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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2008–5126

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Summary and Conclusions

Techniques for estimating flow-duration and low-flow frequency statistics in unregulated streams throughout Oregon were developed in a cooperative study between the USGS and the Oregon Department of Transportation. Major components of the study included computing flow statistics at 466 active and inactive streamflow-gaging stations, computing climatic and physical basin characteristics at these stations, and developing regression equations to predict flow statistics at ungaged sites based on basin characteristics. The flow statistics included annual and monthly flow-duration quantiles for the 5^th, 10^th, 25^th, 50^th, and 95^th percent exceedances and annual and monthly 7-day, 10-year (7Q10) and 7-day, 2-year (7Q2) low flows. Useful in characterizing a range of high- and low-flow conditions in Oregon streams, these statistics are of critical interest to Federal, State, and local agencies involved in activities such as water-quality regulation, biological habitat assessment, and water-supply planning and management. Low-flow statistics, for example, commonly are used as benchmarks when setting wastewater-treatment plant effluent limits and allowable pollutant loads to meet water-quality standards.

The study area, which included all of Oregon and adjacent areas of neighboring States, was divided into 10 regression modeling regions based on ecological, topographic, geologic, hydrologic, and climatic criteria. A total of 910 regression equations were created to estimate seven annual and monthly flow-duration and low-flow frequency statistics in all 10 regions. Equations to predict the five flow-duration statistics were created using Ordinary Least Squares regression. The standard error of estimate of the equations created to predict the high flow (5th percentile) and low flow (95th percentile) statistics had medians of 42.4 and 64.4 percent, respectively. The adjusted coefficient of determination (R²adj) of the 5th and 95th percentile equations had medians of 0.95 and 0.91, respectively. Equations to predict the low-flow frequency statistics were created using Generalized Least Squares regression. The standard error of prediction of the equations created to predict the 7Q2 and 7Q10 statistics had medians of 51.7 and 61.2 percent, respectively. The adjusted coefficient of determination (R²adj) of the 7Q2 and 7Q10 equations had medians of 0.94 and 0.92, respectively.

The regression equations created in the study are not intended to be used at ungaged sites in which the basin characteristics are outside of the range of those used to create the regression equations. Flow statistics predicted by the equations represent natural unregulated flow conditions in Oregon. If the equations are used at ungaged sites on streams regulated by reservoirs or affected by water-supply and agricultural withdrawals, then the estimate would need to be adjusted if actual flow conditions are of interest.

All 910 regression equations developed for this study are included in the USGS StreamStats Web-based tool (http://water.usgs.gov/osw/streamstats/index.html, accessed August 29, 2007). StreamStats provides users with a set of both annual and monthly flow-duration and low-flow frequency estimates for ungaged sites within Oregon in addition to the basin characteristics for the sites. Prediction intervals at the 90-percent confidence level also are automatically computed. A prediction interval at the 90-percent confidence level means that there is 90-percent assurance that the true value of a flow statistic at an ungaged site will be within a plus or minus interval around the predicted flow statistic.

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