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Review of SWRCB Water Availability Analysis

Review of SWRCB Water Availability Analysis. Emphasis on Dry Creek Water Availability Analysis. Purpose. Review Dry Creek Water Availability Analysis for: Defensibility Accuracy Focus of Review Application of Rational Method Specific review of calculations in Dry Creek Analysis

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Review of SWRCB Water Availability Analysis

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  1. Review ofSWRCBWater Availability Analysis Emphasis on Dry Creek Water Availability Analysis

  2. Purpose • Review Dry Creek Water Availability Analysis for: • Defensibility • Accuracy • Focus of Review • Application of Rational Method • Specific review of calculations in Dry Creek Analysis • Alternative methods

  3. Rational Method • Most Commonly used to estimate peak flow • CALTRANS Highway Design Manual

  4. CALTRANS Highway Design ManualChapter 819.2 Empirical Methods “Rational Methods. Undoubtedly, the most popular and most often misused empirical hydrology method is the Rational Formula: Q = 0.28 CiA Q = Design discharge in cubic meters per second. C = Coefficient of runoff. I = Average rainfall intensity in millimeters per hour for the selected frequency and for a duration equal to the time of concentration”.

  5. Typical Flood Hydrograph • Rational Method is used to estimate peak flow, but is not used to estimate area under curve

  6. Conclusion • Rational method is designed to estimate peak flow rates • The Rational method may not be defensible when used to estimate annual runoff volume

  7. Questions • Is there a way to test the applicability of the Rational Method for determining annual runoff volume for WAA? • Is there another simplified method that can be applied?

  8. Rainfall versus Runoff Runoff = Coefficient * Rainfall Volume Runoff = Coefficient * Rainfall * Area Q = CiA C=Q / iA C = Relationship between rainfall and runoff

  9. Testing The Rational Method Compare relationships between rainfall and runoff (C) in gaged watersheds to relationships developed by SWRCB for WAA

  10. Calculating C for Gaged Watersheds • Use approximately 38 Stream gages and approximately 16 Precipitation gages in North Coast Region

  11. Adjustments for Current Use • WAA is assessed using unimpaired flow not historically gaged flow • Adjustment for current use may result in C determined from gaged basins to more closely resemble SWRCB method

  12. Conclusion • Rational method may be useful for screening level analysis • Must be very cautious • Results can vary significantly • Variation in precipitation can have significant effect on results • Variation in basin characteristics can also cause wide variation in results

  13. Can a Better Simplified Method be Developed?

  14. Criteria for Method Development • Defensibility • Purpose • Screening tool • 5%-10% criteria • Level of Detail • Time step • Accuracy • Level of effort for applying method

  15. Possible Alternative MethodUsing Simplified Approach • Develop equation using method employed in USGS analysis “Magnitude and Frequency of Floods in California” -June 1977

  16. USGS Method • Performed analysis using 705 basins in California in 6 regions • 141 stream flow gages used in North Coast Region • Use gage data and corresponding basin characteristics to develop simplified equation by determining how various factors influence runoff

  17. USGS Approach • Use approach developed by USGS for developing equations for peak runoff, but use this approach to develop equations for annual runoff volume. • Focus on basin characteristics that affect runoff volume rather than peak flow

  18. USGS Method • Consideration of Basin Characteristics • Drainage area • Mean annual precipitation • Precipitation intensity • Mean annual potential ET • Main channel slope • Main length • Altitude index • Surface-storage index • Forest cover

  19. Issues • Actual form of equation will be determined based on regression results • Must include adjustment to unimpair flow • Significant work involved in developing equations • Simplified approach may not provide required level of accuracy and detail • Allow for more detail analysis if applicants believe results are incorrect

  20. Limitations of Simplified Methods • Estimation of seasonal runoff • Estimation of February median • Error in estimation may be too large even for screening analysis • Ignores stream-groundwater interaction

  21. Conclusion • SWRCB WAA appears to be adequate for preliminary screening analysis; however when water availability is close to screening criteria additional analysis may be needed

  22. Conclusion and Suggestions • Develop more robust method • Apply a method that uses seasonal or monthly time step • Develop better estimate of February median • Enhance method for estimating historical use • Use more precipitation gages in estimates

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