Multiply this figure by 450 to obtain the flow rate in gallons per minute. Measure the stream in summer, during the greatest irrigation demand and the lowest natural flow rate. They are then detected at some point downstream, and the velocity is determined from the time required by the tracer to flow the measured distance. Because the dye or salt is often diffused throughout the stream, determine the velocity of the first and last portions of the dye and take the average. Multiply the average velocity of the dye by the cross-sectional area of the stream to obtain the flow rate.
Computation is the same as for the float method.
The velocity head rod shown below is a fast and inexpensive method of measuring velocity in a stream. A 26-gauge copper sheet is sometimes fastened to the cutting edge to protect it from abuse. Measure stream depth on scale. Place rod sideways in the water. Stream velocity is proportional to this jump.