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Lecture 4: Streamflow and Stream Gauging
Key Questions1. What is stream discharge and what controls its magnitude?
2. What is a hydrograph?
3. Where is the velocity highest in a stream?
4. Where is the average velocity measured?
5. What is baseflow?
6. What is the USGS Midsection Method?
7. How is a “Real-Time” Hydrograph produced?
8. What relates stage height to discharge?
Stilling well on the North Fork of the Nooksack river
Streamflow is the heartbeat of a watershed
0
50
100
150
200
250
300D
isch
arge
(cfs
)
Oct 1 Apr 1 Sep 30
Austin Creek Hydrograph
Austin Creek Watershed(8.3 square miles)
Click on this web site and read about streamflow measurement
Q = volume of water passing a plane in a unit of time
Q
watershed
Q = stream discharge
Q = average velocity x cross sectional area
average velocity
cross sectional area
Q = average velocity x cross sectional area = ft3/s
Q is usually measured in cubic feet per second (cfs)
velocity is in feet/second (ft/s)
area is in square feet (ft2)
A hydrograph is a plot of stream discharge as a function of time
stre
am d
isch
arge
(cfs
)
time
Q = stream discharge Time
Hydrograph
QQ (cfs)
Base Flow is sustained streamflow in between rain events
base flow
Austin Creek
In lowland streams, baseflow is primarily sustained by groundwater
In mountain streams, baseflow is primarily sustained by snowmelt, glacier melt, and groundwater
Deming Glacier and the Middle fork of the Nooksack River
Photo by John Scurlock
Q = stream discharge
Q
Time
Hydrograph
Q (cfs) rising flow
rain
Q = stream discharge
Q
Time
Hydrograph
Q (cfs)
receding flow
rain stops
Q = stream discharge Time
Hydrograph
QQ (cfs)
Base Flow is sustained streamflow in between rain events
base flow
Q
How is stream discharge measured?
Q = average velocity x cross sectional area
determine average velocity
Measure cross sectional area
How is stream discharge measured?
highest stream velocity (in the middle)
lowest stream velocity
energy lost due to friction along the stream channel
Stream velocity varies through the stream profile
higher stream velocity (near the top)
velocity contour
Click on the web site below and finish the tutorial and TURN IN the certificate on WED January 19.
Measure stream discharge by the USGS Midsection method:
Measure stream discharge by the USGS Midsection method:
• break up the stream into rectangles
Measure stream discharge by the USGS Midsection method:
• break up the stream into rectangles
• measure the average velocity (vi) and area (Ai) in each rectangle
Measure stream discharge by the USGS Midsection method:
• break up the stream into rectangles
• measure the average velocity (vi) and area (Ai) in each rectangle
• estimate the discharge in each rectangle (qi = Ai x vi)
Measure stream discharge by the USGS Midsection method:
• break up the stream into rectangles
• measure the average velocity (vi) and area (Ai) in each rectangle
• estimate the discharge in each rectangle (qi = Ai x vi)
• sum up the discharges in all the rectangles (Q = ∑ qi)
http://www.ecy.wa.gov/programs/wq/plants/management/joysmanual/5meter.html
String a measuring tape across the width of the stream
Measure the depth with a top-setting wading rod
marked in tenths of a foot
Measure the stream velocity in a rectangle with a current meter
Pygmy meter
Marsh-McBirney Flowmate
Measure the stream velocity with a electronic-flow meter
If the water depth is greater than 2.5 feet, then measure the velocity at two locations: 0.2 and 0.8 of the depth below the water surface. The two velocity values are averaged.
4.3 ft
If the water depth is less than 2.5 feet, then measure the velocity at one location: 0.6 of the depth below the water surface.
1.7 ft
Stream Name: Date TimeWeather: Researchers’ Names: Staff Gauge Water Level (to closest 0.01 ft.)
Distance from Bank
(10ths of a ft) Stream Depth
(10ths ft) V (ft 3/s, or cfs)
At 0.6 of the depthComments
Estimate to the closest 0.01 ft.
Estimate to the closest 0.01 ft.
20 or 40 second average
Right bank, left bank, stone in stream affecting flow, etc.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Calculating the discharge in a rectangle
bi
di
qi = vibi+1 – bi -1
2di
Ai
bi = distance from bank
di = stream depth
Software that applies the Midsection Method
North Fork of the Nooksack River
Real Time Hydrographs are used to monitoring stream flow for flood forecasting
North Fork of the Nooksack River
How are real-time hydrographs generated?
Real-time hydrographs are generated using a rating curve that relates the stream stage (or height) to a discharge.
Stage Height (feet)
Dis
char
ge (c
fs)
Austin Creek Stream Gauge
Streamflow stage height measurement
Stilling Well: float and chart recorder
Austin Creek Stream Gauge
Streamflow Stage Height Measurement
10:15 1.1510:30 1.1610:45 1.17
Time Stage (feet)
1.17 feet
pressure transducer
Austin Creek Stage Height: 2010 Water Year
Oct 1 Apr 1
Sta
ge H
eigh
t (fe
et)
Sep 30
Note: A water year in hydrology goes from Oct 1 to Sep 30
10:15 1.15Time Stage (feet)
Measure the discharge at a specific stage height
Austin Creek Rating Curve
Stage Height (feet)
Dis
char
ge (c
fs)
Each red dot . on the plot represents a measured discharge at a respective stage height.
0
50
100
150
200
250
300
Dis
char
ge (c
fs)
Oct 1 Sep 30Apr 1
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5St
age
Hei
ght (
feet
)Stage
Rating Curve Equation
Discharge
Q = 25.643s2 - 10.68s - 0.1558
Austin Creek Hydrograph: 2010 Water YearD
isch
arge
(cfs
)
Oct 1 Apr1 Sep 30
Note: A water year in hydrology goes from Oct 1 to Sep 30
North Fork of the Nooksack River
Real Time Hydrographs are used to monitoring stream flow for flood forecasting
22.76 feet
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