U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2007-3010

February 2007

The National Streamflow Statistics Program: Estimating 

High and Low Streamflow Statistics for Ungaged Sites

Printed on recycled paper

The National Streamflow Statistics (NSS) Program provides an easily accessible graphical user interface to estimate 

high and low streamflow statistics for ungaged sites across the 

United States. Streamflow statistics, such as the mean flow, the 

100-year flood, and the 7-day, 10-year low flow (7Q10), are 

needed and used for a variety of design, planning, and management purposes. These streamflow statistics can be easily computed using the NSS Program. Regression equations developed 

from data for gaged sites found within a statistically significant 

hydrologic region are often used to estimate streamflow statistics for ungaged sites in that region.The equations are developed 

by statistically relating computed streamflow statistics to measured physical and climatic characteristics of the watersheds for 

ungaged sites. The equations typically take a form similar to: 

Q100 = 0.471A0.715 * E 0.827

where

Q100	 is the estimated flood discharge for a flood with

a 100-yr recurrence interval,

A is drainage area, in square miles, and

E is mean basin elevation, in feet.

The characteristics and constants in the equations vary 

regionally to reflect hydrologic conditions. Users enter measured watershed and(or) climatic characteristics into the equations to obtain estimates of streamflow statistics for an ungaged 

site.

The U.S. Geological Survey (USGS) has created a Microsoft Windows1

 computer program named the National Streamflow Statistics (NSS) Program, version 4, to make it easy for 

users to obtain estimates of streamflow statistics for ungaged 

sites throughout the Nation (Ries, 2006). The NSS Program, 

version 4, is a replacement for the National Flood Frequency 

(NFF) Program, version 3 (Ries and Crouse, 2002). The name 

change from NFF to NSS reflects the primary enhancement 

to the program, which now provides estimates for many other 

types of streamflow statistics and not just estimates of flood-frequency statistics, such as the 100-year flood.

Regression equations for estimating flood-frequency statistics are available in the NSS Program for every State, the Commonwealth of Puerto Rico, and the island of Tutuilla, American 

Samoa. Regression equations for estimating other types of 

streamflow statistics, such as those derived from low-flow 

duration and(or) frequency regression equations, are currently 

(2007) available in the NSS Program for less than half of the 

Nation. The regression equations contained in the NSS program 

were compiled from USGS reports that generally were prepared 

in cooperation with individual State Departments of Transportation, and(or) other government agencies, and were published 

either by the USGS or by cooperating organizations.

NSS Program Capabilities

The NSS Program has the following four components: a 

user interface, an equation calculation routine, a database, and 

a help feature. The user interface allows users to control the 

operation of the software and presents results. The equation calculation routine computes streamflow statistics using basin and 

climatic characteristics entered by the user and provides tabling 

and graphing capabilities. The database contains all the information needed, such as the regression coefficients and standard 

errors, to solve more than 2,000 regression equations. The help 

facility contains the NSS Program user’s manual (Ries, 2007), 

a link to the NSS Program Web page, and version information. 

The NSS program is written in the Visual Basic programming 

language.

Version 4 of NSS can be used to:

Estimate rural and urban flood-frequency discharges for 

ungaged streams by use of regression equations, or for six 

States, by region-of-influence analysis;

Estimate a wide range of low-flow duration and frequency 

discharges for ungaged streams;

Estimate discharges for natural streams. The program does 

not account for the effects of water diversions, dams, flooddetention structures, and other manmade works;

Statistically weight estimated peak discharges for ungaged 

sites with drainage basins that span multiple hydrologic 

regions using the percentage of drainage area in each region 

within a given State; 

Statistically weight estimated and observed peak discharges 

for streamgaging stations using the equivalent years of 

record of the regression estimate and the number of years of 

observed record as the weighting factors; 

Statistically weight estimated peak discharges for ungaged 

sites obtained from regression equations and from the flow 

per unit area for an upstream or downstream gaging station;

Plot hydrographs of flood and low flows; and

Generate frequency graphs for both high- and low-flow frequency analyses.

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• 1 The use of trade or product names is for identification purposes only and 

does not constitute endorsement by the U.S. Geological Survey.
Obtaining and Installing NSS Software

NSS can be downloaded from the Internet at the Web 

address listed at the end of this fact sheet. Users will need to 

download at least two files, as follows: (1) NSSv4.exe, which 

contains the NSS Program Setup Wizard and the NSS computer 

program (about 6 Mb), and (2) NSSv4_MM-DD-YYYY.mdb, 

which is the database (about 25 Mb), where MM is the month, 

DD is the day, and YYYY is the year of the most recent release. 

