How does the daily regulation hydropower station reduce the hydrological regime impact? A case study in upper Yellow River

The regulation of hydropower stations has profoundly altered river flow patterns. While studies have extensively assessed the impact of large or multiyear regulated hydropower stations on hydrological regimes using indicators of hydrological alteration (IHA) and range of variability approach (RVA), the impact of daily regulation hydropower stations has received comparatively less attention. This study aims to evaluate the influence of daily regulation hydropower stations on hydrological regime changes. The analysis focuses on the upper Yellow River region in China, which houses cascade hydropower stations, utilizing daily runoff data from the Guide hydrological station spanning from 1954 to 2020. The Mann–Kendall test revealed 27 of 32 IHAs with a significant trend when considering the operation of the multiyear regulated hydropower station (Longyangxia). However, this number decreased to 18 IHAs when daily regulation hydropower stations (Laxiwa and Nina) were included. Evaluation using the RVA method indicated that only 46.87% of IHAs showed high alterations from the natural regime when considering the operation of daily regulation hydropower stations, a decrease from 75.00% when solely considering Longyangxia. Findings suggest that daily regulation hydropower stations can effectively mitigate the adverse effects of multiyear regulated hydropower stations, bringing the hydrological regime closer to a natural state.