Stream Hydrology

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Stream Hydrology
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Hydrologic Cycle
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Average annual water budget

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Runoff Processes
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Runoff Rates
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Stormflow vs. Baseflow
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Gaining vs. Losing Stream
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Ephemeral vs. Intermittent vs. Perennial
Ephemeral
Intermittent / Perennial
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Stream Discharge
Area Velocity Approach Discharge (cms or cfs)
Velocity x Width x Depth
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Measurement of flowrate 3

• In each subarea, a current meter is used to
  measure di Vi..
• Vi(v0.2v0.8)/2
• where v0.2 velocity at 0.2di below water
  surface in subarea Ai.
• If too shallow,
• Vi v0.6

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Why develop a rating curve?

• Measuring discharge Q is inconvenient sometimes
  its impossible.
• Measuring water surface elevation (stage) is
  relatively easy can be automated.
• It seems obvious that Q should be an increasing
  function of stage.
• If we can discover this function, we can measure
  and record stage and infer Q.

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Recording stream gages 2

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Discharge vs. Basin Area
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Hydrograph Flow Regime
Spring Fed
Desert Ephemeral
Typical Mountain Perennial
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Flow Variability
Drying Frequency Flood Frequency Flood
Predictability
Poff and Ward. 1989
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Flow Duration Curve
Percentage of time a given streamflow was equaled
or exceeded over a given period. Usually based
on daily streamflow
Daily mean flow 1,100 cfs Exceeded 20 of the
time
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Return Period Analysis

• At a given stream gage, the largest flowrate
  observed in a year is the annual maximum
  discharge Qam.
• Qam will vary randomly from year to year.
• The N-year annual series is Qam i , i
  1,2,,N.
• Each Qam i can be considered the outcome of a
  statistically independent experiment.

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Probability Return Period

• If N is very large, the probability that Qam q
  , where q is a particular value, is the number of
  times this happens nq divided by the total number
  of years N.
• p 0.1 means that in a very long annual series,
  the average time between Qam q will be
  T 1/p 10 years, where T is the return
  period.
• This does not mean that Qam q happens every 10
  years or even once in every 10 year period.

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Relating Q to T

• Given an annual series of length N, rank the Qam
  from highest to lowest.
• Assign rank m 1 to the largest, 2 to the next,
  etc.
• Estimate return periods using the Weibull
  plotting position formula.
• The approximation becomes better as m increases
  (for events with shorter T).

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Return Period Analysis
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Return Period Analysis
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Return period analysis 3
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Flow Exceedence Probability
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Low-Flow Frequency
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Bankfull Discharge
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Bankful Discharge from Flow Recurrence Data
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Bankful Discharge from Stage / Discharge Data