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Stream gauge
From Wikipedia, the free encyclopedia
A stream gauge, or stream gage, refers to a site along a stream where measurements of volumetric discharge (flow) are made. In the United States, the U.S. Geological Survey (USGS) is the principal federal agency tasked with maintaining records of natural resources. Within the USGS, the Water Resources Discipline carries the responsibility for monitoring water resources.
To establish a stream gauge, USGS personnel first choose a site on a stream where the geometry is relatively stable and there is a suitable location to make discrete direct measurements of streamflow using specialized equipment. Many times this will be at a bridge or other stream crossing. Technicians then install equipment that measures the stage, or elevation of the water surface or, more rarely, the velocity of the flow. Additional equipment is installed to record these values and telemeter the readings to the Water Science Center office where the records are kept. The USGS has a Water Science Center office in every state within the USA.
Automated direct measurement of streamflow discharge is difficult at present. In place of the direct measurement of streamflow discharge, a surrogate or surrogates that are easily measured automatically are used. In the majority of cases, stage is used as the surrogate. For low gradient streams, some are highly influenced by variable downstream channel conditions. Until the recent 10 years, for these cases of highly variable downstream channel conditions a second stream gauge would be installed in order that the slope of the water surface could be computed and used as a surrogate along with the stage to determine the streamflow discharge. More recently, the technological advance of velocity sensors has allowed the use of water velocity as a surrogate for streamflow discharge. These sensors are permanently mounted in the stream and measure velocity at a particular location in the stream.
In those instances where only stage is used as the surrogate, a rating curve must be constructed. A rating curve is the functional relation between stage and discharge. It is determined by making repeated discrete measurements of streamflow discharge using a velocimeter and some means to measure the channel geometry to determine the cross-sectional area of the channel. The technicians and hydrologists responsible for determining the rating curve visit the site routinely, with special trips to measure the hydrologic extremes (floods and droughts), and make a discharge determination by following an explicit set of instructions, called a discharge measurement.
Once the rating curve is established, it can be used in conjunction with stage measurements to determine the volumetric streamflow discharge. This record, then, serves as an assessment of the volume of water that passes by the stream gauge and is useful for many tasks associated with hydrology.
In those instances where velocity is additionally used as a surrogate, called an index velocity determination uses a velocity sensor, often either magnetic or acoustic, to measure the velocity of the flow at a particular location in the stream cross section. Once again, discrete measurements of streamflow discharge are made by the technician or hydrologist at a variety of stages. For each discrete determination of streamflow discharge, the mean velocity of the cross section is determined by dividing streamflow discharge by the cross-sectional area. A rating curve, similar to that used for stage-discharge determinations, is constructed using the mean velocity and the index velocity from the permanently mounted meter. An additional rating curve is constructed that relates stage of the stream to cross-sectional area. Using these two ratings, the automatically collected stage produces an estimate of the cross-sectional area and the automatically collected index velocity produces an estimate of the mean velocity of the cross section. The streamflow discharge is computed as the estimate of the cross section area and the estimate of the mean velocity of the streamflow.
