Stream gauging by salt injection is by csgirla


									                                                                                        Choice of a Measurement
Introduction to Salt                                                                    To apply the technique successfully,
                                                                                        find a stream reach that features
Dilution Gauging for                                                                    complete lateral mixing in a short
                                                                                        distance (Moore 2004b). Longer
Streamflow Measurement                                                                  reaches require a longer injection
                                                                                        duration (and thus volume of salt
Part 2:                                                                                 solution) to achieve steady state.
                                                                                        Selected reaches should have as little
Constant-rate Injection                                                                 pool volume as possible, because the
                                                                                        slow exchange of tracer within the
                                                                                        pool volume will greatly increase the
                                                                                        time to achieve steady state. An ideal
                                                                                        reach begins upstream of a flow
R.D. (Dan) Moore                                                                        constriction (e.g., where the flow
                                                                                        narrows around a boulder, promoting
                                                                                        rapid lateral mixing) and contains no
Introduction                                at steady state (L/L). From Equation        pools or backwater areas below. A
                                            (1), the discharge can be computed
S   tream gauging by salt injection is
    a technique that will work in
many streams in which current-meter
                                                                                        rough guideline is that the mixing
                                                                                        length should be at least 25 stream
                                                                                        widths, but complete mixing may
measurements are unreliable. This            Q=                                   (2)
extension note builds upon a
                                                   RCss                                 require much longer or shorter
                                                                                        distances, depending on stream
previous Streamline article (Moore          Because electrical conductivity (EC) is     morphology. Culverts can be
2004b) and describes field and              linearly related to RC for dilute           convenient measurement locations if
computational procedures for stream         solutions, RCss can be determined           large cobbles are positioned in the
gauging by constant-rate salt               from EC measurements as follows:            flow just below the upstream (input)
injection. The emphasis is on small          RCss = k ( ECss - ECbg)              (3)   end to promote lateral mixing within
streams (less than about 2 m in                                                         the culvert.
wetted width) under low flow                where k is the slope
conditions (discharge less than about       of the relation                                           Methods of
100 L/s). A future article will             between RC and EC,                                        Injection
                                            and ECbg and ECss are       An ideal reach
introduce slug injection, which is                                                                     Several methods are
better suited to gauging higher flows.      the electrical              begins upstream                available for injecting
Users are encouraged to experiment          conductivities of           of a flow                      solution at a constant
with the following procedure to suit        streamwater at
                                            background (i.e.,           constriction and               rate, including siphons
individual circumstances.                                                                              (Johnstone 1988),
                                            prior to injection) and     contains no pools              battery-powered pumps
The Concept                                 at steady state (i.e.,      or backwater                   (Elder et al. 1990), and
A tracer solution injected into a           EC remains constant
                                            in time).
                                                                        areas below.                   Mariotte bottles
stream at a defined rate, q (L/s), will                                                                (Mellina et al. 2002;
become uniformly mixed across the           Combining Equations                                        Story et al. 2003). For
stream at some distance below the           (2) and (3), discharge                                     small streams, a
injection point as a result of turbulent    can be computed as:                         Mariotte bottle constructed from a
flow. After enough time has elapsed, a
                                                           q                            10-L carboy with spigot works well for
steady state (equilibrium) will              Q=                                   (4)   injecting the tracer (Figure 1). The
develop, where the relative                        k ( ECss - ECbg)                     10-L carboy holds an adequate
concentration of the tracer in the                                                      volume of solution appropriate for
                                            To apply Equation (4), we need to
stream is                                                                               small streams, and fits into a large
                                            measure the injection rate of salt
           q  q                                                                         backpack for hiking to remote sites.
RCss =       » ( q << Q) (1)                solution, q, and the background and
                                                                                        Also, it does not require batteries and
          q+Q Q                             steady state values of EC, and to
                                                                                        has no mechanical parts. A
                                            construct a calibration curve of RC
where Q is the stream discharge (L/s)       versus EC to derive k. Table 1 lists the    companion article (Moore 2004a, this
and RCss is the relative concentration      equipment required.                         publication) describes the
                                                                                                               Continued on page 12

