Abstract: The method for calculating eco-flows metrics based on flow duration curves has some limitations, and no evaluation system is yet available for the alterations of hydrologic regimes based on eco-flows metrics. This paper first constructs an index system including twelve month eco-surpluses and twelve month eco-deficits, through refining monthly, seasonal and annual eco-flows metrics based on discharge hydrographs. A formula measuring the degrees of change in these indexes is derived, and an ecological risk assessment method based on monthly eco-deficit is worked out. Then, we develop a new method for assessing the alterations of hydrologic regimes, apply it to the Yichang hydrologic station of the Yangtze River as a case study, and evaluate the changes in its hydrologic regimes caused by the Three Gorges project. Results show that the redefinition of the monthly, seasonal and annual eco-flows metrics eliminates the inconsistencies, significantly improving the similarity of the diversity index of Shannon to its annual eco-deficit based on discharge hydrographs. The formula is effective in evaluating the degree of regime changes, and the ecological risk graph helps greatly in enhancing the applicability of eco-flows metrics.

Key words: eco-flows metrics, eco-surplus, eco-deficit, discharge hydrograph, flow duration curve