Water Resources of the United States

The following documentation was taken from:

U.S. Geological Survey Water-Resources Investigations Report 94-4002:*
Nationwide summary of U.S. Geological Survey regional regression equations for
estimating magnitude and frequency of floods for ungaged sites, 1993*

## STATEWIDE RURAL

### Summary

North Dakota is divided into three hydrologic regions (fig.
1). The regression equations developed for these regions are for
estimating peak discharges having recurrence intervals T that
range from 2 to 500 years. The explanatory basin variables used
in the equations are contributing drainage area (CA), in square
miles; and main channel slope (S), in feet per mile. The
regression equations were developed from peak-discharge records
available for 192 continuous- and partial-record streamflow
gaging stations and are applicable to rural, unregulated streams
draining 1,000 square miles or less. The standard errors of
estimate for the regression equations range from 55 to 98
percent. The equivalent years of record range from 2.0 to 12.0
years. The report by Williams-Sether (1992) includes basin and
flood-frequency characteristics of the streams used to define the
peak-flow relations. The report also includes basin and
flood-frequency characteristics of streams with drainage areas
over 1,000 square miles and that were not used to define the
peak-flow regression relations.

### Procedure

Topographic maps, the hydrologic regions map (fig. 1), and the
following regression equations are used to estimate the needed
peak discharges QT, in cubic feet per second, having selected
recurrence intervals T.

### Region A

### Region B

### Region C

### Reference

Williams-Sether, T., 1992, Techniques for
estimating peak-flow frequency relations for North Dakota: U.S.
Geological Survey Water-Resources Investigations Report 92-4020,
57 p.

Figure 1. Flood-frequency region map for North Dakota.
(PostScript file of Figure 1.)