Streamline Watershed Management Bulletin Vol. 8/No. 1 Fall 2004                                                               11
           Continued from page 11
           construction and application of a                S/cm) will be, to a close                    necessary. This solution will have a
           Mariotte bottle.                                 approximation, proportional to the           relative concentration, RCsec, given by
                                                            concentration of added salt:                              X
                                                                                                         RC sec =
                                                                                                         RCsec                                   (8)
                                                            DEC = bCs                              (6)              Vo + X
                                                            where b is a proportionality factor,         for X = 10 mL, V0 = 1000 mL, RCsec =
                                                            approximately equal to 2100                  9.90 × 103.
                                                            (mS/cm)/(g/L) at 25°C; the precise
                                                            value depends on background                  2. Measure a volume Vc of
                                                            electrical conductivity (Hongve 1987).       streamwater (typically 1000 mL, but
                                                            For example, a steady-state                  greater volumes allow for greater
                                                            concentration of Cs = 20 mg/L (= 2 ×         dilution) into a “calibration tank,” a
                                                            10–2 g/L) should produce a change in         beaker or pail dedicated for this use.
                                                            EC of about 42 mS/cm. Combining              The calibration tank must be clean,
                                                            Equations (5) and (6), we can                and have never carried a strong salt
                                                            estimate the salt concentration in the       solution because residual salt will bias
                                                            injection solution that would be             the calibration. Even though most
                                                            required to generate a desired change        electrical conductivity probes will
                                                            in EC:                                       adjust EC to a standard value of 25°C,
                                                                                                         the calibration tank should be
                                                            Cis = (Qest / q) DEC / b               (7)   immersed in streamwater (e.g., in a
D. Moore

                                                            The injection solution does not need         shallow pool at the stream’s edge), to
                                                            to be made with local streamwater. If        minimize temperature changes during
           Figure 1. Mariotte bottle injecting into         site access is easy, it is often useful to   the calibration. A “corral” constructed
           culvert.                                                                                      from cobbles helps to hold the
                                                            premix the solution to allow generous
                                                            time for dissolution. The accuracy of        calibration tank in place.
           Mixing the Injection                             the method critically depends on the         3. Measure the initial conductivity,
           Solution                                         complete dissolution of the salt. For        EC0, in the calibration tank. This EC
           We use NaCl (table salt) because it is           strong solutions, decanting is advised       will correspond to RC = 0.
           inexpensive, readily available, and              to minimize the presence of
           environmentally benign for the                   undissolved salt.                            4. Use a pipette to add a known
           concentrations and durations                                                                  amount of the secondary solution to
                                                            When mixing the injection solution in        the calibration tank (e.g., 2 mL).
           normally involved in discharge
                                                            the field, we use pre-weighed bags of        Separate pipettes for the injection and
           measurement. The salt concentration
                                                            salt (typically 10 to 100 g per bag,         secondary solutions must be used to
           in the injection solution should be
                                                            depending on the flows to be                 avoid contamination. Mix thoroughly
           high enough to produce a measurable
                                                            gauged) to allow for flexibility in the      and record the EC and water
           increase in electrical conductivity
                                                            volumes and concentrations. The              temperature.
           while remaining less than the
                                                            concentration of salt need not be
           solubility. It should also be low                                                             5. Repeat step 4 until the mixed EC in
                                                            precisely controlled, since calibration
           enough to avoid raising the salt                                                              the calibration tank exceeds ECss. At
                                                            is expressed as the volumetric
           concentration in the streamwater                                                              each step, compute the relative
                                                            concentration of tracer solution in
           above any thresholds associated with                                                          concentration as
                                                            streamwater, not the mass
           negative ecological impacts (Moore
                                                            concentration of salt in the                          RCsec Sy
                                                                                                                   RC sec
                                                            streamwater.                                 RC =                                    (9)
           If Cs represents the maximum desired                                                                   (Vc + Sy)
           salt concentration in the stream (g/L),          Determining k by                             where Sy = the cumulative amount of
           the corresponding concentration in               Calibration                                  secondary solution added to the
           the injection solution (Cis; g/L) can be         Steps:                                       calibration tank (mL).
           estimated as
                                                 1. Create a secondary solution by                       Use at least three additions of
           Cis = (Qest / q)Cs                         (5)
                                                 mixing X mL (typically 5 or 10 mL) of                   secondary solution, even if ECss is
           where Qest is an estimate of stream   injection solution with a measured                      exceeded on the first or second
           discharge based on the product of     volume V0 of streamwater (typically                     addition, to allow a check on the
           estimated width, depth, and velocity. 1000 mL, but different volumes can                      linearity of the calibration and to
           The change in electrical conductivity be used). Use of streamwater for                        protect against errors that could occur
           from background to steady state (DEC; mixing the calibration solution is                      undetected if only one addition were

