Dilution gaging involves adding a non-reactive, dissolved chemical tracer to a flowing stream, and monitoring how this tracer becomes diluted after it has completely mixed throughout the water as it travels downstream. The greater the flow, the greater will be the level of dilution of the tracer.

The technique may be used in streams that are difficult to gage with current meters. Such streams include (a) those which are too shallow and/or slow, and (b) those with an uneven, rocky bottom and/or an irregular wavy surface, as is common for high-gradient mountain streams.

A convenient tracer is common salt, which is cheap, easily available, and readily measured in the stream with an electrical conductivity meter. At the concentrations and measurement durations typically used for dilution gaging, salt is also not harmful to the stream ecosystem.

Other tracers include fluorescent dyes, such as rhodamine, which can be measured at very low concentrations and are thus useful for high stream flows. Detecting the concentrations of such dyes quantitatively, however, requires more-expensive instrumentation (field or laboratory fluorometer).

Dilution gaging can be accomplished by one of two basic approaches:

1. injecting the tracer at a constant rate, and measuring downstream dilution once the concentration there has reached a plateau;
2. injecting the tracer as an instantaneous "slug," and monitoring the passage of the rising-then-falling "tracer wave" downstream.

The constant-rate method generally gives somewhat more-precise results, especially for low flows, but (a) requires additional equipment to control tracer injection, and (b) is impractical for high-volume flows.

The details of of dilution gaging are presented at length in the U.S. Geological Survey manual on the Measurement and Computation of Streamflow (Water Supply Paper 2175).

R.D. (Dan) Moore at the University of British Columbia has written very useful summaries of the salt-dilution approach as shorter articles in the Streamline Watershed Management Bulletin, including introductions to

• the general method (Part I)
• the constant-rate method (Part 2)
• the slug-injection method (Part 3)

Readers are advised to consult these sources for details, but an overview is provided below.

Constant Rate of Injection Method

The diagram to the right illustrates how stream discharge Q is calculated via the constant-rate-of-injection method. A solution with tracer concentration C_is is injected at a rate q into a stream flowing at discharge Q with background tracer concentration C_bg. After mixing downstream, the tracer reaches a plateau of constant diluted concentration C_s.

By conservation of mass, the discharge-concentration products of the stream flow (QC_bg) and the injection solution (qC_is) must add to be the same as the product of the cumulative discharge and concentration downstream ([Q+q]C_s).

The equation may be rearranged to solve for stream discharge Q:

Instantaneous Slug-Injection Method

The basis of the slug-injection method is also conservation of mass, specifically accounting for all of the injected tracer as it passes the downstream sampling point.

After being dumped into the stream, the tracer slug will move downstream via advection with the flowing water, but will also undergo longitudinal dispersion, or downstream stretching, because some parts of the streamflow are faster than others.

Rain that falls onto the ground during a storm can take one of three basic paths:

1. it can be intercepted by plants or by hollows on the ground surface, and ultimately returned to the atmosphere by evaporation;
2. it can infiltrate, or soak downward into the ground; or
3. it can run off the surface toward the nearest surface water body.