U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 084-02

October 2002

USING THE NATIONAL FLOOD FREQUENCY PROGRAM, VERSION 3: 

A COMPUTER PROGRAM FOR ESTIMATING MAGNITUDE 

AND FREQUENCY OF FLOODS FOR UNGAGED SITES

Estimates of the magnitude and frequency of flood-peak 

discharges and flood hydrographs are used for a variety of 

purposes, such as the design of bridges and culverts, flood-control structures, and flood-plain management. These estimates 

are often needed at ungaged sites where no observed flood data 

are available for frequency analysis. Regression equations are 

commonly used to estimate the magnitude and frequency of 

flood-peak discharges and flood hydrographs at ungaged sites. 

These equations are developed by statistically relating the flood 

characteristics to the physical and climatic characteristics of the 

watersheds for a group of streamgaging stations within a region 

that have virtually natural streamflow conditions. Regression 

equations enable the transfer of flood characteristics from gaged 

sites to ungaged sites simply by determining the needed watershed and climatic characteristics for the ungaged site.

In 1994, the U.S. Geological Survey (USGS), in cooperation with the Federal Highway Administration and the Federal 

Emergency Management Agency, released version 1 of the 

National Flood Frequency (NFF) Program (Jennings and others, 

1994). This Microsoft DOS1 program provided engineers and 

hydrologists a practical tool for computing estimates of floodpeak discharges having recurrence intervals of 2 to 500 years for 

user-selected sites on rural and urban streams. The 2-year flood 

occurs, on average, once in 2 years and has a 50 percent chance 

of occuring in any single year, whereas the 500-year flood 

occurs, on average, once in 500 years and has a 0.2 percent 

chance of occuring in any single year. 

Version 1 of NFF contained all USGS-developed regional 

regression equations that were available as of September 1993 

for estimating unregulated flood frequencies. NFF also provided 

the ability to generate flood-frequency plots and plots of typical 

flood hydrographs corresponding to a given rural and/or urban 

peak discharge. Since the release of NFF version 1, new or 

updated equations have been developed by the USGS for most 

of the Nation. Longer periods of record, and improved methods 

for measuring basin characteristics and developing regression analyses, have generally led to improved precision of the 

updated equations over those that they have replaced. The new 

and updated equations have 

been compiled and incorporated into a new, easierto-use, Microsoft Windows1

version of the NFF Program 

(Ries and Crouse, 2002).

Version 3 of the NFF 

software compiles more than 

2,000 flood-flow equations 

developed by the USGS for 

289 regions of the Nation, 

the Commonwealth of Puerto 

Rico, and the island of Tutuilla, American Samoa, into a 

single, user-friendly package. 

The flood-frequency equations contained in NFF were 

taken from statewide floodfrequency reports that were 

prepared generally in cooperation with individual State 

Departments of Transportation, and/or other government 

agencies, and were published 

either by the USGS or the 

State Departments of Transportation.

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Figure 1. View of the main window of NFF.
NFF Program Capabilities

Version 3 of NFF can be used to:

• Obtain estimates of flood frequencies for sites in rural 

(unregulated) ungaged basins.

• Obtain estimates of flood frequencies for sites in urbanized 

basins.

• Estimate maximum floods based on envelope curves developed by Crippen and Bue (1977).

• Create hydrographs of estimated floods for sites in rural or 

urban basins and manipulate the appearance of the graphs.

• Create flood-frequency curves for sites in rural or urban 

basins and manipulate the appearance of the curves.

• 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.

• 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.

• Save output from the program in text and graphic files. 

• Obtain documentation and instruction for use of the program 

from help files.

Obtaining and Installing NFF Software

NFF can be downloaded from the Internet at the Web address 

listed at the end of this report. NFF has two components: 

NFFv3.exe, which is the computer program, and NFFv3.mdb, 

which is the database. Download each file to the local hard drive 

by double-clicking on the file names or icons shown in the Web 

browser window. Next, go to the location of NFFv3.exe on the 

local hard drive using Windows Explorer or My Computer. Double-click on the file name (NFFv3.exe) to start the NFF Setup 

Wizard. The Setup Wizard will prompt for a directory name in 

which to install NFF (typically, C:\Program Files\NFF\), will 

allow selection of the Start Menu folder in which to place the 

program’s shortcut, 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 

NFFv3.mdb file must then be copied to the NFF directory 

when the installation is complete.

Using the NFF Software

NFF can be started by double-clicking on the NFF icon 

on the desktop or by clicking on the NFF menu item under 

Programs in the Start menu. A small window will open that 

allows entry of the user’s name, specification of the NFF work 

file to which data will be stored, and choice of English or 

metric units for computation.