           12                                                                      Streamline Watershed Management Bulletin Vol. 8/No. 1 Fall 2004
used (as in the calibration procedure           Table 1. Equipment list
described by Elder et al. 1990).
                                                Item                                 Purpose
Start out using additions of 2 mL of
secondary solution. If the resulting        Siphon, pump, or Mariotte bottle        injecting at a constant rate
change in EC is small relative to the
                                            1-L volumetric flask                    measuring streamwater
difference ECss – ECbg (i.e., it would
take many additions to cover the            1-L plastic graduated cylinder          providing backup in case volumetric flask breaks
range), then the volume of additions        Plastic measuring cup with handle       pouring streamwater into volumetric flask
can be increased to 5 or 10 mL.
                                            Squirt bottle                           topping up streamwater in volumetric flask
6. Using the RC – EC data from steps
3 to 5, fit a straight line to the points   60-mL Nalgene bottle                    taking a sample of injection solution for pipetting
and find its slope, k. Alternatively, for   5- and 10-mL pipettes
                                                                                    measuring injection solution to mix secondary
field computation, the slope can be                                                 solution
estimated as
                                            Pipette filler (rubber squeeze bulb)    drawing water into pipettes
k=                               (10)       2-L wide-mouth plastic water bottle     mixing the secondary solution
   ECf - ECo
                                            2-L plastic beaker or pail              calibrating tank
where RCf is the relative concentration
for the final calibration mixture, and      2-, 5-, and 10-mL pipette               measuring secondary solution
ECf is the corresponding electrical         Plexiglas rod or tubing, 30 cm long     stirring in calibration tank
                                            100-mL graduated cylinder               measuring injection rate
Summary of Field                            Stopwatch                               measuring injection rate
Procedures                                  1
                                              Separate sets of pipettes need to be used for measuring the injection and secondary
Table 1 lists the equipment required.       solutions.
Steps in conducting a measurement           2
                                              Spare pipettes should be carried in case of breakage in the field. Alternatively, 10-mL
are as follows.                             plastic graduated cylinders or graduated pipettes could be carried as a backup.
1. Select the reach to be measured.
                                       7. Begin constant-rate injection, and                    for ECss, as well as to estimate error
2. Record the background EC and           observe EC at the downstream end                      bounds.
   water temperature at the               of the mixing reach. When EC                      8. After you have recorded ECss and
   downstream end of the mixing           appears to level out, measure it                     the water temperature, measure
   reach, as well as upstream of the      across the section to check for                      the injection rate using a
   injection point.                       lateral mixing. If EC varies across                  graduated cylinder and stopwatch.
3. Measure a volume V0 of water           the stream, try moving farther
                                          downstream. If there are large                    9. Retain some of the injection
   using the volumetric flask and pour
                                          pools downstream, and it appears                     solution in a 60-mL Nalgene
   into the secondary solution bottle.
                                          unlikely that complete lateral                       bottle, then turn off the Mariotte
4. Measure a volume Vc of water           mixing will occur, note the highest                  bottle.
   using the volumetric flask and pour    and lowest EC values across the                   10. Determine k using the procedure
   into the calibration tank.             stream. You can use these values                     described previously, then
5. Immerse the calibration tank in a      to estimate a “most likely” value                    compute the discharge by
   shallow pool at the stream’s edge,                                                          Equation (4).
   downstream of the measurement
   point.                                Example
6. If using a Mariotte bottle set up on         The following example is from measurements made on a stream near Takla
   a tripod within the stream to inject         Lake, B.C. The injection solution was mixed using approximately 300 g of NaCl
                                                in 6 L of water. The secondary solution was mixed from 10 mL of injection
   the salt solution, set up the tripod
                                                solution and 1 L of streamwater. The injection rate was q = 1.53 mL/s.
   at the injection point, then
   remeasure EC at the downstream               The calibration data are summarized in Table 2. The volumes V0 and Vc were
   end of the reach to ensure that              both 1000 mL. From a regression of RC against EC, the slope of the RC – EC
   setting up the tripod has not                relation was k = 1.23 × 10 cm/mS.
                                                               q                          1.5310-3L / s
   altered the background
                                                Q=      k ( ECss- ECbg )  =                    ×
                                                                                                                            L s
                                                                                                                       = 8.3L //s
                                                                          (1. 2310-5 cm /mS )( 221mS / cm - 206mS / cm )
                                                                                                                        Continued on page 14

