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Fact Sheet 2006-3143

December 2006

This factsheet is available as a pdf.


Estimating Magnitude and Frequency of Floods Using the PeakFQ Program



Flood-frequency analysis provides information about the magnitude and frequency of selected flood discharges. Frequency commonly is expressed in terms of exceedance probability (a dimensionless number ranging from 0 to 1.0) or as a recurrence interval (the reciprocal of exceedance probability) in years (table 1). Flood-frequency information is important for defining flood-hazard areas; for managing floodplains; and for designing bridges, culverts, dams, levees, and other structures.

Table 1. Exceedance probabilities and associated recurrence intervals.

    Annual exceedance probability         Recurrence interval (n-year flood)    
0.6667 1.50
0.50 2.00
0.4292 2.33
0.20 5.00
0.10 10.00
0.04 25.00
0.02 50.00
0.01 100.00
0.005 200.00
0.002 500.00

Flood-frequency analysis commonly is performed on records of annual maximum instantaneous peak discharges collected systematically at streamflow gaging stations. Bulletin 17B of the Hydrology Subcommittee of the Interagency Advisory Committee on Water Data (1982) defines procedures recommended to provide a consistent approach for determining flood-flow frequency from peak-flow records. The procedures include methods for improving skew estimates using regional skew information, tests for high and low outliers, adjustments for low outliers and zero flows, and incorporating historic peak-flow information. Historic peaks are noteworthy events occurring before, after, or between periods of record collection.

Bulletin 17B guidelines:

  • Fit logarithms of annual peak discharges to a Pearson Type III distribution using the method of moments to compute mean, standard deviation, and skew of the log-transformed data.
  • Improve skew estimate by averaging it with a regional skew estimate obtained from Bulletin 17B or other source. Weight station and generalized skew to reflect relative accuracy.
  • Test to identify high and low outlier limits and screen data against them.
  • Adjust for low outliers, zero flows, and below gage base peaks based on conditional probability techniques.
  • Incorporate historic or paleoflood information using historically weighted moments approach.
  • Compute confidence limits.
  • Graph frequency curves.
  • Use other approaches as appropriate and justified.


The Peak flow FreQuency analysis program (PeakFQ, Flynn and others, 2006) implements the Bulletin 17B recommended procedures for flood frequency analysis of streamflow records (Kirby, 1981). PKFQWin provides an interactive Windows interface to PeakFQ. PKFQBat provides a command-line interface to PeakFQ for batch-style processing on DOS, UNIX, and Linux operating systems.


Three types of input files are supported by PeakFQ. The annual peaks may be in standard WATSTORE text format or in a Watershed Data Management (WDM) file (Flynn and others, 1995). An optional program specification file (.psf) can be used to identify the file containing the peaks and to specify processing options.


Program PeakFQ provides estimates of flood magnitude for ten annual exceedance probabilities and their corresponding recurrence intervals (table 1.) The output file also provides the logarithmic mean standard deviation, and skew for both the annual peaks and the Bulletin 17B estimate.

PeakFQ can provide a graph with the Bulletin 17B and systematic frequency curves, the systematic peaks, and confidence limits (fig. 1.) See Bulletin 17B for a detailed explanation of the graph components.

Figure 1. Example of PeakFQ probability graph of annual peaks, frequency curves, and confidence limits.

Using PKFWin

PKFQWin is launched by double-clicking on the PKFQWin icon on the desktop or by selecting USGS and then PKFQWin under Programs in the Windows Start menu. The File pull-down menu at the top of the window (fig. 2) contains options to Open an input file, Save Specs to a .psf file, or Exit the program. The main window contains three tabs. The Stations Specification tab and the Output Options tab become active once an input file has been identified and read. The Results tab becomes active after the Run PeakFQ button has been selected.

Figure 2. Station Specifications tab in main window of PKFQWin.

The Station Specifications tab is populated with data read from the file of peak flows and, if used, the .psf file. Default values are used for parameters not specified in the input files. The shaded columns are informational and cannot be modified. Values in the white columns can be modified as appropriate for the analysis.

The Output Options tab is used to specify file and graphics output choices and to name the text output files. The Run PeakFQ button runs the analysis.

The Results tab becomes active after the analysis has been completed and provides access to the text output file and the plots generated by the analysis.

The Save Specs button at the bottom of the window is used to save the specifications and output options to a program specification file (.psf). A .psf file can be edited and used as an input file to rerun the analysis, facilitating evaluation of different parameters.

The Help pull down menu at the top of the window contains three options: About returns the program version number, Send Feedback captures the program and system state and e-mails this information to the developers to help identify problems, and PKFQWin Help opens the userís guide as a Windows help document.

Using PKFQBat

PKFQBat provides a batch-style interface to PeakFQ and accommodates the analysis of a single station or many stations. The program is launched from the command line and includes all of the analysis and output options found in the PKFQWin version.

Software, Data Sources, and Additional Information

The PeakFQ program and documentation and other hydrologic analysis software is available at:

USGS peak flow data can be retrieved in the WATSTORE text format at:

For information on local USGS flood frequency studies and research, contact the USGS Water Science Center Director for the area of interest; a list is available at:


Flynn, K.M., Hummel, P.R., Lumb, A.M., and Kittle, J.L., Jr., 1995, Users manual for ANNIE, version 2, a computer program for interactive hydrologic data management: U.S. Geological Survey Water-Resources Investigations Report 95-4085, 211 p.

Flynn, K.M., Kirby, W.H., and Hummel, P.R., 2006, Users manual for PeakFQ, annual flood frequency analysis using Bulletin 17B Guidelines: U.S. Geological Survey Techniques and Methods Report Book 4, Chap. B4, 42 p.

Interagency Advisory Committee on Water Data, 1982, Guidelines for determining flood flow frequency: Bulletin 17B of the Hydrology Subcommittee, Office of Water Data Coordination, U.S. Geological Survey, Reston, VA., 183 p.

Kirby, W.H., 1981, Annual flood frequency analysis using U.S. Water Resources Council guidelines (program J407): U.S. Geological Survey Open-File Report 79-1336-I, WATSTORE Userís Guide, v. 4, chap. I, sec. C, 56 p.


Prepared by Kathleen M. Flynn, William H. Kirby, Robert Mason, and Timothy A. Cohn

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