FLOOD10479

• County: Statewide
• Basin: Statewide
• Title: Precipitaion - Frequency Maps for Colorado
• Date: 10/1/1967
• Prepared For: Engineering Division, Coil Conservation Service
• Prepared By: Special Studies Branch, Office of Hydrology
• Floodplain - Doc Type: Floodplain Report/Masterplan

.
..

..

itation-F Values. The basic data network for
represents a density 0 1 station per 300 square miles. The
are not evenly distributed but tend to follm. population distri-
Consequently, the density is least in the more rugged terrain ,^1here
it should be greatest to properly depict the large variations in frequency
values that exist there. It was thus necessary to develop procedures for
estimating precipitation-frequency values in order to obtain some knowledge
of their geographic distribution in areas not represented by observations.

IV re
air

are politically
and

Estimatin
Colorado

stations
bution.

precipitation amounts for 6 and 24 hours the type of pre-
cipitation, and 2) the maximum annual 6- resulting from
storms which tvere entirely rain. Amounts the second series
were sometimes the second, third, 'fourth or even some cases the fifth
highest value for a particular year. Comparison of the 2- and 2S-year
return period amounts computed from the two series for these stations for
the 6- and 24-hour durations indicates that the values from the series using
only rainfall data are, on the average, about 10 percent smaller than the
series using all the data. Investigation of the geographical distribution
of these differences indicates that the stations on the relatively level
eastern portions of Colorado and Wyoming shm'l little differences between the
series containing rainfall data only and those containing all data. If
these stations were eliminated from the comparison, the average difference
between values from the two series increases to about IS percent with some
individual differences as large as 40 to 50 percent. Though no definite
line can be drawn separating the regions where snowfall is a definite factor
in producing higher frequency values, a zone of transition along the eastern
face of the Rocky Mountains approximating a generalized 6000-foot contour is
indicated from the southern Colorado border to just north of the Wyoming
border.

for
amounts
tvi thin

without regard
and 24-hour

included
in

Hhite.

method of preparing generalized precipitation-frequency maps
of sparse precipitation data would be one in t-1hich precipitation-
I
frequency values could be estimated for any point on the basis of orographic
factors. If the required relationship cannot be established, more indirect
procedures relating precipitation-frequency values to a combination of topo-
graphic parameters and normal annual or seasonal precipitation or other
meteorological or climatological factors whose geographic variations are
utilized. Since Colorado is a region of diverse topography, it was necessary
to divide the state into approximately homogeneous regions for the develop-
ment of these relations. Consideration was given in this division to
topography and to geographical variations in the meteorological processes
wnich produce precipitation in the state.

A preferred
for regions

Precipitation results from the lifting of moist air. Air masses with
sufficient moisture to produce precipitation over Colorado come from either
the Gulf of Mexico or the Pacific Ocean. Over the eastern portion of the
state, the sonrce region is almost exclusively the Gulf of Mexico. In the
western portion of Colorado, the primary source of moisture ,is the Pacific
Ocean althougb the Gulf of Mexico is a source region in some weather situ-
ations. The importance of the Gulf of Mexico as a moisture source for the
western portion of the state increases during the warm season.

Most of the stations used in the development of
maps do not take snowfall observations on a regular basis. was
necessary to use indirect methods to determine frequency values that
be obtained if a series of rainfall only values were used for each station.
Attempts to determine the type of precipitation occurr1ng at individual
stations from maximum or minimum temperature at individual stations or
hourly temperature readings at nearby stations lead to large uncertainties
for many large precipitation amounts. Since most snowfall occurs during the
colder half year, a series containing only values for the Hay to October
season was compared with the series which was based upon rainfall only. The
two series were in good agreement with a very slight bias toward higher
values for the May to October season. This bias results from some large
ronounts in late October and early May occurring as snow and thus they were
excluded from the rainfall only series. This average difference is slight,
only about 4 percent, with no consistent preference toward higher elevations
or particular geographic regions.

the rainfall-frequency
It therefore
would

"

Relations For Annual Maps. Relations were
estimates of the 2-year 24-hour precipitation.
topography and consideration of the storm types that cause large 24-hour
precipitation amounts suggested separate consideration of the regions east
and west of the Continental Divide. East of the Continental Divide, Colorado
includes the eastern-facing slopes of the Rocky Mountains, where orography
plays an important part in producing precipitation, and the relatively flat
region east of approximately Denver and Pueblo, where the terrain plays

to provide
the

first developed
Examination of

4

Since these investigations indicate that frequency values computed
an annual series of rainfall values only would be different from those
composed of all precipitation values, two separate sets of precipitation-
frequency maps were developed. The first set consists of annual-maps based
on precipitation data from all months of the year without regard to the type
of precipitation; rain, rain and snow mixed, all snow, hail, etc. The
second series takes precipitation values from the months of May through
October. No attempt was made in this second series to differentiate between
types of precipitation occurring within these months but the investigations
mentioned in the preceeding paragraph indicate that these maps will approxi-
mate the values that would be obtained by using an annual series of rainfall
only. Since data for only part of the year were utilized, these maps have
been labeled with the appropriate probabilities rather than with a return
period in years

from

3