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 are damaging to any instruments placed in the flow.
   
3. Cross-sectional areas or velocities are indeterminant or changing.
   
4. There are some unsteady flows such as exist with storm-runoff events on small streams.
   
5. The flow is physically inaccessible or unsafe.
   

From a practical standpoint, such measurements are limited primarily to small streams due to excessively long channel mixing lengths required of larger streams. Very good accuracy can be obtained provided:

1. Adequate mixing length and time are allowed.
   
2. Careful field and laboratory techniques are employed.
   
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.