses on the middle and lower reaches of the Hanjiang River (MLHR), using long-term monthly monitoring hydro-climatic data (1964-2022) from 10 national hydrometeorological stations. Integrated statistical approaches, including the Mann-Kendall (M-K) test, Standard Normal Homogeneity Test (SNHT), Worsley Likelihood Ratio Test (WLRT), and Empirical Orthogonal Function (EOF) analysis, were applied to systematically analyze the spatiotemporal characteristics of water levels and the quantitative contributions of various factors in the MLHR. The results demonstrate that, since the 1960s, significant downward trends in water levels have been observed at most of monitoring stations in the MLHR, and mutation points occurring at all stations except only one station. Fluctuations in water levels due to climatic factors such as precipitation and temperature are short-term and show relatively low correlations. The average water levels increased with mutation points emerging midstream of the Hanjiang River, primarily due to the influence of reservoir operations, while the water level at the lower reaches of the Hanjiang River exhibited a sustained decrease under similar influences. This study verified that water level variation in the MLHR is primarily attributed to hydro-engineering operations rather than climatic factors. By comprehensively evaluating the long-term trends in water level changes and quantitatively assessing the combined contributions of the operation of cascade reservoirs and climate change, this research provides valuable scientific evidence and practical guidance for managing river water resources.