For reservoirs that are operated for multiple purposes such as water supply, flood control, and power generation, any attempts to incorporate environmental flow targets in the reservoir operation rules need to take into account both the human and ecosystem demands. To date, however, none of the reservoir operation schemes that consider environmental flow requirements includes subdaily flow regimes and is able to optimize for multiple reaches. Here, we address the temporal and spatial issues associated with the optimal environmental flow and operation strategies for a multipurpose reservoir system in Taiwan. We propose an environmental flow proportion strategy and three-period release approach, and multireach operation scenarios that simultaneously optimize reservoir performances and environmental flow objectives at subdaily to interannual timescales for a maximum of three connected reaches. Our results imply that taking into account the environmental flow objectives does not necessarily degrade the overall reservoir performance due to the positive effect on flood control, which in turn would compensate for the adverse effects on domestic water supply and hydropower generation. The three-period release approach benefits mainly the subdaily flow regime, while the environmental flow proportion strategy benefits primarily the daily flow regime. Spatially, a mutual exclusion is observed between the reaches above and below a diversion weir, a fact that revises the conventional perception that restoring the flow regimes of a downstream reach would automatically restore those of upstream reaches. An overall evaluation reveals that the three-reach scenario outperforms the two-reach scenarios, which then outperform the one-reach scenarios. The one- or two-reach scenario that incorporates the midstream reach may be taken as an alternative because such scenario would benefit the upstream or downstream reach in addition to the midstream reach.
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