Comparing Flow Duration Curves and Discharge Hydrographs to Assess Eco-flows

Although eco-flows (ecosurplus (ES) and ecodeficit (ED)) based on the flow duration curve (FDC) have been used to assess hydrologic alterations in recent years, they are defective, and their limitations have never been clarified. In this study, the causes of these limitations were analyzed, and the ES and ED on different time scales were redefined using the discharge hydrograph (DH). The monthly ES was redefined as the surplus monthly runoff exceeding the 75th percentile DH divided by its maximum ecological water requirement, and the monthly ED was redefined as the deficient monthly runoff below the 25th percentile DH divided by its minimum ecological water requirement. The seasonal eco-flows were the sum of three corresponding monthly eco-flows, and the annual eco-flows were the sum of twelve consecutive monthly eco-flows. The daily discharge time series from 1956 to 2015 at Xiaolangdi Station was used to test the new method. The results showed that the inconsistencies of eco-flows on different time scales based on FDCs were closely related to the loss of time-dependent information in FDCs. The correlation coefficients and Fréchet distance between Shannon Index (SI) and annual eco-flows showed that DHs performed better than FDCs in assessing eco-flows. Annual EDs calculated from DHs changed synchronously with SI in most years. This novel method for calculating eco-flows are useful for assessing river health in the future.