Abstract: Ecological surplus (ES) and ecological deficit (ED) are indicators used to assess the ecological hydrological conditions of rivers. Previous studies have established calculation methods but lack measurable indicators to determine the extent of runoff surplus or deficiency. This paper introduces a novel method for calculating ES and ED based on discharge hydrograph (DH) as a way to evaluate river ecological runoff. The annual ES is determined by the ratio of annual runoff surplus above the 75^th quantile discharge hydrograph to the annual possible maximum runoff surplus. Similarly, the annual ED is calculated as the ratio of annual insufficient runoff below the 25th quantile discharge hydrograph to the annual possible maximum insufficient runoff. Monthly and seasonal ES and ED are defined in a similar manner. Based on the ED, the ecological risk arising from reduced runoff is categorized into four levels: no risk, low risk, medium risk, and high risk. Data from the Pingshan station in lower Jinsha River were utilized to illustrate the new method. The resulting risk map presents the risk levels for each month, season, and year at the Pingshan station from 1940 to 2012. This new method overcomes the limitations of traditional approaches and provides an assessment method for designing ecological runoff control measures in the future.

Key words: ecological runoff , ecological surplus, ecological deficit , risk assessment