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Techniques and Methods 4-A6

The National Streamflow Statistics Program: A Computer Program for Estimating Streamflow Statistics for Ungaged Sites

Chapter 6 of
Book 4, Hydrologic Analysis and Interpretation
Section A, Statistical Analysis

Compiled by Kernell G. Ries III 1
With sections by J.B. Atkins 1 , P.R. Hummel , M. Gray 2, R. Dusenbury 2,
M.E. Jennings 1 , W.H. Kirby 1 , H.C. Riggs1 , V.B. Sauer 1, and W.O. Thomas, Jr.1

1U.S. Geological Survey.
2Aqua Terra Consultants, Inc.


Cover of Sir-2006-5140


The National Streamflow Statistics (NSS) Program is a computer program that should be useful to engineers, hydrologists, and others for planning, management, and design applications. NSS compiles all current U.S. Geological Survey (USGS) regional regression equations for estimating streamflow statistics at ungaged sites in an easy-to-use interface that operates on computers with Microsoft Windows operating systems. NSS expands on the functionality of the USGS National Flood Frequency Program, and replaces it.

The regression equations included in NSS are used to transfer streamflow statistics from gaged to ungaged sites through the use of watershed and climatic characteristics as explanatory or predictor variables. Generally, the equations were developed on a statewide or metropolitan-area basis as part of cooperative study programs. Equations are available for estimating rural and urban flood-frequency statistics, such as the 1 00-year flood, for every state, for Puerto Rico, and for the island of Tutuila, American Samoa. Equations are available for estimating other statistics, such as the mean annual flow, monthly mean flows, flow-duration percentiles, and low-flow frequencies (such as the 7-day, 0-year low flow) for less than half of the states. All equations available for estimating streamflow statistics other than flood-frequency statistics assume rural (non-regulated, non-urbanized) conditions.

The NSS output provides indicators of the accuracy of the estimated streamflow statistics. The indicators may include any combination of the standard error of estimate, the standard error of prediction, the equivalent years of record, or 90 percent prediction intervals, depending on what was provided by the authors of the equations.

The program includes several other features that can be used only for flood-frequency estimation. These include the ability to generate flood-frequency plots, and plots of typical flood hydrographs for selected recurrence intervals, estimates of the probable maximum flood, extrapolation of the 500-year flood when an equation for estimating it is not available, and weighting techniques to improve flood-frequency estimates for gaging stations and ungaged sites on gaged streams.

This report describes the regionalization techniques used to develop the equations in NSS and provides guidance on the applicability and limitations of the techniques. The report also includes a users’ manual and a summary of equations available for estimating basin lagtime, which is needed by the program to generate flood hydrographs. The NSS software and accompanying database, and the documentation for the regression equations included in NSS, are available on the Web at


Introduction By K.G. Ries III, W.O. Thomas, Jr., and M.E. Jennings
Report Format
How to Obtain the NSS Software and Documentation
History and Overview of Methods for Regionalization of Streamflow Statistics By K.G. Ries III, and W.O. Thomas, Jr
Index-Flood Procedures
Ordinary-Least-Squares Regression
Weighted- and Generalized-Least-Squares Regression
Region-Of-Influence Regression
Estimating Techniques for Rural Areas By K.G. Ries III, W.O. Thomas, Jr., and J.B. Atkins
Watershed and Climatic Characteristics
Hydrologic Regions
Measures of Accuracy
Techniques for Watersheds that Span Regional/State Boundaries
Weighting of Independent Estimates of Rural Flood Frequency
Urban Flood-Frequency Estimating Techniques By V.B. Sauer
Nationwide Urban Flood-Frequency Equations
Local Urban Flood-Frequency Equations
Flood Hydrograph Estimation By V.B. Sauer
Estimation of Extreme Floods By W.O. Thomas, Jr., and W.H. Kirby
Measures of Extreme Floods
Extrapolation for the 500-Year Flood
Testing and Validation of Techniques By K.G. Ries III
Applicability and Limitations By J.B. Atkins, and K.G. Ries III
Summary of Estimating Techniques By K.G. Ries III, H.C. Riggs, and W.O. Thomas, Jr
StreamStats By K.G. Ries III
References Cited
Appendix A. National Streamflow Statistics Program Users’ Manual By P.R. Hummel, M. Gray, R. Dusenbury, and K.G. Ries III
Downloading and Installing the Program
Starting the Program
Main Window
Edit Scenario Window
Frequency Window
Hydrograph Window
Weight Window
Appendix B. Summary of Equations for Estimating Basin Lagtime

This report is available online in Portable Document Format (PDF). If you do not have the Adobe Acrobat PDF Reader, it is available for free download from Adobe Systems Incorporated.

View the full report in PDF (512 KB)

Document Accessibility:  Adobe Systems Incorporated has information about PDFs and the visually impaired. This information provides tools to help make PDF files accessible. These tools and the accessible reader may be obtained free from Adobe at Adobe Access.

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