dilution gauging can be precise to
Introduction to Salt within 5% (Day 1977; Johnstone
1988), equivalent to the accuracy of
Dilution Gauging for current metering at a suitable
Streamflow Measurement: There are two variations on dilution
gauging, depending on whether the
Part 1 tracer is injected into the stream at a
constant rate or as a
near-instantaneous “slug.” In both
cases, the tracer is injected at some
point along the stream, and the tracer
R.D. (Dan) Moore concentration in streamwater is
measured at a downstream point,
where the tracer has become
Editor’s note: This is the first in a series of In addition, flow depths and velocities
planned articles that will explore the uniformly mixed with the
in small streams during low-flow
topic of salt dilution gauging for streamwater. For a given volume or
conditions may be too small for
streamflow. The other articles will discuss rate of injection, greater stream
reliable measurement. An alternative
constant rate injection, slug injection discharges will result in greater tracer
method of stream gauging involves
(solution), and dry salt injection. dilution and lower concentrations
injecting a chemical tracer and
measured at the downstream site.
Introduction determining its dilution following
Equations based on the mass balance
complete mixing into the flow.
D ischarge is an important variable
that governs many aspects of
stream function, such as habitat
Dilution gauging can be effective
where current metering would not be
principle are applied to compute the
accurate, and vice versa, so the This note introduces the general
diversity and rates of nutrient export.
techniques are complementary. An principles and applicability of
It is also a direct measure of the
additional advantage is that dilution streamflow measurement by salt
amount of water available to meet
can be measured without wading dilution. Articles in future issues of
instream and extractive water uses.
across the stream, which can be Streamline will give more detailed
The most common approach to
hazardous in steep streams at higher information on dilution gauging
measuring discharge is the
flows. Under good conditions, salt procedures.
velocity-area method, which involves
measuring water depth and velocity
at points across a stream section with
a current meter. Hydrometric
agencies including Water Survey of
Canada and U.S. Geological Survey
favour this method, which is
endorsed by the provincial Resource
Inventory Standards Committee.
Measuring discharge using a current
meter can be difficult, if not
impossible, in small streams, especially
steep streams with a step-pool or
(Figure 1). These channel
morphologies are common in small
and intermediate-sized catchments
(drainage areas up to about 100 km2),
especially in mountainous areas. In
these streams, the irregular, bouldery
cross-section and strong turbulence
Figure 1. A steep stream unsuitable for gauging with a current meter. Because of the
decrease the accuracy with which significant approach velocity, a standard weir equation may not be accurate, and the weir
depth and velocity can be measured. should be rated using discharge measurements.
20 Streamline Watershed Management Bulletin Vol.7/No.4 Winter 2003/04
160 electrical conductivity of the solution.
This relation can be expressed as
follows for application to dilution
C = k(EC – ECbg)
EC (m S/cm)
where C is the concentration of the
80 salt in stream water, EC is the
electrical conductivity of the
60 streamwater-injection solution
mixture, ECbg is the background or
40 natural electrical conductivity of the
streamwater, and k is a proportionality
constant, to be determined by
0 calibration. In general, the relation
0 5 10 15 20 25 30 depends on the natural streamwater
chemistry (i.e., background
T ( C) conductivity) (e.g., Hongve 1987). For
UNC NLF LF the greatest accuracy, therefore,
calibrations should be conducted for
Figure 2. Dependence of electrical conductivity on temperature for a moderate strength each measurement.
solution. UNC = uncorrected; NLF = corrected to 25°C using a non-linear function;
LF = corrected to 25°C using a linear correction (2%/°C). Measurements were made with a The electrical conductivity of a
WTW LF-340 probe and meter. solution varies with temperature.
Therefore, it is important to use an EC
probe that is capable of adjusting
Advantages of Salt as a in Table 1. This value is also measured EC to a standard
Tracer substantially lower than the U.S. water temperature, typically 25°C. These
quality standard for chloride adjustments can be based on a linear
A range of tracers has been used for
concentration for protection of correction, typically about 2%/ºC, or
dilution gauging. Fluorometric dyes
aquatic life (230 mg Cl/L, equivalent a non-linear function. Both types of
such as Rhodamine WT can be
to a concentration of 373 mg NaCl/L) adjustments appear satisfactory if
measured at very low concentrations,
and the recommended ambient water temperatures vary by up to a few
and thus can be used for higher flows
quality guidelines specified by the degrees Celsius (Figure 2), but the
and greater dilutions than tracers such
B.C. Ministry of Water, Land and Air author has found that the non-linear
as salts. However, dyes require
Protection (Table 2). In addition, the function performs slightly better,
specialized, expensive equipment for
high concentrations usually persist for especially for more dilute solutions.