Streamline Watershed Management Bulletin 8/No. 1 Fall 2004                                                                                13
Continued from page 13
                                                                                                 downstream. Consequently,
  Table 2. Calibration data for example
                                                                                                 streamflow gauged by salt injection
 Vol. of secondary   Cumulative vol.   Electrical          Relative           T                  would measure not only flow within
 added (mL)          of secondary      conductivity (EC)   concentration      (°C)               the channel, but also an additional
                     added (Sg) (mL)   (mS/cm)             (RC) (L/L)
                                                                                                 component flowing in the subsurface.
 0                   0                 206                 0.0                14.7               Thus, salt dilution measurements may
                                                                                                 not be comparable to methods that
 5                   5                 210                 4.93 × 10          14.6
                                                                                                 gauge surface flow in the channel
 5                   10                214                 9.80 × 10
                                                                              14.6               such as a current meter or other
                                                                                                 volumetric method.
 5                   15                218                 1.46 × 10          14.6
                                                                                                 Rainfall can influence the
 5                   20                222                 1.94 × 10–4        14.6
                                                                                                 measurements in two ways. First,
 5                   25                226                 2.41 × 10
                                                                              14.6               unless the calibration tank is
                                                                                                 sheltered, rain falling into it may
 5                   30                229                 2.88 × 10          14.5               dilute the concentrations below the
                                                                                                 calculated values, producing biased
Errors and Limitations                   ±1 mS/cm relative to a difference of 15                 calibrations. Second, rain may
Two key requirements for accurate        mS/cm, or about ±7%. This error                         generate stormflow, resulting in
measurements are that (1) the salt in    could be reduced by injecting at a                      altered discharge and background EC.
the injection solution be completely     higher rate or using a more                             Subsequently, EC may not achieve a
dissolved, and (2) the injection         concentrated injection solution to                      clear plateau, resulting in uncertainty.
solution be fully mixed                                 effect a greater EC at                   The hydrologic response, however,
across the channel. In                                  steady state. Equation                   will depend greatly on characteristics
addition, discharge                                     (7) could be used to                     such as soil depths, textures, and
                               Errors may arise         estimate the desired                     moisture content, so generalizations
should not change
appreciably during the         through                  concentration                            cannot be made.
injection trial.               inaccuracies in          (assuming a fixed
                                                        injection rate).
Errors may arise               measuring the
through inaccuracies in        volumes of               We have used                              Solvej Patschke, Anthony Story, Eric
                                                        constant-rate salt                        Mellina, and Matt Gellis helped me
measuring the volumes          streamwater,
                                                        injection to measure                      experiment with variations on the con-
of streamwater,
injection solution, and
                               injection                flows from 1 to 100 L/s.
                                                                                                  stant-rate salt injection method.
                                                                                                  Michael Church, Greg Henderson,
secondary solution.            solution, and            For higher flows, the
                                                                                                  Robin Pike, and an anonymous
These errors can be            secondary                main limitation is the
                                                                                                  reviewer provided constructive com-
effectively minimized if                                ability to inject tracer at
                               solution.                a sufficient rate and
                                                                                                  ments on earlier drafts. However, any
a volumetric flask is                                                                             errors remain the sole responsibility of
used to measure                                         duration to achieve                       the author.
streamwater and if glass                                steady state with a
pipettes are used to measure the         measurable change in EC. Higher                         For further information, contact:
injection and secondary solutions.       flows may be gauged by using a
However, take plastic ware into the      larger carboy, floating siphon, or                       Dan Moore, Ph.D., P.Geo.
field as a backup in case of breakage.   pump equipment, or alternative                           Associate Professor
                                         methods such as slug injection. For                      Departments of Geography and
The accuracy of the calculated
                                                                                                  Forest Resources Management
discharge is directly related to the     example, Johnstone (1988) measured
accuracy of the injection rate. To       flows up to 100 m3/s using a floating                    1984 West Mall
reduce the magnitude of this source      siphon. However, that approach is not                    University of British Columbia
                                         feasible for remote sites accessed by                    Vancouver BC V6T 1Z2
of uncertainty, conduct several
measurements and average them.           foot.                                                    Tel: (604) 822-3538
Another source of uncertainty is in             At low flows, hyporheic exchange
measuring the difference in EC                  may complicate measurements
between background and steady                   through the infiltration of streamwater
                                                                                                 Elder, K., R. Kattelmann, and R. Ferguson.
state. In the previous example, the             into the bed or banks and                            1990. Refinements in dilution gauging
uncertainty in the difference will be           re-emergence some distance                           for mountain streams. In Hydrology in

