Improved indicators of hydrological alteration for quantifying the dam-induced impacts on flow regimes in small and medium-sized rivers.

Dam construction is the main factor altering the flow regimes, while few studies have described that in small and medium-sized rivers (SMRs). The universal indicators of hydrological alteration (IHA) that are widely used in large rivers calculate the parameters just on the annual scale and omit the intra-annual seasonal differences of the flow regimes in SMRs. To fully quantify dam-induced impacts on the flow regimes in SMRs, this paper proposes the improved IHA (IIHA) based on the universal IHA. Then two methods of range of variable approach (RVA) and histogram matching approach (HMA) are used to assess the flow regime alteration. Finally, two indicators of water quantity level (WQL)/hydrological alteration (HA) defined by the parameters in IIHA are employed to evaluate the influence of flow regime alteration on the riverine ecosystem. The case study of a typical SMR named Liujiaping River in Hunan Province, China, verifies the necessity of improving IHA where more hydrological parameters calculated in different periods can comprehensively reflect the flow regime alteration. We find that the integrated hydrological alteration of Liujiaping River assessed by RVA and HMA are both at a high level, and the flow regimes have been significantly altered after the dam construction. Also, the indicators of WQL and HA have higher correlation coefficients with 77 of IIHA parameters and thus can retain as much information of the flow regimes as possible to evaluate the influence of its alteration on the riverine ecosystem.