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Stream gauging is the process of measuring the water discharge or flow at a particular point on a stream or river.  There are many reasons why someone might take stream gauging measurements, such as implementing a long term study for ecological reasons, establishing a baseline to compare future data to, and studying trends for future growth, to name a few.  Stream gauging measurements can be made using portable equipment or permanent monitoring stations.  The stream gauging instruments used by water professionals don't actually measure the flow of the stream, because making a direct measurement of flow is challenging.  Instead they measure water velocity or level as a substitute and use a rating curve to calculate the actual stream gauge measurement.  More details on stream gauging measurements are below.

Why Take Stream Gauging Measurements?
The main reason to gather stream gauging data is to establish a baseline for use with any future data comparisons.  The historical stream gauge data becomes important when the current data begins to trend outside of the historical data.  The comparison of stream gauge data gives a good idea if the local community needs to be concerned about the trend or if it falls within the general data spread.  Additionally the stream gauging data allows the community to forecast potential effects of future water flows on the local watershed or future projects.  The obvious example of using stream gauging data to forecast is by providing potential flooding levels for the land near the river or stream.  In this case the stream gauging measurements can provide engineers with the data they need to keep the local community safe from flooding situations.  The stream gauging data can indicate how strong engineers need to design structures, such as bridges or levees, near the stream or river to withstand the expected flood level flow.

Unfortunately a forecast becoming much more common is using stream gauge data to predict future drought situations.  Using stream gauging data for these types of forecasts water professionals can estimate how well their existing water supply system will hold up during a drought and determine if improvements are needed.  Additionally ecologists can use the stream gauge measurements to determine the stream's effect on wildlife in the local watershed.  This becomes extremely important if the stream gauge data can be connected to animal or plant growth and death patterns.

Permanent or Portable Stream Gauging Stations
Stream gauging can be done using a permanent gauging station or taking consistent spot measurements with a portable instrument.  The primary purpose for taking stream gauging measurements is to develop a historical record of the stream flow at the measurement site.  The minimum requirement for good stream gauging measurements is to take flow measurements at the same measurement site multiple times throughout the year, every year.  In the case where resources are limited or it has been determined that a permanent stream gauging site isn't necessary these measurements can be taken using a portable flow measurement instrument, such as Global Water's Flow Probe.  In this situation an individual or group visits the site periodically throughout the year to take stream gauge measurements under different environmental conditions and records the data to build the historical record based on these spot measurements.

An alternative to taking data with a portable instrument is to set up a permanent stream gauging station.  When resources are available or more continuous monitoring is desired a stream gauging station is an ideal solution.  A permanent stream gauging station generally involves permanently modifying the measuring site to create a confined path for the water flow, a continuous monitoring facility, or both.  Installing a flume or weir is an example of the first type of permanent stream gauging station.  In this case, the stream flow has been modified to constrain the cross sectional area to a known value which greatly assists with the accuracy of the stream gauge measurements.  With a flume or weir installed spot stream gauging measurements become much easier since you no longer have to measure the cross sectional area as part of the flow measurement.

The ultimate example of permanent stream gauging station built for continuous monitoring is the United States Geological Survey, USGS.  The USGS operates over 25,000 stream gauging stations throughout the US with the primary purpose of monitoring stream flows.  These stream gauging stations are equipped with telemetry equipment which allows for real time monitoring of these measurement sites.  The stream gauging data transmitted is directly responsible for allowing the USGS to provide relevant flood warnings to thousands of communities.

A midlevel stream gauging solution presents itself if continuous monitoring is desired in an area where a permanent station can't be set up.  This type of semi-permanent station acts like a permanent stream gauge station during the time it is being used, but can be easily removed once the study is finished.  In these cases a continuous monitoring instrument is temporarily set up to gather stream gauge data.  Once the instrument's stream gauge measurements are recorded they can be added to the historical record.