14                                                                          Streamline Watershed Management Bulletin Vol. 8/No. 1 Fall 2004
                Mountainous Regions. I - Hydrological
                Measurements; the Water Cycle, IAHS
                Publication No. 193, International           Construction of a Mariotte
                Association for Hydrological Science,
                Proceedings of two Lausanne Symposia,
                August 1990, pp. 247–254.
                                                             Bottle for Constant-rate
            Hongve, D. 1987. A revised procedure for
                discharge measurements by means of
                the salt dilution method. Hydrological
                                                             Tracer Injection into Small
                Processes 1:267–270.
            Johnstone, D.E. 1988. Some recent
                developments of constant-injection salt
                dilution gauging in rivers. Journal of
                Hydrology (New Zealand) 27:128–153.          R.D. (Dan) Moore
            Mellina, E., R.D. Moore, S. Hinch, S.
                Macdonald, and G. Pearson. 2002.
                Stream temperature responses to
                clear-cut logging in British Columbia:
                the moderating influences of
                                                             L   iquid tracers are commonly
                                                                 injected into streams to measure
                                                             streamflow, hydraulic characteristics,
                                                                                                       than the spigot level. The tube should
                                                                                                       be inserted such that it remains below
                                                                                                       the surface of the tracer fluid (as
                groundwater and headwater lakes.
                Canadian Journal of Fisheries and            and rates of nutrient uptake (e.g.,       shown in Figure 2) throughout the
                Aquatic Sciences 59:1886–1900.               Webster and Ehrman 1996; Story et         measurement period; otherwise,
            Moore, R.D. 2004a. Construction of a                                                       tracer solution will not discharge at a
                                                             al. 2003). A Mariotte bottle, based on
                Mariotte bottle for constant-rate tracer
                injection into small streams. Streamline     a device developed by the French          constant rate. The tube allows air to
                Watershed Management Bulletin                physicist Edme Mariotte in the 17th       enter as water drains, thereby
                8(1):15–16.                                  century, provides a simple method for     maintaining a constant water pressure
            Moore, R.D. 2004b. Introduction to salt          injecting tracer at a constant rate.      at the spigot, resulting in a constant
                dilution gauging for streamflow
                measurement: Part I. Streamline              This article describes the construction   outflow rate. The lower end of the
                Watershed Management Bulletin                and application of a Mariotte bottle      tubing should be cut on a bevel, to
                7(4):20–23.                                  appropriate for injecting tracer into     facilitate bubbling.
            Story, A.C., R.D. Moore, and J.S.                small streams.
                Macdonald. 2003. Stream                                                                The spigot is fitted with a tubing
                temperatures in two shaded reaches
                below cut blocks and logging roads:
                                                             Construction                              connector and pipette tip with the
                downstream cooling linked to                                                           end cut off. The non-tapered end of
                                                             A simple Mariotte bottle can be
                subsurface hydrology. Canadian Journal                                                 the tubing connector fits snugly into
                of Forest Research 33:1383–1396.
                                                             constructed from a carboy fitted with
                                                                                                       the spout of the carboy, and the
                                                             a spigot (Figures 1 and 2; Table 1). A
                                                                                                       pipette tip is slid over the tapered end
                                                                            10-L carboy holds a
                                                                                                       of the connector. This set-up “steps
                                                                            sufficient volume of
                                                                                                       down” the outflow rate, and allows
                                                                            tracer for gauging small
                                                                                                       the spigot to be opened fully for
                                                                            streams at low flow,
                                                                                                       delivery of tracer while controlling the
                                                                            and fits into a large
                                                                            backpack for transport     injection rate. Several pipette tips with
                                                                            to remote field sites.     a range of hole diameters allow for a
                                                                            We have measured           range of injection rates. It is important
                                                                            flows as low as 1 L/s      that the outflow be a continuous
                                                                            and as high as 100 L/s     stream rather than discrete drips. If
                                                                            using a Mariotte bottle    the water drips out, air will enter and
                                                                            (e.g., Mellina et al.      create an inconsistent outflow rate.
                                                                            2002; Story et al.
                                                                                                       The Mariotte bottle can be set up on
                                                                            To construct the
                                                                                                       a square of plywood fixed to the top
                                                                            Mariotte bottle, the
                                                                                                       of a tripod, to provide a stable base
                                                                            screw-on cap is
                                                                                                       for the bottle. If the stream is narrow,
                                                                            replaced by a size 13½
                                                                                                       it may be possible to have the tripod
D. Moore

                                                                            one-hole rubber
                                                                                                       legs span the stream.
                                                                            stopper with a length
           Figure 1. Materials required to construct Mariotte bottle.       of Plexiglas tube          After setting up the Mariotte bottle,
           From left to right: 10-L carboy, pipette tip, Quick Disconnect   inserted to a level        open the spigot to begin injection.
           Connector, rubber stopper with Plexiglas tube inserted.          about 10 cm higher         When the spigot is first opened, the
                                                                                                                               Continued on page 16

           Streamline Watershed Management Bulletin Vol. 8/No. 1 Fall 2004                                                                    15

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