measuring their concentrations, and
a few minutes at most, compared
are more expensive and not as readily Constant-Rate
with the 48-hour exposures typically
available as salts. versus Slug Injection
used in toxicity trials. While high
Common table salt (sodium chloride, concentrations may occur at the point Constant-rate injection allows for
NaCl) is popular for dilution gauging of injection, these will decrease rapidly greater accuracy, especially for low
for three reasons. First, table salt is as the tracer disperses into the flows, but requires a pump or other
inexpensive and readily available, downstream flow. device for injecting the tracer. The
even in rural areas. Second, it can be author and his co-workers have
accurately measured in the field using Electrical Conductivity successfully measured flows as low as
an electrical conductivity meter. Third, as a Measure of 1 L/s and up to 100 L/s using a 10-L
it is non-toxic for the concentrations Salt Concentration Mariotte bottle for injection, but
and exposure times typically Electrical conductivity is a measure of lower and higher flows could be
associated with discharge the ease with which an electrical measured at suitable sites with
measurements. For example, in the current can travel through water. For appropriate equipment. Johnstone
author’s experience, peak low solute concentrations, the (1988) measured flows up to 100
concentrations of NaCl at the lower conductivity will vary linearly with the m3/s in a New Zealand stream using a
end of the mixing reach are usually salt concentration; inversely, there siphon-based system to inject tracer at
well under 100 mg/L, far lower than should be a linear relation between about 1 L/s. However, use of
any of the toxicity thresholds shown the concentration of salt and the constant-rate injection will not
Continued on page 22
Streamline Watershed Management Bulletin Vol.7/No.4 Winter 2003/04 21
Continued from page 21
normally be feasible for such high
Table 1. Aquatic toxicity of table salt (NaCl)
flows, especially for remote sites, due
Species Parameter Threshold to the large volumes of tracer solution
concentration (mg/L) and the bulky equipment required to
maintain a high rate of injection. Slug
Rana breviceps (frog) NOEC 400
injection, on the other hand, can be
Daphnia pulex 48-h LC50 or EC50 1 470 readily used for higher flows at
remote sites, but requires that the
Daphnia magna 48-h EC50 3 310 tracer concentrations at the
(water flea) downstream monitoring point be
Myriophyllum spicatum Phytotoxicity 5 962 recorded through time, either
(water milfoil) (EC50 for growth) manually or by using an electronic
data logger. Table 3 summarizes the
Pimephales promealas 69-h LC50 7 650 relative advantages of the two
(fathead minnow) approaches.
Lepomis macrochirus LC50 or EC50 7 846 The traditional approach to salt
(bluegill) dilution gauging by slug injection is to
use a salt solution
Anguilla rostrata 48-h LC50 or EC 50 13 085 as the tracer.
(American eel) However, for flows Salt dilution
Source: U.S. EPA, Ambient Water Quality Criteria for Chloride, 1988, cited by the Salt Institute: greater than about techniques can
http://www.saltinstitute.org 15 m3/s, the
Notes: often be used to
volumes of tracer
NOEC = no observed effect concentration.
EC50 = concentration at which 50% of individuals show a toxic effect.
LC50 = concentration at which 50% of individuals perish. may become streamflow where
unworkable (Kite conventional
Table 2. Recommended ambient water quality guidelines for chloride 1993). To current metering
Water use Guideline overcome this may be
(mg Cl/L) problem, a number
of workers have
Drinking water 250 used dry salt as a
Freshwater aquatic life (maximum concentration) 600 tracer (Elder et al.
1990; Hudson and Fraser 2002).
Freshwater aquatic life (30-day average concentration) 150 While injection of dry salt can allow
Livestock watering 600 higher flows to be gauged, one
potential disadvantage is the need for
a longer mixing length to ensure
Source: http://wlapwww.gov.bc.ca/wat/wq/BCguidelines/chloride.html complete dissolution of salt within the
Table 3. Relative advantages of constant-rate versus slug injection
Injection method Advantages Disadvantages
can easily verify complete mixing !
requires equipment for constant-rate
only requires measurement of background
and steady-state EC values !
maximum flow that can be gauged is
limited by the rate of tracer injection;
at steady-state, losses to transient storage
therefore is best suited to lower range of
zones such as pools and the streambed do
not affect discharge measurement
special equipment for injection not !
requires recording of EC variations
cannot verify directly whether complete
can be used for higher discharges than lateral mixing has occurred unless two
constant-rate injection probes are available
22 Streamline Watershed Management Bulletin Vol.7/No.4 Winter 2003/04
Choice of made using different reach lengths, or References and Useful Sources:
Measurement Reach by measuring EC on each side of the Church, M. and R. Kellerhals. 1970. Stream
stream. If mixing is complete, gauging techniques for remote areas
The success of both constant-rate and consistent discharge values should be using portable equipment. Department
slug injection methods requires a determined. of Energy, Mines and Resources, Inland
reach that provides complete lateral Waters Branch, Ottawa, Ont. Technical
mixing in as short a distance as Streams with significant in-stream Bulletin No. 25.
