Measuring Discharge

Discharge is most often directly measured using the velocity area method.  A flow meter is used measure the velocity and water depth at successive locations along a stream cross section.  These data are then integrated to give the total discharge.

The more locations along the cross section,  the more accurate the measured discharge.  Figure F-5 of Dingman (Physical Hydrology, Prentice Hall, 2002) gives an interesting plot of error in discharge measurement versus number of measurement points along a cross section.  Usually at least 20 locations are recommended.

When working on small streams its often easiest to begin by stretching a tape measure across the stream.  Then select a measuring distance interval that will give about 20 points Xi across the stream.



At each distance Xi , measure the stream depth Yi and the stream velocity vi.  In doing this, follow the USGS “Six-Tenths-Depth Method” whereby you measure the velocity at a depth below the stream surface of 0.6*Yi (or 0.4*Yi above its bottom).

The total discharge of the stream is given by


where the area Ai is defined as


When selecting a location to measure discharge, the USGS seeks out these characteristics (from Dingman, page 609): 

1) Converging flow (i.e., cross-sectional area decreasing downstream) without areas of near
    zero velocity or eddies.

2) Absence of backwater conditions (due, for example, to high water levels in a stream  or lake to
    which the gaged stream is a tributary.

3) Smooth cross-section with minimal flow obstructions upstream or downstream

4) Velocities and depths not exceeding the range for which the velocity- and depth-measuring 
    devices give accurate results and for which one can safely negotiate the section.

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