Themes > Science > Earth Sciences > Geology > Water and Water Cycles > Surface water > Surface water > How streamflow is measured (part 2)

This page gives a more detailed explanation of how the amount of water flowing in a stream (streamflow) is measured. The flow is referred to as stream discharge. Discharge is usually expressed in the number of cubic feet of water flowing past a point each second (cfs).

In order to accurately measure streamflow, someone has to go to the stream, measure its width and depth, and measure the speed (velocity) of the water at many points across the stream. To develop a stream-stage/stream-discharge relation, we need to measure the stream discharge at many different stream stages. After many different streamflows (which are taken at many different stream stages) are measured we can then estimate streamflows at virtually any stream stage (generally, the height of the water surface above an arbitrary reference point, called "Datum"). More simply, if we measure a stream at stages of 3.5, 6, 7.1, 9, and 10.2 feet, then we can estimate the discharge at 8 feet -- that is the goal.

For our example, let's say we need a measurement of Big Creek when it is at a stream stage of approximately 3 feet. First, someone has to go out to the stream when the stage is near 3 feet. The diagram below shows a cross-section of Big Creek at a 5-foot stage. Big Creek is about 10 feet wide. The stream-measurement procedure is to go across the stream at selected intervals and measure the total depth and the velocity of the water at selected depths at each interval across the stream.

In the diagram, the hydrologist would take a measurement of how fast the water is moving at every green 'X'. He/she would then determine the areas between all of the measured intervals, such as the one shown by the purple box.

Cross section of stream showing where flow measurements are taken

In the diagram, water depth/velocity measurements are obtained horizontally across the stream at 1, 3, 5, 7, and 9 feet (the vertical lines in the diagram). At each location, measurements of velocity and total depth are obtained. Depending on the depth and flow conditions, one or more velocity reading(s) are obtained in each vertical. For our example, a water depth/velocity measurement is obtained at a point 5 feet from the edge of the stream. The total depth is 5 feet and velocity readings are obtained at depths of 1 foot and 4 feet (the big 'X's). The purple box represents an area that is midway between this measurement point and the measurement points on either side. The purple area is 2 feet across and 5 feet high, or 10 square feet. The measured velocity at the upper big X is 2 feet per second, and 1 foot per second at the lower X, for an average velocity of 1.5 feet per second. To compute the amount of water flowing in that purple area each second, multiply the area of the purple box times the velocity of the water:

  

(1)  2 feet wide  x  5 foot high = 10 square feet
(2)  10 square feet  x  1 1/2 feet per second = 15 cubic feet per second.
To compute the total stream discharge the hydrologist has to create imaginary purple boxes between all of the 'X's and, using the average velocity of the water in every box, compute the discharge for each purple area. Summing the discharges for all the purple areas will give the total stream discharge.

Actually, the example above is a simplified explanation of how stream discharge is measured. When a real measurement is made, the hydrologist really takes measurements at about 20 points across the stream. The goal is to have no one vertical cross-section contain more than 5 percent of the total stream discharge.


Information provided by: http://wwwga.usgs.gov