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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

MONTANA


STATEWIDE RURAL

Summary

Montana is divided into eight hydrologic regions (fig. 1). 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; mean annual precipitation (P), in inches; basin high elevation index (HE+10), which is the percentage of the total basin area above 6,000 feet, plus 10; and mean basin elevation (E), in feet, divided by 1000 (E/1000). The constant 10 is added to HE and E is divided by 1000 in the computer application of the regression equation. The user should enter the actual values of HE and E. The variable P is taken from a map developed by the U.S. Soil Conservation Service (1980). The other variables can be measured from topographic maps. The regression equations were developed from peak-discharge records available as of 1988 for 476 stations in Montana and 46 stations in adjacent states and Canada. The regression equations apply to unregulated streams having a drainage area ranging from 0.04 to 2,554 square miles, but are not valid where unique topographic or geologic features affect floods. The standard errors of prediction of the equations range from 22 to 128 percent. The report by Omang (1992) includes graphs of flood characteristics along seven major streams, and a table showing basin and flood characteristics and maximum floods of record at gaging stations.

Procedure

Topographic maps, the hydrologic regions map (fig. 1), the mean annual precipitation map in U.S. Soil Conservation Service (1980), and the following equations are used to estimate the needed peak discharges QT, in cubic feet per second, having selected recurrence intervals T.

Northwest-Foothills Region
Regression equations for estimating magnitude and frequency of floods in Northwest-Foothills Region in Montana

Northeast Plains Region
Regression equations for estimating magnitude and frequency of floods in Northeast Plains Region of Montana

East-Central Plains Region
Regression equations for estimating magnitude and frequency of floods in East-Central Plains Region of Montana

Southeast Plains Region
Regression equations for estimating magnitude and frequency of floods in Southeast Plain Region of Montana

West Region
Regression equations for estimating magnitude and frequency of floods in West Region of Montana

Northwest Region
Regression equations for estimating magnitude and frequency of floods in Northwest Region of Montana

Southwest Region
Regression equations for estimating magnitude and frequency of floods in Southest Region of Montana

Upper Yellowstone-Central Mountain Region
Regression equations for estimating magnitude and frequency of floods in Upper Yellowstone-Central Mountain Region of Montana

Reference

Omang, R.J., 1992, Analysis of the magnitude and frequency of floods and the peak-flow gaging network in Montana: U.S. Geological Survey Water-Resources Investigations Report 92-4048, 70 p.


Figure 1. Flood-frequency region map for Montana.

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

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