Water-Supply Paper 2502

Glossary

Although much of the terminology used in this report is widely understood, some terms have specialized meanings in hydrology or are unfamiliar outside of hydrologic usage. Most of the definitions given here are from Langbein and Iseri (1960), some with slight modifications, and explain the terms as they are generally used by hydrologists in the U.S. Geological Survey.

Absorption. The entrance of water into the soil or rocks by all natural processes. It includes the infiltration of precipitation or snowmelt.

Area-weighted rainfall. Method of converting point rainfall data into an average value for a certain area or basin.

Bank. The margins of a channel. Banks are called right or left as viewed facing the direction of flow.

Cubic feet per second. A unit expressing rates of discharge. One cubic foot per second is equal to the discharge of a stream of rectangular cross section, 1 foot wide and 1 foot deep, flowing water an average velocity of 1 foot per second.

Current meter. An instrument for measuring the velocity of flowing water. The U.S. Geological Survey uses a rotating cup meter.

Discharge. In its simplest concept, discharge means outflow; therefore, 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. If the discharge occurs in some course or channel, it is correct to speak of the discharge of a canal or of a river.

Drainage area. The drainage area of a stream at a specified location is that area, measured in a horizontal plane, that is enclosed by a drainage divide.

Drainage basin. A part of the surface of the Earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.

Flash flood. A sudden, violent flood, as after an intense rain.

Flood. An overflow or inundation that comes from a river or other body of water (Barrows, 1948, p. 4) and causes or threatens damage. Any relatively high streamflow overtopping the natural or artificial banks in any reach of a stream (Leopold and Maddock, 1954, p. 249-251).

Flood plain. The lowland that borders a river, usually dry but subject to flooding (Hoyt and Langbein, 1955, p. 12).

Flood stage. The stage at which overflow of the natural banks of a stream begins to cause damage in the reach in which the elevation is measured.

Hydraulic head. Difference in height between a point and the free water surface above or below.

Isohyetal map. A map or chart showing lines that join points that receive the same amount of precipitation.

Overland flow. The flow of rainwater or snowmelt over the land surface toward stream channels.

Regulation. The artificial manipulation of the flow of a stream by the use of dams, gates, or canals to impede, release, or redirect the natural flow of water.

Reservoir. A pond, lake, or basin, either natural or artificial, for the storage, regulation, and control of water.

Stage. The height of a water surface above an established datum plane (also gage height).

Stage-discharge curve. A graph showing the relation between the gage height, usually plotted as ordinate, and the amount of water flowing (discharge) in a channel, expressed as volume per unit of time, usually plotted as abscissa.

Stage-discharge relation. The relation expressed by the stage-discharge curve.

Streamflow. The discharge that occurs in a natural channel. Although the term discharge can be applied to the flow of a canal, the word "streamflow" uniquely describes the discharge in a surface stream course. The term "streamflow" is more general than runoff, as streamflow may be applied to discharge whether or not it is affected by diversion or regulation.

Streamflow-gaging station. A gaging station where a record of discharge of a stream is obtained.

Surface runoff. That part of the runoff that travels over the soil surface to the nearest stream channel. It also is defined as that part of the runoff of a drainage basin that has not passed beneath the surface following precipitation.

Surface water. Water on the surface of the Earth.

Water equivalent of snow. The amount of water that would be obtained if the snow should be completely melted. Water content may be merely the amount of liquid water in the snow at the time of observation (Wilson, 1942, p. 153-154).

Water year. In U.S. Geological Survey reports, water year is the 12-month period, October 1 through September 30. The water year is designated by the year in which it ends. Thus, the year ending September 30, 1989, is called the "1989 water year."