Measurement of discharge using tracers

The development of fluorescent dyes and fluorometers that can measure these dyes at very low concentrations has made dye-dilution methods practical for measuring discharge. These methods are particularly useful for determining discharge under certain flow conditions that are unfavorable for current meter measurements. These include small streams, canals, and pipes where
1. Turbulence is excessive for current-meter measurement but conducive to good mixing.
2. Moving rocks and debris may damage instruments placed in the flow.
3. Cross-sectional areas or velocities are indeterminate or changing.
4. The flow is unsteady, such as the flow that exists with storm-runoff events on small streams and urban storm-sewer systems.
5. The flow is physically inaccessible or unsafe.
From a practical standpoint, such methods are limited primarily to small streams, because of the excessively long channel-mixing lengths required for larger streams. Very good accuracy can be obtained provided that
1. Adequate mixing length and time are allowed.
2. Careful field and laboratory techniques are used.
3. Dye losses are not significant.
This manual describes the slug-injection and constant-rate injection methods of performing tracer-dilution measurements. Emphasis is on the use of fluorescent dyes as tracers and the equipment, field methods, and laboratory procedures for performing such measurements. The tracer-velocity method is also briefly discussed.