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

California is divided into six 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 100 years. The explanatory basin variables
used in the equations are drainage area (A), in square miles;
mean annual precipitation (P), in inches; and an altitude index
(H), which is the average of altitudes in thousands of feet at
points along the main channel at 10 percent, and 85 percent of
the distances from the site to the divide. The variables A and H
may be measured from topographic maps. Mean annual precipitation
(P) is determined from a map in Rantz (1969). The regression
equations were developed from peak-discharge records of 10 years
or longer, available as of 1975, at more than 700 gaging stations
throughout the State. The regression equations are applicable to
unregulated streams but are not applicable to some parts of the
State (see fig. 1). The standard errors of estimate for the
regression equations for various recurrence intervals and regions
range from 60 to over 100 percent. The report by Waananen and
Crippen (1977) includes an approximate procedure for increasing a
rural discharge to account for the effect of urban development.
The influences of fire and other basin changes on flood
magnitudes are also discussed.

### Procedure

Topographic maps, the hydrologic regions map (fig. 1), the
mean annual precipitation from Rantz (1969), and the following
equations are used to estimate the needed peak discharges QT, in
cubic feet per second, having selected recurrence intervals
T.

#### North Coast Region

#### Northeast Region

#### Sierra Region

#### Central Coast Region

#### South Coast Region

#### South Lahontan-Colorado Desert Region

In the North Coast region, use a minimum value of 1.0
for the altitude index (H). Equations are defined only for basins
of 25 mi² or less in the Northeast and South
Lahontan-Colorado Desert regions.
### Reference

Waananen, A.O., and Crippen, J.R., 1977,
Magnitude and frequency of floods in California: U.S. Geological
Survey Water-Resources Investigations Report 77-21, 96 p.
### Additional Reference

Rantz, S.E., 1969, Mean annual
precipitation in the California region: U.S. Geological Survey
Open-File Map (Reprinted 1972, 1975).

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