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FOR462: Watershed Science & Management FOR462: Watershed Science & Management

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Streamflow AnalysisStreamflow Analysis

Module 8.7

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FOR462: Watershed Science & Management

Frequency Analysis – Why?

• Common dialog to describe events• Quantify extreme events• Flood flow return periods

• Engineering design• Watershed planning• Zoning• Geomorphic processes

• Water resources management• What is the probability of a given flow being

available?

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FOR462: Watershed Science & Management

How is Flow Variability Quantified?

2 Methods:1. Recurrence interval / Flow probability

• General technique to describe the probability of an event occurring

• Frequently used for flood peaks• Just like we did for precipitation

2. Flow duration• Indicates the amount of time a given flow is

equaled or exceeded• Describes availability of water resources

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FOR462: Watershed Science & Management

Frequency Analysis

How’s it done?

1. Rank (sort) events

2. Calculate return interval (Tr)

3. Calculate probability (p)

m

nTr

1+=

rTp

1=

year1hr depth

(in)

1972 2.321971 1.161970 1.751969 1.551968 1.611967 1.551966 1.781965 0.861964 0.651963 0.911962 1.531961 1.381960 1.821959 2.061958 1.041957 2.081956 1.071955 1.131954 1.411953 1.02Example: Maximum hourly rainfall per year

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FOR462: Watershed Science & Management

Frequency Analysis

How’s it done?

1. Rank (sort) events

2. Calculate return interval (Tr)

3. Calculate probability (p)

4. Plot event vs. recurrence interval

year1hr depth

(in) RankReturn

Interval (yrs) probability

1972 2.32 1 21.0 0.051957 2.08 2 10.5 0.101959 2.06 3 7.0 0.141960 1.82 4 5.3 0.191966 1.78 5 4.2 0.241970 1.75 6 3.5 0.291968 1.61 7 3.0 0.331969 1.55 8 2.6 0.381967 1.55 9 2.3 0.431962 1.53 10 2.1 0.481954 1.41 11 1.9 0.521961 1.38 12 1.8 0.571971 1.16 13 1.6 0.621955 1.13 14 1.5 0.671956 1.07 15 1.4 0.711958 1.04 16 1.3 0.761953 1.02 17 1.2 0.811963 0.91 18 1.2 0.861965 0.86 19 1.1 0.901964 0.65 20 1.1 0.95

1. 2. 3.

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FOR462: Watershed Science & Management

Flood Frequency Plot

Viessman Fig. 3.9

Source: Viessman & Lewis, Introduction to Hydrology

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FOR462: Watershed Science & Management

Flow Duration Analysis

Variant of Flow Frequency Analysis• Probability presented as:

percent of time a given flow is equaled or exceeded

• Often constructed for a complete series of flows• e.g. daily average streamflow

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FOR462: Watershed Science & Management

Flow Duration Curve

Source: Viessman & Lewis, Introduction to Hydrology

James River at Cartersville, VA

Viessman Fig. 3.14

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FOR462: Watershed Science & Management

Conceptual Example

Dis

char

ge

(cfs

)

Percent of time flow exceededExceedance probability

0 100

Which system has greater flow variability?

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FOR462: Watershed Science & Management

Oct 1 Dec 1 Feb 1 Apr 1 Jun 1 Aug 1 Oct 1 Dec 1 Feb 1 Apr 1 Jun 1 Aug 10

20

40

60

80

100

120

140

160

180

200

Discharge (cfs mi

-2

)

Metolius Siletz

Drainage area:Metolius OR = 316 mi2

Siletz OR = 202 mi2

Oregon Hydrographs

Relation of Flow Variability to Riparian Zones

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FOR462: Watershed Science & Management

Metolius River

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FOR462: Watershed Science & Management

Siletz River

How do disturbance regimes differ?

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FOR462: Watershed Science & Management

Runoff Distribution