Discharge (Q)
Define (cfs; m3/s or “cumecs”)
Why is Q Important?
How is it measured?
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What is the usefulness of Q?
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Calculation of mass loading (L = Q*C)
Stream power determination (Q & slope)
Infer watershed hydrodynamics:
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Shape of storm event hydrograph
Watershed characteristics.
 Predominant water source of streams.
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Interannual hydrograph
Flood frequency and magnitude
 Changing conditions, e.g., land-use impacts.
 Modeling and forecasting water resource needs.
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Hydrographs
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Plot of Q over time;
Peak(s) with a storm event.
Watershed Morphology
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Longitudinal profile (changing slope)
Basin shape and timing of peak flow.
Downstream progression
Crotty Cr. = 1.2 km2
Acheron R. = 619 km2
Goulbon R. = 8601 km2
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Water
Supply
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Groundwater
supply in the
Au Sable R.
Runoff supply
in the Raisin R.
Note spring
rain storms.
More People; more development;
less infiltration = flashy watershed
Q
Urban
stream
Forested
Stream
Time after rain storm →
Why the
change in
baseflow?
Urbanization
Senaca Creek, MD
Greater maximum discharge for storm events of similar
frequency before and after urbanization.
Dam
Impacts
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Lower
Missouri R.
Dammed in
1950s.
Variability
reduced.
Q controlled
for barges.
Changing Interannual conditions:
Climate vs Land-use
Acheron
How is Q measured?
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Direct Volumetric (L/s)
“V”-notch Weir
Dye Dilution
Velocity x Area
Stage to Discharge
Manning’s Equation
“V”=notch Weirs
Calibrate artificial structure.
Brakensiek Equation:
Q = 1.342 H 2.48
Dye Dilution Estimates of Q
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Dye Slug vs Continuous Addition
Concentration of added dye is known.
Concentration is measured in stream over time.
Q = Area x Velocity
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Measure height to cable per interval.
Note height to cable for “wet in” and “wet out”.
Measure velocity per interval.
Calculate Qi ; i.e. discharge per interval.
Q = Qi
Velocity Methods
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Manually timed distance:
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Near-neutrally buoyant floats
Dyes
Current meters:
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Mechanical (propeller or cups)
Electromagnetic (our FlowMate)
Acoustic Doppler
Continuous Monitoring of Q
“The Q Rating Curve”
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Water height (Stage) vs Q relationship.
Stage determined by staff plate.
Stage by float mechanism.
Stage by pressure gauge:
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Incorporated in a water quality Sonde
Requires correction for or venting for
barometric pressure changes.
East Branch Codorus Creek
Graydon Station Q-Rating Curve
(Spring 2003)
800
Stage
700
Q (cfs)
600
500
y = -10.493x 3 + 115.87x 2 - 216.83x + 136.65
R2 = 0.9999
400
300
200
100
0
0.00
1.00
2.00
3.00
4.00
water height (ft)
5.00
6.00
Manning’s Equation
Q = 1/n
2/3
AR
1/2
S
A = cross-section area
 S = longitudinal slope of channel
 n = Manning’s n

(bottom friction constant)
Manning’s n
n = (n0 + n1 + n2 + n3 + n4)m5