Lecture 4: Streamflow and Stream Gauging
Key Questions 1. What is stream discharge and what controls its magnitude? 2. What is a hydrograph? 3. Where is the velocity highest in a stream? Stilling well on the North Fork 4. Where is the average velocity measured? of the Nooksack river 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? Streamflow is the heartbeat of a watershed
Austin Creek Watershed (8.3 square miles)
Austin Creek Hydrograph 300
250
200
150
Discharge (cfs) Discharge 100
50
0 Oct 1 Apr 1 Sep 30 Click on this web site and read about streamflow measurement watershed
Q = stream discharge
Q
Q = volume of water passing a plane in a unit of time cross sectional area
average velocity
Q = average velocity x cross sectional area Q is usually measured in cubic feet per second (cfs)
velocity is in feet/second (ft/s) area is in square feet (ft2)
Q = average velocity x cross sectional area = ft3/s A hydrograph is a plot of stream discharge as a function of time stream discharge (cfs)
time Base Flow is sustained streamflow in between rain events
Hydrograph
Q
Q (cfs) base flow
Q = stream discharge Time In lowland streams, baseflow is primarily sustained by groundwater
Austin Creek Deming Glacier and the Middle fork of the Nooksack River
In mountain streams, baseflow is primarily sustained by snowmelt, glacier melt, and groundwater
Photo by John Scurlock rain
Hydrograph
Q
Q (cfs) rising flow
Q = stream discharge Time rain stops
Hydrograph
Q receding flow
Q (cfs)
Q = stream discharge Time Base Flow is sustained streamflow in between rain events
Hydrograph
Q
Q (cfs) base flow
Q = stream discharge Time How is stream discharge measured?
Q How is stream discharge measured?
Measure cross sectional area
determine average velocity
Q = average velocity x cross sectional area Stream velocity varies through the stream profile
higher stream velocity (near the top) highest stream velocity (in the middle)
velocity contour lowest stream velocity
energy lost due to friction along the stream channel 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
x • estimate the discharge in each rectangle (qi = Ai 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
x • estimate the discharge in each rectangle (qi = Ai vi)
• sum up the discharges in all the rectangles (Q = ∑ qi) String a measuring tape across the width of the stream
http://www.ecy.wa.gov/programs/wq/plants/management/joysmanual/5meter.html 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 Measure the stream velocity with a electronic-flow meter
Marsh-McBirney Flowmate 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 Time Weather: Researchers’ Names: Staff Gauge Water Level (to closest 0.01 ft.)
Distance from Bank Stream Depth V (ft 3/s, or cfs) Comments (10ths of a ft) (10ths ft) At 0.6 of the depth Estimate to the closest Estimate to the closest 20 or 40 second Right bank, left bank, stone in stream 0.01 ft. 0.01 ft. average 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
bi = distance from bank bi+1 –bi -1 qi = vi di di = stream depth 2
Ai
Software that applies the Midsection Method
North Fork of the Nooksack River Real Time Hydrographs are used to monitoring stream flow for flood forecasting How are real-time hydrographs generated?
North Fork of the Nooksack River Real-time hydrographs are generated using a rating curve that relates the stream stage (or height) to a discharge. Discharge (cfs)
Stage Height (feet) Streamflow stage height measurement
Austin Creek Stream Gauge Stilling Well: float and chart recorder Streamflow Stage Height Measurement
Austin Creek Stream Gauge Time Stage (feet) 10:15 1.15 10:30 1.16 10:45 1.17
1.17 feet pressure transducer Austin Creek Stage Height: 2010 Water Year Stage Height (feet)
Oct 1 Apr 1 Sep 30
Note: A water year in hydrology goes from Oct 1 to Sep 30 Time Stage (feet) 10:15 1.15
Measure the discharge at a specific stage height Austin Creek Rating Curve Discharge (cfs)
Stage Height (feet)
Each red dot . on the plot represents a measured discharge at a respective stage height. Stage 4.5
4.0
3.5
3.0
2.5
2.0
1.5 Stage Height (feet) Height Stage
1.0
0.5
0.0
Rating Curve Equation Q = 25.643s2 - 10.68s - 0.1558
Discharge
300
250
200
150 Discharge (cfs) 100
50
0 Oct 1 Apr 1 Sep 30 Austin Creek Hydrograph: 2010 Water Year Discharge (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