A file to help the user install and operate the software (NSSreadme.txt) can also be downloaded from this site; the instructions in this file should be read before attempting to install the 

program. Each file should be downloaded to the local hard drive 

by double-clicking on the file names or icons for the files shown 

in the Web browser window. Next, go to the location of the file 

“NSSv4.exe” on the local hard drive using Windows Explorer 

or My Computer. Double-click on the file name “NSSv4.exe” 

to start the NSS Program Setup Wizard. The Setup Wizard will 

prompt for a directory name in which the NSS Program will be 

installed (the default is, C:\Program Files\NSS\); allow selection of the Start Menu folder in which to place the program’s 

shortcut (the default is C:\Programs\USGS); and ask whether 

or not to create a desktop icon for the program. After all of the 

selections are completed, click on the Install button to complete 

the installation. The database file must then be copied to the 

NSS Program directory when the installation is complete.

Region-of-influence analysis methods are currently (2007) 

available for estimating peak discharges for six States. Additional files that contain data needed for these analyses must be 

downloaded from the NSS Program Web site and placed in the 

directory where the NSS Program is stored.

Using the NSS Software

The NSS Program can be started by double-clicking on 

the NSS icon on the desktop or by selecting the NSS menu item 

under All Programs in the Start menu. This will cause a small 

window to open (fig. 1) that allows the entry of the user’s name, 

specification of the NSS Program work file to which data will 

be stored, and choice of English or metric units for computation. The main NSS Program user interface window is shown in 

figure 2. Three pull-down menus (File, Graph, and Help) at the 

top of the user interface allow users to (1) open and save work, 

and create reports for printing; (2) create hydrographs and frequency plots; and (3) access the help file, the NSS Web site, and 

version information for the program, respectively. Just below 

the pull-down menus are a pair of boxes that allow selection of 

the State (mandatory) in which the site of interest is located, and 

naming of the site (optional).

Two large frames—one for rural estimates (on the left) 

and the other for urban estimates—fill the center of the main 

window. The box at the top left of each frame shows the name 

of the current scenario (a set of estimates for a site). To the right 

Figure 2.  Main NSS

Program window.

Figure 1.  NSS Program startup window.
of the box is a set of three buttons (New, Edit, and Delete), 

allowing users to (1) create a new scenario, (2) edit the current scenario, and (3) delete the current scenario, respectively. 

Below the buttons are two text boxes. The top box shows input 

parameters used to evaluate the regression equations for the 

scenario. The bottom box shows the output, including the recurrence intervals, the estimated peak flows, the standard errors of 

estimate or prediction, and the equivalent years of record for the 

estimates. The standard errors and the equivalent years of record 

provide indications of the reliability of the estimates.

Clicking on either of the New buttons opens the Edit Scenario window (fig. 3), which allows users to select the hydrologic region of interest and enter the basin and climatic characteristics needed to evaluate the equations for that region. The 

scenario name can be specified by typing it into the box at the 

top of the window, or the program will automatically name the 

scenarios based on the type of scenario (rural or urban) and the 

number of scenarios that have previously been run. A list of the 

hydrologic regions available within the State is presented on the 

left side of the window. Selection of a region will cause a list of 

the basin and climatic characteristic that are needed to solve the 

regression equations for the region to appear in a grid within the 

window. Users should enter characteristic values in the boxes 

next to the characteristic names. Placing the cursor over one of 

these boxes or clicking in a box causes the minimum and maximum values of the basin and climatic characteristics that were 

used to develop the equations to appear in a pop-up text box and 

in a text line below the list of basin and climatic characteristics. 

Values outside the ranges shown can be entered, but the resulting peak-flow estimates will be extrapolations with unknown 

errors.

Sauer and others (1983) developed regression equations for 

estimating flood frequencies for urban ungaged sites throughout the United States that are included in the NSS Program. 

Additional local urban equations are available in the NSS Program for several States and municipalities. The national urban 

regression equations and most local urban regression equations 

use the rural estimates as an explanatory variable in the equations; therefore, usually generation of rural estimates is required 

before urban estimates can be obtained. The NSS Program generates a warning when urban estimates are requested for a site, 

and the rural estimates for the site have not yet been determined.

Three buttons at the bottom of the main window (fig. 2) 

access the Frequency Plot, Hydrograph, and Weight options. 

Clicking on any of these buttons after a rural scenario is created 

Figure 3.  Edit Scenario

window.

Figure 4.  Frequency Plot window.

causes another dialogue window to open. The Frequency Plot

window allows selection of the plotting scenario in a secondary window. Once a selection is made, clicking the Plot button 

causes the selected plotting scenario to be generated in the Frequency Plot window, as shown in figure 4. Pull-down menus in 

the plot window allow printing and saving the plots, modifying 

of axes, titles, line styles and legends, listing of the data used to 

create the plot, and closing of the plot window.

