This page collects short working definitions for recurring terms used across the site.
Primary reference for several reservoir and operations terms: USBR Operating Agreement Glossary (PDF).
ACRE-FOOT
A unit of water volume equal to the amount of water needed to cover one acre to a depth of one foot.
ACTIVE CAPACITY
The portion of reservoir capacity normally usable for storage and regulation of reservoir inflows to meet operating requirements.
WATER YEAR (WY)
Period of time beginning October 1 of one year and ending September 30 of the following year and designated by the calendar year in which it ends. It corresponds to the typical water cycle from winter snow accumulation through spring and summer runoff.
HYDROLOGIC YEAR
See Water Year.
ACTUAL INFLOW
Observed inflow reflecting the managed upstream system as actually operated.
DEAD STORAGE
Reservoir storage from which water cannot be withdrawn by gravity.
DRAWDOWN
Lowering of a reservoir’s water level or the process of depleting reservoir storage.
NATURAL FLOW
Modeled flow representing the river absent upstream regulation and adjusted to include water that was consumptively used upstream. In that sense, natural flow is not the same as unregulated inflow.
UNREGULATED INFLOW
Modeled or estimated inflow representing no upstream regulation (counterfactual or naturalized condition).
CONSUMPTIVE USE
Water removed from immediate return to the system, for example through irrigation or municipal use, usually as defined in basin accounting reports.
BANK STORAGE
Water exchanged between the reservoir and adjacent geologic materials, which can affect apparent mass balance.
RELEASE
The amount of water released after use. In reservoir accounting, the difference between delivery and release is often associated with consumptive use.