Evaluation of instream ecological flow with consideration of ecological responses to hydrological variations in the downstream Hongshui River Basin, China

Due to increasing anthropogenic activities, especially with cascade reservoirs having been constructed, the hydrological regimes of the Hongshui River (HSR) Basin which were mainly shaped by precipitation in the undisturbed period, have inevitably changed, making further impacts on riverine ecological environment. Although previous researches to analyze hydrological alterations of flow regimes are numerous, insightful investigation into ecological impacts behind them is needed for watershed management and riverine ecosystem protection. Employing two generalized eco-flow indicators (ecodeficit, ecosurplus), fluvial biodiversity Shannon Index (SI), and multiple hydrological indicators: Indicators of Hydrologic Alteration (IHA), integrated degree of hydrological variation (D0) and Dundee Hydrological Regime Alteration Method (DHRAM), this study focuses on the evaluation of temporal variations of ecological flow in multiple time scales while considering ecological responses to evolutionary characteristics of flow regimes which were chronologically subject to rainfall and reservoirs-disturbances; in addition, an effective method that combines eco-flow indicators and ecologicallyrelevant hydrological indicators (ERHIs) to evaluate eco-hydrological regime was proposed. The results indicate that: (1) dam-disturbance has broken the close relationships between ecological flow and precipitation in the near-natural period, causing high level of ecodeficit (0.53 the largest) in flood season and astonishing ecosurplus (1.99 the maximum) in dry season; (2) flow regimes have greatly altered and the degree of integrated hydrological variations reaches 60.22%, with an ecological hazard class at level 3, indicating the riverine ecological environment is at risk of facing moderate damage; (3) contributions of seasonal deficit or surplus of ecological flow have been greatly influenced by reservoirs operations, although with astonishing increment of total seasonal ecosurplus, SI has been abnormally decreasing after 1992 when major reservoir came to operate, indicating the degradation of fluvial biodiversity in the HSR Basin due to dam regulation; (4) the combination of eco-flow indicators and ERHIs, which is capable of mirroring the key hydrological information and related ecological connotation, is prove to be an effective method to evaluate the eco-hydrological regimes. The findings of the study could be instructive to relative researches in other cases and provide scientific guidance for local watershed management.

Automated summary

This study investigated the impacts of anthropogenic activities, especially cascade reservoirs, on the hydrological regimes and ecological environment of the Hongshui River Basin. By using eco-flow indicators and ecologically-relevant hydrological indicators, the study proposed an effective method to evaluate the eco-hydrological regimes, providing insights for watershed management and riverine ecosystem protection.