Analysis of the Magnitude and Frequency of Floods in Colorado

Regionalized flood-frequency relations
need to be updated on a regular basis (about
every 10 years). The latest study on regionalized
flood-frequency equations for Colorado used
data collected through water year 1981. A study
was begun in 1994 by the U.S. Geological Survey,
in cooperation with the Colorado Department
of Transportation and the Bureau of Land
Management, to include streamflow data
collected since water year 1981 in the regionalized
flood-frequency relations for Colorado.
Longer periods of streamflow data and improved
statistical analysis methods were used to define
regression relations for estimating peak
discharges having recurrence intervals of 2, 5, 10,
25, 50, 100, 200, and 500 years for unregulated
streams in Colorado. The regression relations
can be applied to sites of interest on gaged and
ungaged streams. Ordinary least-squares regression
was used to determine the best explanatory
basin or climatic characteristic variables for each
peak-discharge characteristic, and generalized
least-squares regression was used to determine
the best regression relation. Drainage-basin area,
mean annual precipitation, and mean basin slope
were determined to be statistically significant
explanatory variables in the regression relations.
Separate regression relations were developed for
each of five distinct hydrologic regions in the
State. The mean standard errors of estimate and
average standard error of prediction associated
with the regression relations generally ranged
from 40 to 80 percent, except for one hydrologic
region where the errors ranged from about 200
to 300 percent. Methods are presented for determining the magnitude of peak discharges for
sites located at gaging stations, for sites located
near gaging stations on the same stream when
the ratio of drainage-basin areas is between
about 0.5 and 1.5, and for sites where the drainage
basin crosses a flood-region boundary or a State
boundary. Methods are presented for determining
the magnitude of peak discharges for sites located
at gaging stations, for sites located near gaging
stations on the same stream when the ratio of
drainage-basin areas is between about 0.5 and 1.5,
and for sites where the drainage basin crosses a
flood-region boundary or a State boundary.