Impact of intra-day peak regulation of hydropower stations on fish habitat suitability

Abstract

[Objective] Regulating reservoir releases profoundly alters river hydrology and hydrodynamics. While intra-day peak regulation by daily-regulating hydropower stations can re-regulate the flow processes generated by upstream regulating reservoir releases, the impact of this re-regulation on fish habitats downstream of the dam remains unclear. [Methods]The Luding Nanqiao spawning reach was selected as the research object, and a two-dimensional planar hydrodynamic model was constructed. Based on the hourly inflow and outflow hydrographs on typical days during the spawning season(March and April) of Schizothorax prenanti from 2018 to 2021, the hydrodynamic characteristics of the reach under various typical flow conditions and intra-day hourly flow variations were simulated. Furthermore, the physical habitat simulation method was used to analyze the spatial distribution patterns of suitable spawning habitats and assess the impact of intra-day peak regulation on the intra-day variations of the weighted usable area(WUA). [Results] Flow variations significantly altered the distribution of flow velocity and water depth in Luding Nanqiao spawning ground reach. The WUA exhibited a unimodal response to increasing discharge, peaking at 33 800 m² at 80 m³/ s. Regarding overall habitat suitability, compared to inflow scenarios, the average values of WUA on most typical days under outflow scenarios showed a slight decrease, with a reduction rate of 1. 96% ~ 10. 45%, while the intra-day minimum WUA increased significantly, with an increase rate of 13. 82% ~ 40. 89%. In terms of habitat stability, both the coefficient of variation for the habitat and the average intra-day WUA variation decreased on most typical days under outflow scenarios, with increases observed only on certain days in 2020. Concerning the risk of short-term habitat degradation, the maximum hourly WUA reduction effectively decreased by 903 ~ 11 747 m² on most days. However, the maximum hourly WUA reduction increased during the typical days of March and April 2020 under outflow scenarios. [Conclusion] The impact of intraday peak regulation at Luding hydropower station on spawning habitat suitability varies with the operational regime. For most typical days, daily regulation has a limited influence on the overall habitat suitability of the Nanqiao spawning reach. However, it improves habitat stability to some extent and reduces the risk of short-term habitat degradation. Nevertheless, under certain operating scenarios, hydropower regulation may increase habitat fluctuations and exacerbate the risk of short-term habitat degradation within the spawning reach.

Keywords

fish habitat / intra-day peak regulation / ecological operation / hydrodynamic simulation / Schizothorax prenanti / fish spawning season / Luding hydropower station / middle reaches of Dadu River