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

The State of Rhode Island is considered to be one hydrologic
region. The regression equations developed for the State are for
estimating peak discharges QT with recurrence intervals of 2 and
5 years. The 10-, 25-, and 50-year floods are computed as ratios
of the 2-year peak discharge. These ratios are taken from USGS
Water-Supply Paper 1671. The explanatory basin variables used in
the equations are drainage area (A), in square miles; mean basin
elevation (E), in thousands of feet; and forest cover (F)
expressed as 0.01 plus the decimal fraction of the drainage area
covered by forests. The constant of 0.01 is added to F in the
computer application of the equations. The user should enter the
actual value of F. All these variables can be measured from
topographic maps. The regression equations were developed from
peak-discharge records from 1966-1971 for 38 stations and are
applicable only to rural streams having no significant storage
with drainage areas less than 10 square miles. The standard error
of estimate of the regression equation for the 2-year flood is 46
percent, but the results must be considered preliminary because
of the short records on which they are based. Comparison of flood
records for long-term (1941-1967) and short-term (1966-1971)
indicates that the equation derived from the short-term records
should include an additional coefficient of 0.79.

### Procedure

Topographic maps and the following equations (which should be
adjusted for time bias by the factor 0.79) are used to estimate
the needed peak discharges QT, in cubic feet per second, having
selected recurrence intervals T.

### Reference

Johnson, C.G., and Laraway, G.A., 1976, Flood magnitude
and frequency of small Rhode Island streams: U.S. Geological
Survey Open-File Report 76-883, 22 p.