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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occurrence of the jet stream in a favorable location to influence large
precipitation amounts and other factors which cause precipitation. This
region was divided, therefore, into three approximately homogeneous sections
the Upper Colorado, Gunnison, and Green River Basin below the confluence
with the Yampa River; the San Juan River; and the Yampa and Green River
above the confluence with the Yampa River. TI1is la~t region includes a
portion of southwestern \-lyoming west of the Continental Divide. In the last
two regions satisfactory relationships between precipitation-frequency
values and topographic factors could not be developed. Relations (table 2)
were developed therefore between 2-year 24-hour precipitation and normal
annual precipitation and elevation.

..

In the application of these relations it is sometimes necessary to
extrapolate beyond the range of the data used in the development. When the
equation used in Arizona and New Mexico was applied to the higher elevations
and larger normal annual precipitation amounts on the steep slopes of the
San Juan Mountains unreasonably large values were estimated. A new relation-
ship was developed restricting the data used to the southerly facing slopes
of the mountains in Colorado. The equation used previously in other
portions of the San Juan Drainage was used in the valleys and on the lower
slopes of the mountains. The tw'o relations give approximately the same
results in the lower elevations where the normal annual precipitation
amounts are within the range of the development data.

In the Upper Colorado, Gunnison and Green River Basin below the con-
fluence with the Yampa River, the normal annual precipitation, distance from
the point to the confluence of the Colorado and San Juan Rivers, and the
distance along this line from the point of interest to the nearest barrier
more than 2000 feet above the elevation of the point provided the best
procedure for estimating the 2-year 24-hour precipitation values. These two
geographic factors do have a physical basis. Moisture for this region comes
primarily from the Pacific Ocean in a southwesterly flow. The distance to
the confluence of the C~lorado and Green Rivers then measures the d~inution
of moisture with increasing distance from the moisture source. The second
factor is a reflection of the decrease of precipitation behind mountaiu
ranges. As the distance from a barrier incr~ases there is less effect of
the downwind barrier. Within this region the slopes are such that points
\'lith an elevation within 2000 feet of the crest are not in the region of
precipitation depletion behind the mountain.

~

After satisfactory relationships were developed to estimate the 2-year
24-hour precipitation values for regions of sparse or no data, similar
relations were investigated for estimating the 100-year 24-hour amounts.
Little evidence was found of direct relations between normal annual precipi-
tation or topographic facto~s and lOO-year 24-hour p~ecipitation amounts.
The best relations developed related the 100-year 24-hour amounts to the
2-year 24-hour values and elevation. The same geographic regions used to
develop relations for estimating 2-year 24-hour values were used to develop
those (table 3) for estimating 100-year values

with records of adequate
24-hour precipitation

In eastern Colorado the number of stations
length to provide reliable estimates of the 2-year

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