When Water Arrives at the Wrong Time: Storage Deficits and Market Misalignment in Hydropower

Hydropower schedule-following performance depends on the interaction among turbine capacity, reservoir storage, inflow seasonality, and electricity-market priorities. Mismatch between the timing of water availability and high-price electricity-market periods can substantially constrain market-responsive hydropower operation. This study integrates reservoir topography, including area–volume–elevation relationships, hydrological conditions, hourly electricity prices, and turbine capacity within an hourly water-balance simulation framework. Two complementary indicators are used to quantify operational–market alignment: the schedule-fulfilment ratio (SFR), defined as the percentage of planned operating hours during which the simulated feasible turbine release follows the price-ranked reference release schedule, and the annual proportion of operating hours aligned with market-priority periods. The framework is applied to Iran’s Karun-4 hydropower system, which has an installed capacity of 962 MW and a total turbine discharge capacity of 684 m³ s⁻¹. Under normal-year conditions and the current reservoir configuration, the SFR at the existing turbine capacity is approximately 45.2%, with 616 operating hours aligned with the reference schedule, corresponding to about 7.0% of the year. Limited active storage and the seasonal concentration of inflows during relatively low-price periods require releasing approximately 841 MCM of water during spring to reduce the risk of later spillway releases. A refined storage-sensitivity analysis shows that SFR increases progressively with additional active storage and reaches a plateau of 99.71% at +1,725 MCM, the smallest tested storage increment producing this level of schedule fulfilment. The results show that increasing active-storage flexibility substantially improves schedule fulfilment and reduces avoidable spillway releases, while the proposed indicators provide a transparent basis for diagnosing configuration-specific hydropower–market misalignment.