Introduction to Salt Dilution Gauging for Streamflow Measurement

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					                                                                                             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
                                                                                             stream discharge.
                                                   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
boulder-cascade morphology
(Figure 1). These channel
morphologies are common in small
and intermediate-sized catchments
(drainage areas up to about 100 km2),
                                                                                                                                         D. Whyte

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
              120                                                                              gauging:
                                                                                                       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                                                                        15 m3/s, the
Notes:                                                                                                                       often be used to
                                                                                                    volumes of tracer
NOEC = no observed effect concentration.
                                                                                                    solution required
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
 Wildlife                                                                        600
                                                                                                    a longer mixing length to ensure
Source:                                  complete dissolution of salt within the
Table 3. Relative advantages of constant-rate versus slug injection

 Injection method                               Advantages                                                Disadvantages
 Constant-rate              !
                                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

 Slug                       !
                                special equipment for injection not                      !
                                                                                             requires recording of EC variations
                                required                                                 !
                                                                                             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                   
during a steady discharge can be               E-mail:                       ydro/index.htm.

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                                                                                                                     Continued on page 24

Streamline Watershed Management Bulletin Vol.7/No.4 Winter 2003/04                                                                   23

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