Water Science for Maryland, Delaware and the District of Columbia
Discharge= Velocity x Depth x Width
|Previous slide||Next slide||Back to first slide||View graphic version|
The most practical method of measuring stream discharge is through the velocity-area method. Discharge is determined as the product of the cross-sectional area of the water times velocity. Measuring the average velocity of an entire cross section is impractical, so the USGS uses what's called the mid-section velocity-area method. Using this method, the width of the stream is divided into a number of increments, each usually containing no more than 5% of the total discharge. For each incremental width, stream depth and average velocity are measured. The current meter is placed at a depth where average velocity is expected to occur. For shallow sections, this is at 0.6 of the distance from the water surface to the streambed. When depths are large, the average velocity is best estimated by measuring velocity at 0.2 and 0.8 the distance from the water surface to the streambed. The product of velocity, depth and width of the section is the discharge through that increment of the cross section. The total of the incremental discharges equals the discharge of the stream.