PCWA-L 467.pdf - PCWA Middle Fork American River Project ...

7. DISCHARGE-TRACER-DILUTION METHOD 251
into a stream, the discharge can be computed by use of the ratios of
the conductances ofthe injected and sampled salt solutions, instead of
the ratios of the salt concentrations themselves. The procedure followed
in the salt-d.ilution method is analogous to that used in the
dye-dilution method in that relative conductance values are used
rather than absolute values.
The factors that enter into the selection ofthe reach ofchannel for a
salt-dilution measurement of discharge are identical with those discussed
earlier on pages 215--223. Although loss of salt in the measurement
reach by sorption is a negligible concern, reaches that have
areas of slack water should be avoided, if possible, because the storage
and slow release of tracer from those areas greatly prolongs the
time required for the entire salt cloud to pass the sampling site. The
sampling site should be free ofexcessive turbulence because the operation
of the electrode cell that is placed in the stream to measure
relative conductance is adversely affected by the presence of air bubbles.
The salt-dilution method of measuring discharge in open channels
is Iittled used in the UB.A. but is popular elsewhere, as in the
U.S.S.R. The description of the salt-dilution technique that follows,
and the sample computation that concludes the discussion, have been
taken from a description of salt-dilution technique that follows, and
the sample computation that concludes the discussion, have been
taken from a description of saltwdilution stream gaging in the
U.S.S.R. (World Meteorological Organization, 1962, p, 47-49). The
subject is discussed under the following three headings:
1. Preparation and injection of the concentrated salt solution.
2. Measurement of relative conductance at the sampling site.
3. Computation of discharge.
PREPARATION AND INJECTION OF THE CONCENTRATED SALT SOLUTION
The following equipment is needed for the preparation and injection
of the concentrated salt solution at the injection site:
L Two tanks, each having a capacity of60-70 L; one tank is used for
preparation of the solution and the other for injection of the
solution.
2, A measuring vessel of lO-L capacity. having a mark showing the
exact level for 10 L of liquid.
3. A wooden or aluminum paddle for mixing the solution.
4. A vessel having a capacity of0.3-0.5 L for adding small volumes of
solution to the 10-L measuring tank.
5. A glass flask with a ground glass stopper, having a capacity of
20-30 mL, in which a sample of the solution is retained.
The quantity of salt (NaC!) required for injection depends on the