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

Illinois is divided into four hydrologic regions (fig. 1),
each of which is assigned a regional factor. The regression
equations developed for these regions are for estimating peak
discharges (QT) having recurrence intervals T that range from 2
to 500 years. The explanatory basin variables used in the
equations are drainage area (A), in square miles; main-channel
slope (S), in feet per mile; rainfall (I), in inches, which is
the 2-year 24-hour precipitation (fig. 2); and a regional factor
(Rf). The constant 2.5 is subtracted from I in the computer
application of the regression equations. The user should enter
the actual value of I from figure 2. The variables A and S can be
measured from topographic maps and I can be determined from
figure 2. The regional boundaries can be determined from figure 1
and a table of regional factors (Rf) is given below. The
regression equations were developed from peak-discharge records
for 268 gaged sites in Illinois, Indiana, and Wisconsin. The
regression equations are applicable to streams with drainage
areas ranging from 0.02 to 10,000 square miles. Standard errors
of prediction for the regression equations range from 35 to 50
percent. The report by Curtis (1987) also includes graphical
relations of flood characteristics to drainage area for the
regulated Big Muddy, Fox, and Illinois Rivers.

### Procedure

Topographic maps, the hydrologic regions map (fig. 1), the
table of regional factors (Rf), the 2-year 24-hour precipitation
map (fig. 2), and the following equations are used to estimate
the needed peak discharges QT, in cubic feet per second, having
selected recurrence intervals T.

### Regional factors (Rf)

### Reference

Curtis, G.W., 1987, Technique for
estimating flood-peak discharges and frequencies on rural streams
in Illinois: U.S. Geological Survey Water-Resources
Investigations Report 87-4207, 79 p.

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

Figure 2. The 2-year 24-hour precipitation in Illinois.
(PostScript file of Figure 2.)