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 Alternative methods

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

CALTRANS Highway Design Manual Chapter 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”.

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

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

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?

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

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

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

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

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

Can a Better Simplified Method be Developed?

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

Possible Alternative Method Using Simplified Approach Develop equation using method employed in USGS analysis “Magnitude and Frequency of Floods in California” -June 1977

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

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

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

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

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

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

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