Reservoir ecological operation based on the improved indicators of hydrologic alteration

Dam construction significantly alters the downstream flow regime. However, current reservoir operation strategies often rely on economic or single ecological objectives that inadequately reflect comprehensive ecological demands. Universal indicators of hydrologic alteration (IHA), which calculate parameters solely on an annual scale, fail to capture seasonal variations in flow regimes. To address these limitations, this study proposes improved indicators of hydrologic alteration (IIHA) that advances beyond traditional annual-scale statistics to better characterize seasonal flow variations. Two ecological objectives, water quantity level (WQL) and hydrological alteration (HA), are defined based on the IIHA to reflect the multi-level ecological demands of rivers and are integrated into the operation model to optimize both economic and ecological objectives. A case study of the Liujiaxia Reservoir in Gansu Province, China, was used to verify the effectiveness of the reservoir ecological operation model based on the IIHA. The results indicated that, the improved IHA offers a richer characterization of flow regime alterations across different temporal scales than the universal IHA. Compared to other conventional ecological evaluation objectives, WQL and HA can better preserve information from the full indicators of hydrologic alteration. Compared to the historical strategy, the optimal strategies achieve average reductions in HA of 44.04 %, 26.85 %, and 26.05 % for dry, normal, and wet years, respectively, along with increases in WQL of 2.05 %, 1.25 %, and 3.75 %, respectively. Moreover, the selected balanced operation strategy developed through this model achieved superior performance by maintaining a high hydropower production (HP) while preserving the natural flow characteristics, thus providing a practical guidance for eco-friendly reservoir operation through optimal strategies that balanced ecological and economic benefits.