U.S. Department of the Interior

U.S. Geological Survey

FS-2006-3143

December 2006

ESTIMATING MAGNITUDE AND FREQUENCY OF FLOODS 

USING THE PeakFQ PROGRAM

FLOOD-FREQUENCY ANALYSIS

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.

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, 

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

limits.

and incorporating historic peak-flow 

information. Historic peaks are noteworthy events occurring before, after, or 

between periods of record collection.

PROGRAM PeakFQ

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.

PeakFQ INPUT OPTIONS

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.

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

PeakFQ OUTPUT 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 

Bulletin 17B guidelines: components.

• 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.

5

4

3

2
Using PKFQWin

PKFQWin is launched by doubleclicking 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.

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.

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

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 

user’s 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: http://water.usgs.

gov/software/ .

USGS peak flow data can be retrieved 

in the WATSTORE text format at: http://

nwis.waterdata.usgs.gov/usa/nwis/peak .

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: http://water.usgs.gov/district_chief.

html .

References:

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 User’s Guide, v. 4, chap. I, 

sec. C, 56 p.

Prepared by Kathleen M. Flynn, William H. 

Kirby, Robert Mason, and Timothy A. Cohn

Beginning

Sqr Err

Run PEAKFQ