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Hydrology Definitions


The science dealing with the waters of the earth, their occurrence and movement on the surface and underground, and the cycles involving evaporation, precipitation, flow to wetlands, ponds, lakes, rivers and the seas. 

Acre-Feet (AF)

An expression of water quantity.  One acre-foot of water will cover one acre of ground one foot deep.  An acre-foot contains 43,560 cubic feet, 1,233 cubic meters, or 325,829 gallons (U.S.).

Cubic Feet Per Second (cfs)

A unit expressing the rate of discharge (or rate of flow) of water.  One cubic foot per second is equal to the discharge through a rectangular cross-section one foot wide and one foot high, flowing at an average velocity of one foot per second.  One cubic foot per second equals 448.8 gallons per minute, and 1.98 acre-feet per day.


The volume of water represented by a flow of one cubic foot per second for 24 hours.  It is equivalent to 86,400 cubic feet, approximately 1.9835 acre-feet, or about 646,000 gallons or 2,447 cubic meters.


 The crest is the top of a dam, dike, weir, or spillway, which water must reach before passing over the structure.  (The highest elevation reached by flood waters flowing in a channel is also called the crest.)


The conversion of water level to a discharge (flow) is made by the use of a stage-discharge relation.  The physical element that controls the relation is known as a control.  A natural control may consist of a change in slope of the water surface just down-stream from the gage or a constriction which may result from the local rise in the stream bed, as a natural riffle or rock outcrop.  Examples of artificial controls are weirs.


An expression of the volume of water in a reservoir or lake in acre-feet.  Contents is a function of stage (i.e., water-surface elevation) and physical dimensions of the reservoir or lake.


Reduced area of flow in a weir due to the weir's crest and sides being sufficiently removed from the bottom and sides of the weir box or channel in which it is set.


The horizontal movement of water.

Current Meter

A device for the measurement of the speed of flowing water.   Link to more information on current meters.

Discharge (Q)

In the simplest form, it means an outflow of water.  The use of this term is not restricted as to course or location and it can be applied to describe the flow of water from a pipe or from a drainage basin.


In the simplest form, it means to remove water from a water body.


The lowering of the water level caused by pumping.  It is measured in feet for a given quantity of water pumped during a specified period, or after the pumping level has become constant.


An overflow or inundation from a source expressed in volume with respect to time.


A specially-shaped open-channel flow section which may be installed in a canal, stream, or ditch to measure the rate of flow (i.e., gallons per minute or cubic feet per second) of water.   Link to more information on flumes.

Free Flow

When used to describe the operation of a flume, free flow is when discharge depends solely upon the width of the throat and the depth of water in the converging section.

Gage Height

Also known as stage.  The water surface elevation referred to an arbitrary or predetermined gage datum.

Gaging Station

A particular site in a stream, canal, lake, or reservoir where systematic observations of hydraulic data are obtained.


The height of a column of water above a reference point.


The force over a unit area.

Rating Table

A table listing gage heights with their corresponding discharge values.  It is used in obtaining instantaneous discharges.

Recorder, Stream Data

A mechanical or electrical apparatus which records a continuous record of water level.  An example is a STEVENS A-71 chart-type recorder.   Link to more information on other stream data recorders.

Reference Gage

Usually a non-recording gage; for example, a staff gage.  Gage readings are used for checking and resetting the gage height indicated on the stream data recorder.


A shallow reach in a stream that produces a stretch with increased velocity and ruffled or choppy water.


A concentrated discharge of ground water coming out at the surface as flowing water.

Staff Gage

Either vertical or inclined.  The standard U.S. Geological Survey vertical staff gage consists of porcelain-enameled iron sections, each 4 inches wide, 3.4 feet long, and graduated every 0.02 foot.  They are used to measure water-surface elevations.


The stage of a stream or lake is the height of water surface above an established datum plane.  The water surface elevation referred to an arbitrary or predetermined gage datum is called the "gage height".

Stilling Well

A circular pipe that is connected to a canal, ditch, or stream by an inlet pipe.  A float in the well is coupled to a stream data recorder in order to obtain water stage data that is not affected by surges or choppy water that may occur in the channel.

Stream Gaging

A process of determining the rate of flow (i.e., the discharge) of streams.

Submerged Flow

When used to describe the operation of a flume, submerged flow is when the resistance to flow due to increased discharge becomes sufficient to reduce the velocity, increase the flow depth, and cause a backwater effect at the flume.  Discharge is not reduced until the submergence ratio exceeds at least 50 percent.


When used to describe the operation of a flume, submergence is the ratio of the downstream head (Hb) to the upstream head (Ha).  The ratio is expressed as a percentage.


A device that converts electrical energy into pressure pulses or the converse.

Water Year

A 12-month period, usually October 1 through September 30.  The water year is designated by the calendar year in which it ends and includes 9 of the 12 months.  Thus the year ending September 30, 1999, is called the "1999 Water Year".


A structure placed in a canal, stream, or ditch to measure the rate of flow of water.  In its simplest form, a weir consists of a bulkhead with an opening of fixed dimensions cut into its top edge.  Link to more information on weirs.

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