Available capacityThe amount of water held in the soil that is available to the plants. See water holding capacity.
AverageA stock market average is a mathematical way of reporting the composite change in prices of the stocks that the average includes.
Average annual runoffFor a specified area, the average value of annual runoff amounts calculated for a selected period of record that represents average hydrologic conditions.
Average degree of consolidationThe ratio of the total volume change in a soil mass at a given time to the total volume change anticipated in the soil mass due to primary consolidation.
Average energyThe total power generation produced by a powerplant during all of the years of its actual or simulated operation divided by the number of years of actual…
Average year supplyThe average annual supply of a water development system over a long period.
Average year water demandDemand for water under average hydrologic conditions for a defined level of development.
Avoid HarmExecutive Order 13158 mandates that each federal agency whose actions affect an MPA shall, to the extent permitted by law and to the maximum extent…
Avoirdupois weightAn English and American system of weights based on a pound of 16 ounces.
AWARDSAgricultural Water Resources Decision Support.
AxisA straight line passing through a spherical body between its two poles. The central line of the body or any of its parts. The vertebral column.
Axis of DamA vertical plane or curved surface, appearing as a line in plan or cross section, to which horizontal dimensions can be referred.
Axis of dam (concrete)A vertical reference surface coincident with the upstream face at the top of the dam.
B-44 Form, Cooperative Station ReportA Weather Service form documenting station management, exposure, topography, driving instructions, payment information, hydrometeorologic equipment, and…
Back pressureA pressure that can cause water to backflow into the water supply when a user's water system is at a higher pressure than the public water system.
BackfillMaterial used in refilling excavation, or the process of such refilling. Material used to fill an excavated trench.
Backfill concreteConcrete used in refilling excavation in lieu of earth material.
BackflowA reverse flow condition, created by a difference in water pressures, which causes water to flow back into the distribution system.
BackfurrowThe first cut of a plow, from which the slice is laid on undisturbed soil.
BacksightA rod reading taken on a point of known elevation, a benchmark or a turning point.
BacksiphonageA form of backflow caused by a negative or below atmospheric pressure within a water system.
BackwaterShallow area of a river that is sometimes isolated, often being located behind a sand bar or other obstruction in the river.
Backwater CurveThe longitudinal profile of the surface of a liquid in a non-uniform flow in an open channel, when the water surface is not parallel to the invert owing…
Backwater EffectThe effect which a dam or other obstruction or construction has in raising the surface of the water upstream from it.
Backwater FloodingUpstream flooding caused by downstream conditions such as channel restriction and/ or high flow in a downstream confluence stream.
BaffleA device, such as a steel plate, used to check, retard, or divert a flow of a material.
Baffle block (dentate)One of a series of upright obstructions designed to dissipate energy as in the case of a stilling basin or drop structure.
BailerA 10- to 20-foot-long pipe equipped with a valve at the lower end. A bailer is used to remove slurry from the bottom or the side of a well as it is being…
Balanced head conditionThe condition in which the water pressure on the upstream and downstream sides of an object are equal (such as an emergency or regulating gate).
Ball-millingThe repeated churning action of cobbles, gravel, and sand caused by the force of water in a stilling basin or other structure by which severe concrete…
BankAmerica has a "dual banking system" which means that both states and the federal government issue bank charters for the need and convenience of their…
Bank fullAn established river stage at a given location along a river which is intended to represent the maximum safe water level that will not overflow the river…
Bank StorageWater that has inflitrated from a reservoir into the surrounding land where it remains in storage until water level in the reservoir is lowered.
Bankfull StageAn established river stage at a certain point along a river which is intended to represent the maximum safe water level which will not overflow the river…
Bankfull Stage/ElevationAn established river stage/water surface elevation at a given location along a river which is intended to represent the maximum water level that will not…
BarrageAny artificial obstruction placed in water to increase water level or divert it. Usually the idea is to control peak flow for later release.
Barrage (gate-structure dam)A barrier built across a river, comprising a series of gates which when fully open allow the flood to pass without appreciably increasing the flood level…
BarrelA volumetric unit of measure for crude oil and petroleum products equivalent to 42 U.S. gallons.
BasaltVolcanic rock (or lava), characteristically dark in color and containing 45 to 54% silica and generally rich in iron and magnesium (mafic composition).
baseOne of the components of nucleosides, nucleotides and nucleic acids . Four different bases are found in naturally occurring DNA - the purines A (adenine )…
Base courseA layer of specified or selected material of planned thickness constructed on the subgrade or subbase for the purpose of serving one or more functions…
Base exchangeThe physicochemical process whereby one species of ions adsorbed on soil particles is replaced by another species.
Base FloodThe flood having a 1% chance of being equaled or exceeded in any given year.
Base flood plainThe flood plain innundated by the 100-year flood.
Base flowGround water inflow to the river. Portion of stream discharge that is derived from natural storage.
Base safety condition (BSC)The level of loading above which a dam failure does not contribute an incremental loss of life.
Base StationA computer which accepts radio signals from ALERT gaging sites, decodes the data, places the data in a database, and makes the data available to other…
Base width or thicknessThe maximum thickness or width of a dam measured horizontally between upstream and downstream faces and normal to the axis or centerline crest of the dam…
BaseflowStreamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel.
Baseline profileUsed for a survey of the environmental conditions and organisms existing in a region prior to unnatural disturbances.
BaseloadThe minimum level of electric power demand of a utility, region, or utility customer delivered or required over a given period of time at a steady rate…
Baseload plantPowerplant normally operated to carry baseload; consequently, it operates essentially at a constant load.
BaseloadingRunning water through a powerplant at a roughly steady rate, thereby producing power at a steady rate.
Basin: (1) A depression in the Earth's surface that collects sediment. (2) The area of land that drains to a particular river.
Basin BoundaryThe topographic dividing line around the perimeter of a basin, beyond which overland flow (i.e.; runoff) drains away into another basin.
Basin LagThe time it takes from the centroid of rainfall for the hydrograph to peak.
Basin RechargeRainfall that adds to the residual moisture of the basin in order to help recharge the water deficit. i.
Basin runoff modelAny one of the computer programs that mathematically models basin characteristics to forecast reservoir inflow from rainfall and/or streamflow data.
Basin StatesArizona, California, Colorado, New Mexico, Utah, Wyoming.
BeachingThe action of water waves by which beach materials settle into the water because of removal of finer materials.
Bear-trap gateAny of a family of crest gates consisting of two leaves, an upstream leaf hinged and sealed at the upstream edge and a downstream leaf hinged and sealed…
Bed elevationHeight of streambed above a specified level.
Bed layerThe flow layer, several grain diameters thick (usually taken as two grain diameters thick), immediately above the bed.
Bed LoadSediment that moves by rolling or sliding along the bed and is essentially in contact with the streambed in the bed layer.
Bed materialUnconsolidated material, or sediment mixture, of which a streambed is composed.
Bed-load dischargeThe quantity of bed load passing a cross section of a stream in a unit of time.
Bed-material dischargeThat part of the total sediment discharge which is composed of grain sizes found in the bed.
BeddingIn fishing, this term refers to bedding fish during the spawning period.
Bedding planeA separation or weakness between two layers of rock, caused by changes during the building up of the rock-forming material.
BedloadCoarse sediments carried along near the bottom of a river.
BedrockGeneral term referring to the solid rock or ledge underlying other unconsolidated material, i.e., soil, loose gravel, etc.
Beginning of FreezupDate on which ice forming a stable winter ice cover is first observed on the water surface.
Beginning of the BreakupDate of definite breaking, movement, or melting of ice cover or significant rise of water level.