If surface velocity of a stream at a vertical is vs the average velocity in the vertical will be about
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Streamflow discharge is defined as the volume rate of flow of water that includes any substances dissolved or suspended in the water. Discharge is usually expressed in units of cubic feet per second (cfs or ft3/sec). With rare exception, stream discharge is not measured directly, but is computed indirectly from velocity and water level (stage) measurements. If the mean water velocity normal to the direction of flow (V) and the crossectional area (A) of water flow is known, then the discharge (Q) can be computed as Q=VA. As previously discussed in Lesson 1, determining the mean stream velocity is a labor intensive activity, and usually only performed to establish or refine a relationship between stage, which is easy to measure, and discharge. The discharge rating is a relationship between the stage and discharge or between stage, discharge, slope, rate of change of state, or other factors.
Developing a stage-discharge relationship for a stream requires a set of observed flow (discharge) measurements. Discharge measurements are made at each gaging station to determine the discharge rating for that site. Discharge measurements are initially made are made frequently over a wide range in stages. Periodic measurements are then made (usually monthly) to validate the rating or to identify any changes in the rating caused by changes in the stream channel or stream bed.
The objective of this lesson is to review the techniques and instruments used to measure stream velocity and to calculate stream discharge. The lesson will emphasize discharge measurement via current meters and tracer dilution. We review the methods for determining the vertically averaged velocity at a point and the factors affecting the accuracy of discharge measurement. The interested reader is referred to Rantz et al. (1982) for a review of discharge measurement using the moving boat methodology.