Using a relation between stage and discharge, discharge is derived from the stage data. The stage and discharge relation for a specific stream location is defined from periodic discharge measurements made at known stages. The most practical method of measuring the discharge of a stream is through the velocity area method. This method requires the physical measurement of the cross sectional area and the velocity of the flowing water. Discharge is determined as the product of the area times the velocity.
Velocity is measured by using a current meter.
The meter consists of a propeller that is rotated by the action of flowing water. Given the number of revolutions in a given time interval, velocity can be determined for the location of the current meter. Many streamflow data users must make operational decisions daily. For these users, streamflow records are computed and made available on a provisional basis. Computers in the district office convert the depth reading to a value for flow and plot a graph of flow versus time, usually showing the previous 7 days.