The Hydrograph window allows selection of the scenario 

and the recurrence interval to be plotted, as well as a lag time. If 

the scenario is based on the national urban equations (Sauer and 

others, 1983), the lag time can be estimated by specifying the 

basin length in miles. Other methods for estimating lag times 

are provided in an appendix at the end of the user’s manual 

included with the program, as described later. Once a selection 

is made, clicking the Plot button causes a window to open with 

the hydrograph plot (fig. 5). The pull-down menus in the plot 

window provide for printing and saving the plot, modifying the 

axes, titles, line styles, and legend of the plot, as well as listing 

the data used to create the plot, and closing the plot window.

The Weight window can be selected to compute weighted 

estimates for both gaging stations and ungaged sites. A rural 

scenario of regression estimates must first be obtained for a 

gaging station before weighted estimates can be obtained for 

the station. When the Weight window is opened, a button at the 
top of the window is initally set to calculate weighted estimates 

for a gaging station. Entering the years of actual record for the 

gaging station and the peak-flow estimates determined from 

observed data will cause weighted estimates to be calculated for 

the gaging station based on the length of years of observed data 

and the equivalent years of record for the estimates. Clicking 

on the Apply button saves the weighted estimates as a weighted 

scenario and makes them appear in the output box of the rural 

frame in the main NSS Program window.

The weighting feature for ungaged sites requires that the 

drainage area for the ungaged site must be within 0.5 and 1.5 

times the drainage area for an upstream or downstream gaging station. Before the weighted estimates for the ungaged site 

can be obtained, first use the NSS Program to create a scenario 

of weighted estimates for the gaging station, and then create a 

rural scenario for the ungaged site. When the Weight window 

is opened, click on the button at the top right of the window 

that allows weighting for an ungaged site using weightedgaged values. Select the rural weighted scenario for the gaging 

station from the pull-down list that will appear in the window, 

then click on Apply to compute the weighted estimates for the 

ungaged site and make them appear in the output box of the 

rural frame in the main NSS Program window. These weighted 

estimates combine the weighted estimates for the gaging station 

with the regression equation estimates for the ungaged site on 

the basis of drainage area.

Documentation of Equations and Parameters

The user’s manual can be accessed through the Help menu 

at the top of the NSS Program main window. It provides complete documentation of the technical methods used to develop 

the functionality included in the software and detailed directions 

for using it.

The NSS Program Web site provides links to reports that 

document the software, reports, and summaries that document 

the equations included in the software, and the information 

necessary to solve the equations. The NSS Program Web site 

and links are 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.

By D. Phil Turnipseed and Kernell G. Ries III

References

Crippen, J.R., and Bue, C.D., 1977, Maximum floodflows in the 

conterminous United States: U.S. Geological Survey WaterSupply Paper 1887, 52 p. 

Jennings, M.E., Thomas, W.O., Jr., and Riggs, H.C., comps., 

1994, Nationwide summary of U.S. Geological Survey 

regional regression equations for estimating magnitude and 

frequency of floods for ungaged sites, 1993: U.S. Geological Survey Water-Resources Investigations Report 94-4002, 

196 p. + 1 CD.

Ries, K.G., III, in press, 2007, 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 5, Chapter A6, 45 p.

Ries, K.G., III, and Crouse, M.Y., comps., 2002, The National 

Flood Frequency Program, Version 3: A computer program for estimating magnitude and frequency of floods 

for ungaged sites, 2002: U.S. Geological Survey WaterResources Investigations Report 02-4168, 53 p.

Sauer, V.B., Thomas, W.O., Jr., Stricker, V.A., and Wilson, K.V., 

1983, Flood characteristics of urban watersheds in the United 

States: U.S. Geological Survey Water-Supply Paper 2207, 

63 p.

Figure 5.  Hydrograph Plot window.

USGS hydrologic analysis software is available for electronic retrieval 

through the World Wide Web (WWW) at http://water.usgs.gov/software/. 

The WWW page from which the NSS software and user documentation 

can be retrieved is http://water.usgs.gov/software/nss.html. NSS requires a 

computer running Windows1 98/NT Version 4.0 or higher with service pack 

5 or higher. For optimal performance, a processor running at 400 megahertz 

or faster with at least 64 megabytes of memory is recommended. A video 

graphics array or a higher quality color monitor is also recommended.

Additional earth science information is available from the USGS 

through the WWW at http://www.usgs.gov/, or by calling 1-888-ASK-USGS.

Contact For Additional Information:

U.S. Geological Survey

Office of Surface Water

415 National Center

Reston, Virginia 20192

(703) 648-5301