How Does the USGS Measure Streamflow?
In order to measure streamflow, the depth and width of a stream and the velocity (speed at which water travels) need to be determined. The area (depth multiplied by the width) multiplied by the velocity gives the discharge (volume of water per unit of time) that passes a given location along a stream. A discharge measurement, then, is a measurement of the volume of water that passes a given stream cross section (total width of the stream) within a given period of time.
Three types of discharge measurements are made by USGS and DEQ technicians on a stream: wading, suspension, and boat. A wading measurement is made by a technician who walks into the water on foot (see cover photograph). A suspension measurement is made by a technician from a bridge (bridge measurement) or a cable (cableway measurement) that crosses the stream (fig. 3). A boat measurement is made by a technician in a boat when a bridge or cable is not available. Using the latest technology-an acoustic Doppler current profiler (ADCP)-the technician can make discharge measurements from a boat, bridge, or cableway. Acoustic technology allows the technician to measure a wide range of streamflows accurately without wading into the stream.
The wading measurement is still the most common type of discharge measurement and requires the use of a current meter, wading rod, earphones, stopwatch, tagline, and notepad (fig. 4). To make a wading measurement, the technician divides the cross section into small units, called "stations." At each station, the technician measures the depth of the stream, the width of the station, and the velocity of the water by use of the current meter. A current meter, similar to a wind-speed measuring device, consists of a wheel with six rotating cups that slides up and down the wading rod. The technician records the number of revolutions of the current meter and the elapsed time on a notepad for each station; the greater the number of revolutions of the current meter, the greater the velocity of the water. These records are used to calculate the velocity of the water at each station. The technician then multiplies the depth of the stream, in feet, by the distance between each station, in feet, by the velocity of the water at each station, in feet per second. The product of the three numbers gives the discharge for that station, in cubic feet per second. The discharges calculated for all of the stations are then summed to get the total discharge for the stream at the cross section.
Continuous measurements of water level of the stream are made inside a gagehouse. A typical gagehouse is a concrete structure located on the bank of a stream, with a pipe extending into the stream (fig. 4). The pipe allows water from the stream to enter the gagehouse so that the gage height (level of the water surface in the gagehouse) is equal to that of the stream. Inside the gagehouse, electronic devices record the gage height, usually at 15-minute intervals. These gage-height records are transmitted via telephone or satellite to the USGS office in Richmond. These data are processed and made available on the Web in near real-time (updated at 1- to 4-hour intervals). The many uses of streamflow data are enhanced by making these data available quickly to a wide audience. Recreationists can get current information to plan their outings. During floods, real-time streamflow data contribute to saving lives and property. Gage-height records and real-time data are provisional (subject to revision after review). The data can be accessed by clicking the "Real-Time Streamflow Data" link on the Virginia District home page at va.water.usgs.gov. After review, the data are published annually in "Water Resources Data-Virginia."
The continuous gage-height records along with the discharge measurements made by the technician are used to create a rating table (a chart showing the relation of gage height to discharge) for a stream. Rating tables are used to estimate the discharge of the stream by using only the gage-height records collected from the gagehouse. The advantage of having a rating table for a stream is that once the gage height is known, the discharge can be inferred without having to go to the stream and make a discharge measurement. Over time, however, the relation between the gage height and discharge can change, requiring periodic discharge measurements to be made to redefine that relation.