Evaluation of Methods Used for Estimating Selected Streamflow Statistics, and Flood Frequency and Magnitude, for Small Basins in North Coastal California

By Michael P. Mann, Jule Rizzardo, and Richard Satkowski



Scientific Investigations Report 2004-5068

Sacramento, California 2004


Prepared in cooperation with the
California State Water Resources Control Board

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    Accurate streamflow statistics are essential to water resource agencies involved in both science and decision-making. When long-term streamflow data are lacking at a site, estimation techniques are often employed to generate streamflow statistics. However, procedures for accurately estimating streamflow statistics often are lacking. When estimation procedures are developed, they often are not evaluated properly before being applied. Use of unevaluated or underevaluated flow-statistic estimation techniques can result in improper water-resources decision-making. The California State Water Resources Control Board (SWRCB) uses two key techniques, a modified rational equation and drainage basin area-ratio transfer, to estimate streamflow statistics at ungaged locations. These techniques have been implemented to varying degrees, but have not been formally evaluated. For estimating peak flows at the 2-, 5-, 10-, 25-, 50-, and 100-year recurrence intervals, the SWRCB uses the U.S. Geological Survey’s (USGS) regional peak-flow equations. In this study, done cooperatively by the USGS and SWRCB, the SWRCB estimated several flow statistics at 40 USGS streamflow gaging stations in the north coast region of California. The SWRCB estimates were made without reference to USGS flow data. The USGS used the streamflow data provided by the 40 stations to generate flow statistics that could be compared with SWRCB estimates for accuracy. While some SWRCB estimates compared favorably with USGS statistics, results were subject to varying degrees of error over the region. Flow-based estimation techniques generally performed better than rain-based methods, especially for estimation of December 15 to March 31 mean daily flows. The USGS peak-flow equations also performed well, but tended to underestimate peak flows. The USGS equations performed within reported error bounds, but will require updating in the future as peak-flow data sets grow larger. Little correlation was discovered between estimation errors and geographic locations or various basin characteristics. However, for 25-percentile year mean-daily-flow estimates for December 15 to March 31, the greatest estimation errors were at east San Francisco Bay area stations with mean annual precipitation less than or equal to 30 inches, and estimated 2-year/24-hour rainfall intensity less than 3 inches.




Purpose and Scope


Description of Study Area


Selection of Study Stations

SWRCB Flow-Statistic Estimation Methods

SWRCB Rational Equation Method

SWRCB Drainage Basin Area-Ratio Method

USGS Flood-Frequency Equations

USGS Flow-Statistic Determination Methods

Extension of Study-Station Statistics to Long-Term Conditions

Selection of Index Stations

Mean-Daily-Flow Regression Analyses

Adjustment of Flow Statistics via Regression Equations

December 15 to March 31 Mean Daily Flows for 25- and 50-Percentile Years

Flood Frequency Analysis

Low-Outlier Thresholds

Two-Station Comparison

Historical Adjustment

Comparison Between SWRCB Estimated Flow Statistics and USGS Flow Statistics

Annual and Seasonal Runoff and February Median Flow Statistics

SWRCB Annual and Seasonal Runoff and February Median Flow-Statistics Estimates versus USGS Statistics

Annual and Seasonal Runoff and February Median Flow Estimates: Station Percent Differences versus Basin Characteristics

Peak-Flow Statistics

SWRCB Peak-flow Estimates vs. USGS Peak Flows

Peak-flow Estimates: Station Percent Differences versus Basin Characteristics

December 15 to March 31 Mean Daily Flows for 25- and 50-Percentile Years

SWRCB Mean-Daily-Flow Estimates versus USGS Published Mean Daily Flows

Average December 15 to March 31 Mean-Daily Flow Comparisons

December 15 to March 31 Mean-Daily-Flow Estimates for 25- and 50-Percentile Years: Relative Standard Errors versus Basin Characteristics

Summary and Conclusions

Recommendations for Future Studies



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