Optimizing environmental flow operations based on explicit quantification of IHA parameters

Reservoir operations are increasingly being asked to consider environmental flow, which is needed to sustain a healthy river ecosystem. The Indicators of Hydrologic Alteration (IHA) is a tool that is used widely to describe environmental flow regimes, but few studies have explicitly included its parameters in multi-objective reservoir operation models. With the goal of incorporating detailed environmental flow requirements into reservoir operations, this study proposes a two-objective reservoir operation model that includes explicit IHA constraints. A series of formulae is developed to calculate IHA parameters without using a loop or conditional statement, which allows operators to manage environmental flow directly. An experimental operation of the Jinghong reservoir in the upstream portion of the Mekong Basin is conducted to apply the method. The economic objective is defined by hydropower production (HP), while the environmental flow objective is represented by a weighted aggregate eco-index (EI) based on IHA parameters. Five scenarios with different objective functions and constraints are compared, and the results show that the scenario with "HP–EI" as its objective achieved optimal benefits for both indices. Hard EI and explicit IHA constraints led to significant loss of HP that can be attributed to variations of inflow. To make this model more convenient for practical use, operation rule curves are regressed from the optimized results of the model. Finally, policy implications of the operation with economic and environmental objectives and some limitations are discussed. The quantification method of IHA parameters provides significant reference value for reservoir environmental operation issues.