Intensified regulation of large reservoirs accelerates river ecohydrological regime transition: based on the long–short-term memory model

Abstract

Abstract Although previous studies have analyzed ecohydrological changes, external pressure sources on the ecohydrological regime have rarely been clarified, and knowledge about the individual impacts of large reservoir regulation is still insufficient. In this study, we reconstructed the natural flow in the middle reaches of the Yangtze River (MRYR) not regulated by the Three Gorges Reservoir (TGR) based on a long–short-term memory model. We then evaluated the dynamics of ecohydrological regimes using environmental flow components and ecohydrological risk indicators and quantified the impacts of the TGR. The results showed that the hydrological stability has increased after TGR construction, with significant decreases in the frequency and duration of extremely low-flows, large floods, and small floods. In addition, there is a high ecohydrological risk in the MRYR, with a higher ecodeficit and showing a continuous upward trend. The impact of the TGR on environmental flow components decreases along the river, averaging 42.1%, with the strongest impact on small floods, accounting for 56.2%. Overall, reservoir regulation has counteracted the increased eco-risk caused by climate change. Considering only the TGR, the ecosurplus in the spring and winter seasons increased, while the ecodeficit increased in the autumn season, with corresponding contribution rates of 81.4, 82.7, and 53.1%, respectively.