The main NFF user interface window is shown in figure 

1. Three pull-down menus 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 NFF Web site, and version information for the program. Just below the pull-down menus are a 

pair of boxes that allow selection of the State (required) in 

which the site of interest is located, and naming of the site 

(optional). 

Two large frames — one for rural estimates and the other 

for urban estimates — fill the center of the main window. 

A 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 of 

each box are three buttons, allowing users to (1) create a new 

scenario, (2) edit the current scenario, or (3) delete the current 

scenario. Below these buttons are two text boxes. The top 

box shows input parameters used to evaluate the regression 

equations for the selected 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 (figure 2), 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 

Figure 2. View of the Edit Scenario window.

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it into the box at the top of the window, or the program will 

automatically name the scenario based on the type of scenario 

(rural or urban) and the number of scenarios that have previously been run. When a region is selected, the names of the 

basin and climatic characteristics needed to solve the equations 

for that region are shown in the window. The values are entered 

in the boxes next to the basin and climatic 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, which 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 NFF. Additional urban 

equations are available in NFF for several states and municipalities. The urban equations typically use rural estimates for the 

same site as predictor variables in the urban equations. NFF 

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 access the 

Frequency Plot, Hydrograph, and Weight options. Clicking 

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

another window to open. The Frequency Plot window, allows 

selection of the plotting scenario. Clicking the plot button 

causes the selected plotting scenario to be generated in the Frequency Plot window, as shown in figure 3. Pull-down menus 

allow printing and saving of the plots, modifying of the 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 

���������� View of the Hydrograph Plot Window.

���������� View of the Frequency Plot Window.

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�������������� be estimated by specifying the basin 

length, in miles. Clicking the Plot button causes a window to open containing the hydrograph plot (figure 4). The 

pull-down menus provide capabilities 

including printing, saving, and 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 compute 

weighted estimates for both gaging 

stations and ungaged sites. A rural 

scenario of regression estimates 

must first be obtained for the gaging station before weighted estimates 

can be obtained. 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 

places them in the output box of the 

rural frame in the main NFF window.

To obtain weighted estimates for 

an ungaged site, first create weighted 

estimates for the gaging station to be 

used for estimating peak flows for 

the ungaged site, then create a rural 

scenario for the ungaged site. The 
gaging station must be on the same stream as the ungaged site, 

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

and 1.5 times the drainage area for the gaging station for the 

weighting to be applied. The weighting feature for ungaged sites 

requires weighted estimates for the nearest gaging station to be 

used in estimating peak flows, and requires that a scenario for 

the ungaged site also be created. Then, a rural scenario for the 

ungaged site must also be created. When the Weight window 

is open, click on the button at the top right of the window that 

allows weighting for an ungaged site using weighted gaged 

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 NFF window. These weighted estimates 

for the ungaged site combine the weighted estimates for the 

gaging station with the regression equation estimates for the 

ungaged site on the basis of the drainage areas at the gaged and 

ungaged sites. 

Documentation of Equations and Parameters

The Help file can be accessed through the Help menu at the 

top of the NFF main window. It provides complete documentation of the technical methods used to estimate peak flows in the 

software.

The NFF Web site provides links to online reports, fact 

sheets, and state summaries that document the software and 

the equations in NFF. Full documentation of the equations and 

information necessary to solve them is provided in the individual reports for each state. Many of the state reports published 

since the release of NFF version 1 are available for download 

from the Web. In addition, fact sheets are available that summarize the reports for 20 states with new or corrected equations 

developed since the release of NFF version 1. Summaries from 

the original NFF report (Jennings and others, 1994) are available online for the states that have not developed new equations 

since the previous software release. 

The NFF Web and site 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. 

Prepared by Craig D. Perl and Kernell G. Ries III

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

the conterminous United States: U.S. Geological Survey 

Water-Supply Paper 1887, 52 p. 

Jennings, M.E., Thomas, W.O., 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.

Ries, K.G., III, 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, 42 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. 

For more information contact:

U.S. Geological Survey

Office of Surface Water

415 National Center

Reston, Virginia 20192

(703) 648-5301

USGS hydrologic analysis software is available for electronic retrieval through the World Wide Web (WWW) at 

http://water.usgs.gov/software/, and through anonymous 

File Transfer Protocol (FTP) from water.usgs.gov (directory: /pub/software). The WWW page and anonymous 

FTP directory from which the National Flood-Frequency 

software and user documentation can be retrieved are http:

//water.usgs.gov/software/nff.html and /pub/software/

surface_water/nff, respectively. NFF software version 3 

requires Windows 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 VGA or better color monitor is 

also recommended.

Additional earth science information is available from 

the USGS at http://water.usgs.gov/ or by calling 

1-888-ASK-USGS.

1

 The use of trade or product names in this fact sheet is for identification purposes only and does not constitute endorsement by the U.S. Geological Survey.

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References