Reservoir Conditions

An "off-stream reservoir" is constructed away from the main river channel in an adjacent natural valley or tributary basin.

Water is diverted from a distant main river via artificial tunnels or canals into the reservoir for storage (e.g., Sun Moon Lake from Zhuoshui River, Xinshan Reservoir from Keelung River, Baoshan Second Reservoir from Shangping River, and Wushantou Reservoir from Zengwen River).

The primary advantage is the ability to close diversion gates during heavy flood peaks, preventing muddy river sediment from entering the reservoir and greatly extending its operational lifespan.

Pumped-storage hydropower utilizes the elevation difference between an upper and a lower reservoir as a "giant rechargeable water battery" for grid energy storage:

1. [Peak Electricity Demand (Generation)]: Water is released from the upper basin (Sun Moon Lake) to the lower basin (Mingtan/Daguan II power plants) to generate electricity, causing upper water levels to drop quickly. 2. [Off-Peak or Solar Surplus (Pumping)]: Powerful pumps push water from the lower basin back up to the upper reservoir, restoring water levels.

Therefore, daily multi-meter level fluctuations in Sun Moon Lake reflect normal grid balancing operations rather than leaks or drought.

A weir or barrage is a "run-of-river diversion facility" (e.g., Gaoping River Weir, Jiji Weir, Shigang Dam, and Zhitan Dam).

Its primary purpose is to raise the upstream river level just enough to divert water directly into purification plants or irrigation canals, rather than retaining multi-month storage reserves.

Flow operates on a real-time "divert as it arrives, release the rest" basis with negligible storage volume, which is why monitoring focuses on real-time inflow and intake rates rather than storage percentages.

An underground reservoir does not involve excavating a massive hollow underground chamber; rather, it utilizes natural porous aquifers, gravel layers, and permeable strata to store water.

Engineers construct subsurface cutoff walls or infiltration lakes to intercept subterranean flows during rainy seasons and extract water via wells during dry periods (e.g., Penghu Chikan Underground Reservoir, Wukan Reservoir, and Pingtung Great Chaozhou Aquifer Recharge Project).

The greatest benefit is zero evaporative loss from sun and wind exposure, making it ideal for offshore islands and arid regions.

This is a common observation in open telemetry datasets due to three engineering reasons:

1. [Hydropower Tailrace Flow]: Much of the discharged water passes through underground penstocks and turbines to generate electricity before entering downstream channels. This is counted in total outflow but does not appear in spillway gate apertures. 2. [Dedicated Water Supply Intakes]: Direct diversions to tap water purification plants or agricultural irrigation canals draw water through dedicated intake tunnels rather than crest gates. 3. [SCADA Telemetry Timestamp Offsets]: Real-time data syncs across primary hydrological master stations and individual gate sensors can carry slight 5-to-30-minute timestamp variances.

Prior to typhoons or heavy rainstorms, reservoir operators initiate regulated releases to vacate flood retention capacity and safeguard dam integrity.

During or following flood peaks, bottom desilting outlets are opened for hydraulic flushing, washing sediment downstream to preserve reservoir lifespan.

Every year between June and September during flood season, Agongdian Reservoir operates under an official "empty-reservoir desilting" regime.

The reservoir is intentionally kept empty or near base level so storm currents flush deposited silt directly into the sea while reserving detention storage to prevent downstream flooding in Gangshan.