Evaluating the risk levels of ecohydrological conditions at different time scales based on discharge hydrographs

Abstract

Eco-flows (ecosurplus (ES) and ecodeficit (ED)) calculated with flow duration curves (FDCs) have some limitations on their use in assessing the ecohydrological conditions of rivers. A new method of defining eco-flows based on discharge hydrographs (DHs) instead of FDCs is presented in this study. In this method, the annual ES is the ratio of the surplus annual runoff above the 75th percentile DH to the probable maximum surplus annual runoff above the 75th percentile DH. The annual ED is the ratio of the deficient annual runoff below the 25th percentile DH to the probable maximum deficient annual runoff below the 25th percentile DH. The monthly and seasonal eco-flows are redefined similarly. A methodforranking the risks posed by certainecohydrological conditions is proposed that includes four risk levels: no risk, low risk, moderate risk, and high risk. Data from the Yellow River were used to illustrate these new methods. Results indicate that the new method based on DHs addresses the limitations and that the annual EDs based on DHs have the strongest correlativity to the Shannon diversity index. The risk graph produced with this method displayed the risk levels for each month, season and year overthe past 60 years in an intuitive format. The changes in the dominant risk levels between the pre and post-impact periods indicated the degree to which reservoirs impacted ecohydrological conditions at monthly, seasonal and yearly scales. This study can be used in the future to design ecological regulation measures to improve ecohydrological conditions during adversely impacted months and seasons.