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Discharge (Q)

Discharge (Q) is a crucial metric in hydrology, measuring water flow in cubic feet per second or cubic meters per second. This parameter is important for various reasons, such as calculating mass loading and determining stream power. Understanding how to measure Q through methods like "V"-notch weirs and velocity measurements is essential for accurately assessing watershed dynamics and water resource needs. Dive into the world of hydrographs, watershed morphology, and water supply to grasp the significance of discharge in managing changing hydrological conditions.

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Discharge (Q)

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  1. Discharge (Q) Define (cfs; m3/s or “cumecs”) Why is Q Important? How is it measured?

  2. What is the usefulness of Q? • Calculation of mass loading (L = Q*C) • Stream power determination (Q & slope) • Infer watershed hydrodynamics: • Shape of storm event hydrograph • Watershed characteristics. • Predominant water source of streams. • Interannual hydrograph • Flood frequency and magnitude • Changing conditions, e.g., land-use impacts. • Modeling and forecasting water resource needs.

  3. Hydrographs • Plot of Q over time; • Peak(s) with a storm event.

  4. Watershed Morphology • Longitudinal profile (changing slope) • Basin shape and timing of peak flow.

  5. Downstream progression • Crotty Cr. = 1.2 km2 • Acheron R. = 619 km2 • Goulbon R. = 8601 km2

  6. Water Supply • Groundwater supply in the Au Sable R. • Runoff supply in the Raisin R. • Note spring rain storms.

  7. Why the change in baseflow? More People; more development; less infiltration = flashy watershed Urban stream Forested Stream Q Time after rain storm →

  8. Urbanization Senaca Creek, MD Greater maximum discharge for storm events of similar frequency before and after urbanization.

  9. Dam Impacts • Lower Missouri R. • Dammed in 1950s. • Variability reduced. • Q controlled for barges.

  10. Changing Interannual conditions:Climate vs Land-use Acheron

  11. How is Q measured? • Direct Volumetric (L/s) • “V”-notch Weir • Dye Dilution • Velocity x Area • Stage to Discharge • Manning’s Equation

  12. “V”=notch Weirs Calibrate artificial structure. Brakensiek Equation: Q = 1.342 H2.48

  13. Dye Dilution Estimates of Q • Dye Slug vs Continuous Addition • Concentration of added dye is known. • Concentration is measured in stream over time.

  14. Q = Area x Velocity • 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

  15. Velocity Methods • Manually timed distance: • Near-neutrally buoyant floats • Dyes • Current meters: • Mechanical (propeller or cups) • Electromagnetic (our FlowMate) • Acoustic Doppler

  16. Continuous Monitoring of Q“The Q Rating Curve” • Water height (Stage) vs Q relationship. • Stage determined by staff plate. • Stage by float mechanism. • Stage by pressure gauge: • Incorporated in a water quality Sonde • Requires correction for or venting for barometric pressure changes.

  17. Manning’s Equation Q = 1/n AR2/3 S1/2 • A = cross-section area • S = longitudinal slope of channel • n = Manning’s n (bottom friction constant)

  18. Manning’s nn = (n0 + n1 + n2 + n3 + n4)m5

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