vegetation are not suitable candidates Day, T.J. 1977. Field procedures and
possible. The reach should also have evaluation of a slug dilution gauging
minimal pool volume. An ideal for salt-dilution gauging. The method in mountain streams. Journal of
situation occurs where the tracer is vegetation will suppress mixing, and Hydrology (New Zealand) 16:113–133.
injected just upstream of a flow may adsorb or absorb salt (L. Tolland, Elder, K., R. Kattelmann, and R. Ferguson.
pers. comm., 2003; J. Fraser, pers. 1990. Refinements in dilution gauging
constriction (e.g., where the flow for mountain streams. In Hydrology in
narrows around a boulder) and the comm., 2003).
mountainous regions. I - Hydrological
reach below contains no pools or measurements; the water cycle, IAHS
backwater areas. Summary Publication No. 193, International
Association for Hydrological Science,
A rough guideline for the required Salt dilution techniques can often be Proceedings of two Lausanne Symposia,
length of the mixing reach is 25 times used to measure streamflow where August 1990, pp. 247–254.
the stream width (Day 1977). conventional current metering may be Herschy, R.W. 1995. Streamflow
inaccurate. Salt is a useful tracer measurement. E & FN Spon, London.
However, the actual mixing length Hongve, D. 1987. A revised procedure for
can vary significantly from this value because it is (1) inexpensive and
discharge measurements by means of
depending on channel morphology, readily available, (2) easily measured the salt dilution method. Hydrological
and should be verified for each site in the field as electrical conductivity, Processes 1:267–270.
and (3) non-toxic at the Hudson, R. and J. Fraser. 2002. Alternative
and flow condition. For constant-rate methods of flow rating in small coastal
injection, lateral mixing can be concentrations that occur during
streams. B.C. Ministry of Forests,
checked once “steady state” dilution gauging. Future articles in Vancouver Forest Region, Nanaimo,
conditions have been achieved (i.e., Streamline will provide more detail on B.C. Extension Note EN-014. 11 p.
EC has reached a “plateau” and has field procedures and computations. Johnstone, D.E. 1988. Some recent
developments of constant-injection salt
stopped increasing) by measuring EC For further information contact: dilution gauging in rivers. Journal of
at different points across the stream Hydrology (New Zealand) 27:128–153.
cross-section. For slug injection, Kite, G. 1993. Computerized streamflow
Dan Moore, Ph.D., P.Geo. measurement using slug injection.
complete mixing can be verified by Associate Professor Hydrological Processes 7:227–233.
using two probes, located either on Departments of Geography and Forest Resource Inventory Standards Committee
each side of the stream, or at two Resources Management (RISC). 1998. Manual of standard
different distances downstream of the 1984 West Mall operating procedures for hydrometric
injection point. If only one probe is surveys in B.C. Version 2.1. Available
University of British Columbia from:
available, consecutive measurements Vancouver, BC V6T 1Z2 srmwww.gov.bc.ca/risc/pubs/aquatic/h
during a steady discharge can be E-mail: email@example.com ydro/index.htm.
Workshops, Events, and News
May 1–5, 2004 http://faculty.forestry.ubc.ca/richardson/ Knowledge for Better Adaptation. CWRA.
BC Water & Waste Association 2004 AGM. NABS2004.htm Montreal, PQ
Whistler, BC June 11–15, 2004 http://www.ouranos.ca/acrh/
http://www.bcwwa.org/agm2004/index.php BCLSS Annual Conference. June 22–25, 2004
May 2–6, 2004 108 Mile Resort, 108 Mile House, BC International Instrumented Watershed
Fourth World Fisheries Congress. http://www.nalms.org/bclss/ Symposium 2004. University of Alberta,
Vancouver, BC Edmonton, AB
June 13–16, 2004
2004 International Mountain Logging
May 12–14, 2004 IIWS.htm
Conference - Forest Operations under
BC Land Summit 2004 Conference. Mountainous Conditions. June 27–30, 2004
University of British Columbia, Vancouver, BC
Vancouver, BC International Conference on Sediment and
http://www.feric.ca/en/wd/home/events/ Geochemical Budgets in Geomorphology to
mountainlogging/MountainLogging.htm Honour Professor Olav Slaymaker. University
June 6–10, 2004 of British Columbia, Vancouver, BC
June 16–18, 2004
North American Benthological Society, http://www.geog.ubc.ca/department/
52nd Annual Meeting. University of British 57th Annual Water Resources Conference:
Water and Climate Change, activities/
Columbia, Vancouver, BC
Continued on page 24
Streamline Watershed Management Bulletin Vol.7/No.4 Winter 2003/04 23