National Streamflow Statistics (NSS)

Note: The original release of NSS, The NSS Program, is no longer supported. Documentation can be found at https://water.usgs.gov/water-resources/software/NSS/.

Overview of NSS

The U.S. Geological Survey (USGS) develops and publishes regression equations for estimating streamflow statistics for every State, the Commonwealth of Puerto Rico, and a number of metropolitan areas in the United States. These equations have been compiled into the National Streamflow Statistics (NSS) Application for use by engineers, hydrologists, and others for planning, management, and design applications. NSS is an easy-to-use interface that operates in an online environment.

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 many of the states.

Each equation used in the NSS Application 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. 

Method

NSS is an online application that can be used to:

1. Obtain estimates of various streamflow statistics for sites in rural (non-regulated) ungaged basins.
2. Obtain estimates of flood frequencies for sites in urbanized basins.
3. Obtain indicators of the errors associated with the estimated streamflow statistics, which may include any combination of the standard errors of estimate or prediction, the equivalent years of record, or 90-percent prediction intervals.
4. Obtain estimates for ungaged sites by use of the Region-of-Influence method, which generates a custom regression equation for each ungaged site, in areas where the method has been implemented.
5. Estimate maximum floods based on envelope curves developed by Crippen and Bue (1977).
6. Create hydrographs of estimated floods for sites in rural or urban basins and manipulate the appearance of the graphs.
7. Create flood-frequency curves for sites in rural or urban basins and manipulate the appearance of the curves.
8. Obtain improved flood-frequency estimates for gaging stations by weighting estimates obtained from the systematic flood records for the stations with estimates obtained from regression equations.
9. Obtain improved flood-frequency estimates for ungaged sites by weighting estimates obtained from the regression equations with estimates obtained by applying the flow per unit area for an upstream or downstream gaging station to the drainage area for the ungaged site.
10. Save output from the program in text and graphic files.
11. Obtain documentation and instruction for use of the program from help files.

Note: References to commercial vendors of software products or services is for informational purposes only and does not imply endorsement by the U.S. Government. 

Database

NSS works in concert with a Microsoft Access database. The database contains the information needed to solve the regression equations for each State. To protect the integrity of the database, it is password protected and not intended to be opened directly by users.

As new equations become available, the NSS database will be updated and links to new documentation will be provided from the NSS publications page. Users should check often to determine if the database has been updated with new equations for their areas of interest. 

Data requirement

NSS requires user input of physical and climatic characteristics used as independent variables in the equations. Manual or automated methods for measuring the input parameters are described in documentation provided for each state. 

Output options

Output is provided in the NSS user interface, and includes input parameters, flow estimates, standard errors, and equivalent years of record. This output can be saved to a text file or printed. Hydrographs and frequency plots are available.