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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




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.


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.

Regression equations for estimating magnitude and frequency of floods in Rhode Island


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.