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Article |
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Realtime Streamflow Stations |
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Uses
of Streamflow Data
Data
Collection Process
Discharge
Graph Explanation
Gage
Height Graph Explanation
Using
the National Atlas to Get Streamflow Information |
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The USGS, in cooperation with more than 800 state,
local and other federal agencies, operates approximately 7,000 continuously
active streamflow measurement and data collection sites, called
streamgages. Almost 5,000 of the USGS's approximately 7,000 streamgages
are equipped with telemetry that transmits a reading of stream depth
("stage") to a district office via satellite or telephone.
This "realtime" data is used for a multiplicity of purposes:
including flood hazard mitigation by the National Weather Service,
the U.S. Army Corps of Engineers, and the Federal Emergency Management
Agency; and for resource planning, and infrastructure design of
reservoirs and dams.
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L. Topinka, U.S. Geological Survey |
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The data typically are updated every 4 hours, but
this can vary depending upon the equipment used and the flow status
-- more frequent updates may be made in times of flooding. Real-time
hydrologic data are considered provisional
data. Please be aware of the limitations this imposes.
The National Atlas online interactive Map Maker includes
the current map of the streamgages equipped with telemetry, which
are referred to as realtime streamflow stations. The map usually
is regenerated every business day. A "robot" program examines each
of the web servers that serve realtime data, compiles a list of
realtime stations, and uses the list to regenerate the realtime
map. The Map Maker Identify tool will link to an interactive site
to let you plot a graph of flow versus time, called a hydrograph.
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Uses of Streamflow Data |
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- Enhancing public safety by providing data for forecasting and
managing floods
- Allocating water for municipal, industrial and irrigation uses
- Scheduling power production
- Delineating and managing flood plains
- Advising recreational users of likely conditions to be found
on streams
- Characterizing current water-quality conditions
- Determining input rates of various pollutants into lakes, reservoirs,
or estuaries
- Understanding the biological effects of contamination
- Setting permit requirements for discharge of treated wastewater
- Computing the loads of sediment and chemical constituents
- Operating and designing multipurpose reservoirs
- Designing highway bridges and culverts
- Developing or operating recreation facilities
- Designing, operating, and maintaining navigation facilities
- Setting minimum flow requirements for meeting aquatic life goals
- Monitoring compliance with minimum flow requirements
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Data Collection Process |
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The basic piece of data obtained at a station is the
stage, which is the height of the water surface above a reference
elevation. If the stage of the streambed is known and is subtracted
from the water surface stage, then the result is the depth of water
in the stream. Although stage of a stream is useful in itself in
planning uses of flood plains, most users of streamflow data need
to know the discharge of the stream. Discharge is defined as the
volume of flow passing a specified point in a given interval of
time and includes the volume of the water and any sediment or other
solids that may be dissolved or mixed with the water. The units
of discharge usually are measured in cubic feet per second. Stage
can be measured continuously with automatic sensors, but discharge
can be directly measured only by on-site observers. Using a relation
between stage and discharge, discharge is derived from the stage
data. The stage and discharge relation for a specific stream location
is defined from periodic discharge measurements made at known stages.
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Measuring Discharge
The most practical method of measuring the discharge of a stream
is through the velocity area method. This method requires the physical
measurement of the cross sectional area and the velocity of the flowing
water. Discharge is determined as the product of the area times the
velocity. Velocity is measured by using a current meter. The meter consists
of a propeller that is rotated by the action of flowing water. Given
the number of revolutions in a given time interval, velocity can be
determined for the location of the current meter.
Automatic Data Dissemination
Many streamflow data users must make operational decisions daily.
For these users, streamflow records are computed and made available
on a provisional basis. A reading of stream depth (stage) is transmitted
to a district office via satellite or telephone. Computers in the district
office convert the depth reading to a value for flow and plot a graph
of flow versus time, usually showing the previous 7 days. Some stations
also transmit values for temperature, conductance, or other parameters.