Determining the Stream Gauge Measurement
Stream Gauging is the process of measuring water flow which has the equation Q=V*A.  Measuring flow, Q, directly is challenging so stream gauging is typically done by measuring both the water velocity, V, and the cross sectional area, A, at the monitoring site.  Measuring water velocity can be done in several ways with the most popular being the use of a water velocity or current meter, such as Global Water's Flow Probe.  These meters provide the water velocity measurement, but require the user to measure the cross sectional area manually to calculate the stream discharge measurement.  Another popular way to determine water velocity is to use Doppler technology.  Doppler systems use sound waves to measure the velocity of particles in the water.  Some Doppler meters can automatically measure the cross sectional area or be programmed with it to provide the required stream gauging data.  In general Doppler instruments are not the best choice for permanent stream gauging at sites with natural flows because they must typically be mounted underwater.

For applications requiring continuous stream gauging data of natural channels neither the current meter nor the Doppler meter are ideal solutions.  These types of stream gauging applications are typically made using water level as a substitute measurement.  To use water level for a stream gauging measurement a rating curve is needed.  A property of any gravity fed open channel is that its stream gauge value is directly related to the height of the water in that channel.  A rating curve is the equation or table that relates water level to the stream gauge measurement.  In the case of flumes or weirs the rating curve is generally well known and the stream gauge measurement is easily determined from the water level.  For natural rivers and streams the main challenge to determining stream gauging data is the cross sectional area of the monitoring site.  One solution is to build a constrained channel at the stream gauging monitoring site which will fix the cross sectional area to a known value.  Another solution is to manually measure the area at the stream gauging site knowing that the area measurement will be a source of error as the stream erodes the site over time.  With either solution a stream gauging rating curve must be developed.

A typical way to develop the stream gauging rating curve is to take spot measurements throughout the year under different water level conditions.  The more data points collected the more accurate the stream gauge rating curve will be.  Once enough data points are available a rating curve equation can be created allowing future stream gauge measurements to be calculated based on water level measurements alone.  An added benefit is that if you have a water level recorder already installed at your stream gauging site once you have your rating curve created all the data that the water level recorder has previously recorded can be used to calculate the stream gauge data for that same time period.

Water level for stream gauge measurements can be measured in many ways.  The simplest way is to mount a staff gauge or ruler at the stream gauge site.  These gauges are permanently mounted in the stream or on the flume or weir.  They provide the ability to easily record the current water level for stream gauge calculations.  For permanent stream gauging stations a stilling well is often installed for water level measurements.  A stilling well is a well that is connected directly to the stream in such a way that water level measurements can be taken without the typical turbulence associated with the stream environment.  An older technology to measure water level for stream gauging is to use a stage encoder or water float.  These instruments are attached to pulleys which convert the float level inside the stilling well into electronic readings for stream gauge conversion.  Submersible pressure sensors, like the WL400 sensor, are a more modern instrument used in stream gauge station stilling wells.  These instruments use a flexible membrane to measure the water pressure and convert it into a water level reading.  The sensor is typically mounted near the bottom of the stilling well and a cable is routed into the stream gauging station's monitoring area where it is connected to either a recording device like the WL16 water level logger or to a telemetry system, such as Global Water's RM100 radio system or SIT65 satellite system.  Submersible sensors are also commonly used with smaller stream gauging stations because of their low power requirements.  The smaller stream gauging stations can also be connected to telemetry systems.  An additional method of measuring water level as a substitute for stream gauge measurements is to use ultrasonic or radar sensors.  These sensors are ideal in stream gauging situations where power isn't an issue and installing a submersible sensor may not be the best solution.
Obtain Rating Equation

Using the Flow Probe depth/flow relationship data obtained for your gauging station site, enter water level (h's) in Column A and flows (Q's) in Column B (h1 opposite Q1, etc).

Highlight all Q’s and h’s.

Click on Graph icon.

Click on “X-Y (Scatter)”.

Click on image with curves and dots.  Then click “Finish”.

Right click on an point on the curve.

Select “Add Trendline”.

Under “Type”, pick “Power”

Under “Options”, check “Display equation”

Click "OK" or “Complete” and your rating equation will be displayed.  The equation is represented as "y" = a function of "x", where "y" is your "Q" data trend and "x" is your "h" data trend.

Apply Rating Equation to Convert Water Level Logger Data

Apply your rating equation to "h" data downloaded from the Water Level Logger data to calculate corresponding “Q” flow.
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