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Measuring Streamflow in the Field <br />Stream discharge (Q) is defined as the volume of water that flows past a given channel location per unit time <br />(e.g., cubic feet per second, or cfs). It is calculated as the product of the cross - sectional area of the wetted <br />channel times the average water velocity. <br />Because local stream velocities vary greatly with depth and distance from the river edge, discharge <br />measurements in the field are based on dividing a river cross section into numerous subsections. For each <br />subsection, a cross - sectional area is calculated from width and depth measurements and the velocity is measured <br />for each subsection using a current meter (Figure 11). The calculated discharge for each subsection (i.e., <br />subsection cross - sectional area times the average velocity of the subsection) is then summed to obtain the total <br />stream discharge or flow. <br />If individual flows are measured for several river <br />stages, it may be possible to construct a flow "rating <br />curve." A rating curve is a graphical plot of river stage <br />(i.e., vertical water level) vs. flow (Figure 12). Once a 30,000 Discharge (cfs) <br />rating curve is established, it is possible to estimate flow <br />from a measure of river stage, such as marks on a 25,000 <br />bridge pier. 20,000 <br />Flow measurements in cfs give the appearance of <br />great accuracy and are sometimes reported to one or two 15,000 <br />decimal places. However, as the above procedures <br />should make clear, several aspects of flow measurement 10,000 <br />introduce error into the calculation. People who have a 5,000 <br />great deal of experience measuring flows in the field are <br />always careful to qualify the accuracy of their o <br />measurements. Those of us who rely on their 0 2 a s s 10 12 14 <br />measurements should respect this caution. As a rule of Stage (ft) <br />thumb, any given flow measurement, even if calculated Figure 12. Example of a rating curve from the San Pedro <br />via sound methods, is only an estimate of the actual flow River, Arizona. The table allows researchers to use stage <br />and may easily be off by five to ten percent. This notion information (vertical height of water) to determine flow at a <br />is particularly useful to remember at the negotiating table transect site. <br />when competing water users are arguing over a small <br />difference in flows. <br />uie prouu- of veiociry Times Tile cross sectional area. in the field, <br />measurements are made for each subsections and then summed. <br />19 <br />