Why the Capacity of Taiwan’s Zengwen Reservoir Appeared to Grow: Full-Water Level Adjusted in Three Stages

Observers who track Taiwan’s reservoirs noticed that Zengwen Reservoir suddenly appeared to hold 17.95 million more cubic meters. The increase did not reflect an expansion of the reservoir. It came from a seasonal adjustment to its permitted full-water level.

The Southern Region Water Resources Office said Zengwen uses three full-water levels, unlike most reservoirs, which generally use one. The system balances dam safety with water supply needs during the wet season and the following agricultural year.

From May through August, the full-water level is set at 228 meters, with an effective capacity of 450.04 million cubic meters. The level rises to 229 meters in September, increasing effective capacity to 467.99 million cubic meters. From October through April, authorities raise it again to 230 meters to store water for agriculture, households and industry, bringing effective capacity to 486.54 million cubic meters.

A low-pressure system and southwest winds brought substantial inflows from Aug. 21. The office began regulating releases on the evening of Aug. 24 and later opened the spillway. At 8 a.m., storage stood at 476.81 million cubic meters, still above the September effective capacity, so releases were continuing to protect the dam.