pondage

Pondage is the volume of water that a dam or reservoir can store for release for purposes such as hydroelectric generation, irrigation, flood control, and municipal supply. It refers to the usable, or active, storage of a reservoir, as distinct from dead storage, which lies below the intake and cannot be released through normal outlets.

Active storage is typically defined as the difference between the full supply level and the dead storage level. The pondage determines how much water can be drawn down and released during periods of high demand or low inflow, and it governs the reservoir's operational flexibility.

Operations rely on predictions of inflow, evaporation, seepage, sedimentation, and withdrawal demands. Pondage can be replenished by inflows such as rainfall and river flow, and is reduced by evaporation and sediment accumulation, which gradually lower the usable capacity.

Measurement is in volume terms, commonly cubic meters or acre-feet, and is often represented by a capacity curve that relates water level to storage. Changes in pondage over time can result from sedimentation, dam aging, and changes in water rights or operational rules.

Historically, pondage has been central to the planning of hydroelectric schemes and broader water-resource management, enabling storage across seasons and helping to balance supply and demand. It remains a key concept in dam engineering and reservoir operations, reflecting the practical capacity to store and release water as needed.