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

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West of the Continental Divide the primary moisture source is the
Pacific Ocean rather than the Gulf of Mexico. In addition, many of the
characteristics of storms producing large point values of precipitation
during early spring, fall and winter over western Colorado are different
f~Qm those for eastern Color~do. There are differences also thoughout the
western portion of the state caused by differences in distance from moisture
source, height and number of barriers depleting the moisture, frequency of

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The region from the crest of the Continental Divide to the generalized
7000-foot contour includes the northern portion of the Rio Grande Basin.
Within the region where both relations (table 2) were applicable, the results
they yielded were in good agreement.

West of the generalized 7000-foot contour and east of the Continental
Divide lies a region of considerable variation in topography. Large differ-
ences in elevation, slope and orientation of the terrain, and other
topographic factors occur within short distances. In this region, station
values are usually representative of only a small area surrounding the
station. The slopes of the mountain ranges in this region generally face
eastward and present a relatively solid north-south barrier to moisture-
bearing winds from the Gulf of Mexico. The northern limit of this region is
in southern Wyoming, at the northern edge of the South Platte River Basin.
North of this boundary the general orientation of the Rocky Mountains is
more northeast than east. The southern boundary is at the southern edge of
the Sangre de Cristo Mountains. South of this line the mountain ranges are
less massive and present less of a barrier to the flow of moist air. The
primary mechanisms producing precipitation are relatively the same throughout
this region, and the Gulf of Mexico is the primary source of moisture. The
factors found most important in estimating the 2-year 24-hour precipitation
for a point were the distance from the point to the generalized 7000-foot
contour and the sum of the highest elevations to the east, southeast and
south of the point (a total of 3 elevations). These are the directions from
'~hich moisture-bearing winds enter this region. These selected elevations
were nearly always within SO miles of the point, and an elevation was not
chosen if the contour did not have at least a lateral extent of 10 miles.
In others, elevations of isolated peaks were excluded The final equation
developed together with the number of stations used, the correlation
coefficients, and the standard errors of estimate can be found in table 2.

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a much less important role. Though it is difficult to define a specific
elevation where the topography becomes important, it can be approximated in
this region by a generalized 70GO-foot contour. East of this boundary the
terrain is for the most part a gradual upslope from the east and southeast,
and there are few significant differences in elevation and slope within
short distances. Precipitation in this region results primarily from metco-
rological processes, and occurrences are more randomly distributed than in
regions with rugged topography. Variations betwecn adjoining stations in
these regions of relatively level topography are usually much less than in
regions with rugged topography, and point rainfall data usually have more
areal representativity~ The observed rainfall data were considered adequate
for delineating the geographic distribution of the 2-year 24-hour precipi-
tation values, and no relationships with physiographic factors were developed

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