1. Start Audio Lecture! FOR462: Watershed Science & Management 1 Streamflow Analysis Module 8.7

2. FOR462: Watershed Science & Management 2 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?

3. FOR462: Watershed Science & Management 3 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

4. FOR462: Watershed Science & Management 4 Frequency Analysis How’s it done? 1.Rank (sort) events 2.Calculate return interval (T r ) 3.Calculate probability (p) Example: Maximum hourly rainfall per year

5. FOR462: Watershed Science & Management 5 Frequency Analysis How’s it done? 1.Rank (sort) events 2.Calculate return interval (T r ) 3.Calculate probability (p) 4.Plot event vs. recurrence interval 1.2.3.

6. FOR462: Watershed Science & Management 6 Flood Frequency Plot Viessman Fig. 3.9 Source: Viessman & Lewis, Introduction to Hydrology

7. FOR462: Watershed Science & Management 7 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

8. FOR462: Watershed Science & Management 8 Flow Duration Curve Source: Viessman & Lewis, Introduction to Hydrology James River at Cartersville, VA Viessman Fig. 3.14

9. FOR462: Watershed Science & Management 9 Conceptual Example Discharge (cfs) Percent of time flow exceeded Exceedance probability 0100 Which system has greater flow variability?

10. FOR462: Watershed Science & Management 10 Relation of Flow Variability to Riparian Zones

11. FOR462: Watershed Science & Management 11 Metolius River

12. FOR462: Watershed Science & Management 12 Siletz River How do disturbance regimes differ?

13. FOR462: Watershed Science & Management 13 Runoff Distribution