A revised range of variability approach for the comprehensive assessment of the alteration of flow regime

- Assessment of flow regime alteration is significant for riverine ecosystem.
- The inner characteristics of hydrological year are collectively reflected by 32 IHA.
- Treating the set of IHA values separately cannot reflect the alteration of ICOHY.
- We develop a revised RVA for comprehensive assessment of flow regime alteration.

Assessment of flow regime alteration is significant for the scientific protection and restoration of rivers. The range of variability approach (RVA) and its revised versions, based on a suite of Indicators of Hydrological Alteration (IHA), have been widely used for flow regime alteration assessment. In these methods, the IHA of each specific year are treated separately, and their individual alterations are used to reflect the alteration of flow regime. However, it is known that one hydrological year is an important hydrological period, and the alteration of inner characteristics of one hydrological year (ICOHY), collectively characterized by the 32 IHA values of the year, can also lead to serious ecological degradation of rivers. In other words, treating the 32 IHA individually and separately in RVA and its revisions can not reflect the alteration of ICOHY. To overcome this limitation, a revised RVA is proposed. In the revised method, the 32 IHA values for each specific year are considered as a set, and the alteration of the set (SA) is measured by the set pair analyses method. The assessment of individual alteration (IA) of each IHA is carried out according to RVA. SA and IA are incorporated into a single index to reflect the overall flow regime alteration (OA). The proposed method is applied to a case study of the Xi Dayang Reservoir in China to test its effectiveness. The results suggest that the RVA underestimates the alteration degree and that the new method could give a more comprehensive assessment.