By using telemetry, data are transmitted around the clock by means
of two geostationary operations environmental satellites (GOES)
that are positioned above the Equator over the eastern Pacific
Ocean and Brazil. These data then are retransmitted by means of
a domestic satellite, and the resulting signal is received by the
USGS and other users. The transmission and receipt of the signals
are automated, as are the provisional discharge computations that
are available for meeting current data needs. The data literally
come "from the stream to your screen".
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Discharge Graph Explanation |
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Below is a hydrograph. This a graph showing variation
of stage, discharge, velocity, or other properties of water with
respect to time. The information on the graph is from a device called
a gage which measures water features such as surface elevation.
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1. Discharge - The volume of
water (and anything mixed with the water) passing a specific
point at a given interval of time. Streamflow is the discharge
that occurs in a natural channel, such as in a surface stream.
2. Unique identification number -
Identifies a particular site, in downstream order, on a stream,
canal, lake, or reservoir, where long-term, systematic observation
of hydrologic data are obtained.
3. Dates - The data from 4,200
stations typically are transmitted at intervals of 3 or 4 hours
by an earth-satellite-based communications system. Those data
are referred to as "realtime", and are record over
the given dates on the chart with the most recent stream measurement
time identified with the data.
4. Cubic foot per second - The
rate of streamflow representing a volume of 1 cubic foot passing
a given point during 1 second. A cubic foot per second is equivalent
to 7.48 gallons per second or 448.8 gallons per minute or 0.646
million gallons per day. Abbreviated as ft3/s or CFS. One cubic
foot equals 0.0283 cubic meters.
5. Median Daily Streamflow -
Takes the arithmetic daily mean value (all measurements on
a given day [one per "bump"], then divide by the
number of measurements) and calculates the mean of those values
for the same date over a given period of years. For example,
total all daily mean values for every Jan. 21 records over
the years of record, then divided by the years of record (48
in this case).
6. Years of record - Number
of years data was collected. Variations in continuity or type
of data collected at each site are described under "period
of record" and "gage" below each specific streamflow
site link.
7. Optional data values -
Depending on season or flow conditions, additional data lines
may be shown such as: 90% flow - The low flow that is exceeded
90% of the time for the days in the period of record. Flood
stage - The water level where flooding may begin, as determined
by the National Weather Service.
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Gage Height Graph Explanation |
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1. Gage height - The water-surface
elevation above some arbitrary gage datum is called gage height.
This is often used interchangeably with the more general term "stage",
although gage height is more appropriate when used with a reading
on a gage.
2. Unique identification number -
Identifies a particular site, in downstream order, on a stream,
canal, lake, or reservoir, where long-term, systematic observation
of hydrologic data are obtained.
3. Dates - The data from 4,200
stations typically are transmitted at intervals of 3 or 4 hours
by an earth-satellite-based communications system. Those data
are referred to as "realtime", and are record over the given
dates on the chart with the most recent stream measurement
time identified with the data.
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Using the National Atlas to Get Streamflow Information |
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You can use the National Atlas to check today's flow conditions
for a stream near you. More than 4,000 locations are included
in the Realtime Streamflow Stations map layer. You might use this
service to check the current flow of your favorite recreational
stream or to find up-to-date information on flood conditions.
For more uses see the section - Uses of Streamflow Data. Here
are the steps for identifying realtime streamflow stations.
- Go to the National Atlas Map
Maker.
- In the Map Layers frame on the right, click the symbol
or the word Water to open the Water theme
- Next, turn on the Realtime Streamflow Station map layer by clicking
in the box next to the map layer name
- Use either Zoom to State or the Zoom In function to go to the
location where you want information
- Click the Identify button and center the cursor over a realtime
streamflow station (a red dot) and click
- In a pop-up window, the Identify results are displayed
To get the current conditions at the realtime streamflow station,
click on the hyperlinked station name and ID. This will open a
new page that includes current and historic stage, discharge,
and velocity information and other properties of water with respect
to time.
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