# National Streamflow Statistics Program

The National Streamflow Statistics (NSS) Program is a computer program that provides a simple method for applying regional flood-peak streamflow estimates and low flow frequency/duration streamflow estimates.

## Overview

The U.S. Geological Survey (USGS) has developed and published regression equations for every State, the Commonwealth of Puerto Rico, and a number of metropolitan areas in the United States. The NSS should be useful to engineers, hydrologists, and others for planning, management, and design applications. The 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 NFF Program, which it replaces.

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 100-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, 10-year low flow (7Q10)) 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.

Each equation used in the NSS program is the result of considerable effort by USGS Hydrologists to study and document the behavior of streams in their State or region. As such, each equation has limitations that should be understood by the NSS user prior to applying results from NSS. This information can be found in the latest report listed on the NSS publications page. 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.

## Software

The NSS program, an accompanying data base, and documentation can be downloaded from the Web at http://water.usgs.gov/software/nss.html. Documentation needed to solve the equations for individual States is provided through links from the main NSS Web page to reports, or fact sheets.

The NSS Web page and links will be updated as new equations become available. It is recommended that users check these sites periodically to determine whether changes in the software have been made or new equations have been developed for areas of interest that will require obtaining an updated version of the database and new documentation. The version history link on the main NSS Web page will document any changes made to the data base or the software.

## Contact

Problems with the NSS program, suggestions for improving the program, questions about the hydrologic procedures, or requests for additional information can be addressed to Todd Koenig (573-308-3924).

## Documentation

A brief description of NSS is provided in:

Complete documentation of the NSS, including program history, content, theory, application, and a user's manual, is contained in:

- Ries, K.G., III, 2006, The National Streamflow Statistics Program: A computer program for estimating streamflow statistics for ungaged sites: U.S. Geological Survey Techniques and Methods Report TM Book 4, Chapter A6, 45 p.

Note: The USGS publications listed above describe the NSS program up to version 4. For the most up-to-date documentation on the NSS program, please see the Help files associated with the downloaded NSS program.