Mid-section Measurements
The mid-section method has been the primary method for measuring discharge within the USGS for decades. The mid-section method involves measuring the channel area and water velocities of a stream at a cross section. The channel is divided into a number of vertical subsections. Most natural channels must be divided into at least 25 sub-sections to adequately characterize their irregular geometry.
The depth and average velocity are measured at each subsection and are applied to a sub-area the width of which extends halfway to the preceding and halfway to the following observation points. The area of each subsection is determined by directly measuring width and depth. The average water velocity in each sub-section is estimated using the measured velocity at selected locations in the vertical. Velocities can be computed using velocities from a variety of instruments including point velocity meters like current meters and Acoustic Doppler Velocimeters (ADVs) as well as velocity profilers like Acoustic Doppler Current Profilers (ADCPs). The total discharge within the stream is the sum of the individual subsection discharges as demonstrated in the illustration below.
Mid-section Measurement Resources
Guidance: Published guidance reports, technical memorandums, and best practices.
Software: USGS utilities and post-processing programs, public domain software, manufacturer's firmware and software.