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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PRECIPITATION-FREQUENCY MAPS FOR COLORADO
Introduction. Precipitation-frequency maps for Colorado based on data
for the entire year (annual) and for the May to October season, were pre-
pared as part of a study of the variation of rainfall-frequency values with
physiographic factors in western United States. The maps will eventually be
published in reduced size as part of a Technical Report of the Environmental
Science Services Administration.
Because of the greater amount of 24-hour data, primary emphasis was
olaced on develonment of mans for thi s dur:lt':ion for hot-h :In Annu:ll ;:mil for
... . . ------,1. ---- -........ --&:- --- ----- ----------- --- ----- --- --------- ---- ---
the May to October maps. The maps for the 24-hour duration and frequency
data based on observed 6-hour precipitation amounts were used in constructing
the 6-hour maps.
~sic Data. Precipitation data were obtained from 276 recording gages
and 456 non-recording gages. Table 1 shows the distribution of these gages
by state. Data from states other than Colorado were used to develop index
relations for river basins which were at least partly within Colorado.
Table l. Distribution of precipitation gages by states.
Recording Non-recording
State gages gages
Colorado 85 176
Arizona 38 115
Idaho 0 2
Kansas 3 8
Nebraska 4 11
Nevada 33 0
New Hexico 71 101
Oklahoma 1 3
Texas 0 1
Utah 35 29
\.Jyoming 6 10
Total 276 456
Geographic variations in precipitation-frequency values are best detected if
differences resulting from varying periods of record are eliminated. In
1

precipitation
these stations

2

There are approximately 40 stations in and near Colorado where data on
amounts and daily snowfall values are both available. For
two series of data were tabulated 1) the maximum annual

Rain versus snow. Precipitation over Colorado occurs as rain, snow, or
as a mixture of rain and snow. Snowfall occurs primarily in the colder
6 months of the year from November to March. The importance of snowfall in
determining the precipitation-frequency values has been investigated
previously with inconclusive results. Since a large portion of Colorado
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fall it was desirable to investigate whether frequency amounts estimated
using all precipitation data \vithout regard to the character of the precipi-
tation were significantly different from those v.:hich \",ould be obtained from
using only rainfall values

Although 15 years provide reasonable estimates for the 2-year
return period, this length record is too short to provide reliable esti-
mates for the 100-year return period. In a l5-year period some stations
will have precipitation amounts that have a return period of 50 or 100 years
or longer. Other stations in the same 15-year period will not experience an
amount as large as a true IS-year value. Thus when values for the longer
return periods are estimated from a l5-year sample, the dispersion from
station to station becomes quite large. To increase the reliability of the
estimate for the longer return periods the data collection was extended to
obtain as long a record as possible for each station without violation of
the 100-foot or 5-mile criteria.

of data
of

the collection of data from many stations there are some stations
for which the data are missing or are of questionahle reliability for all or
part of one or more years. In this study the data for each such case
individually evaluated in order to keep spurious data to a minimum
instance, the maximum value recorded for a year was used even
months of data were missing if the value appeared reasonable
with other years. This could result in an underestimate
values but it is felt that such errors are small and of

though few
\oJhen compared
of the accepted
little consequence

order to maintain homogeneity in the basic data, one period of
chosen, and the data for that period were used for determining
graphic variation of the 2-year precipitation-frequency amounts.
l5--year period 1949-1963 was selected as the base period for determining
2-year precipitation-frequency values. 'l'his period was considered suf-
ficiently long to adequately define the 2-year return period precipitation
amount and still provide a reasonable number of stations. To provide more
complete geographic coverage of data, however, it was necessary to use
stations that had less than a complete IS-year period of record. About
one-third of the stations had less than 15 years of record, but none had
less than 10 years of data. No station was used if a move involved more
than a 100-foot change in elevation or a change in lateral distance of
5 miles. If a change in site resulted in an exposure incompatible with the
old location, e.g., from one side of a barrier to the other, the station ,vas
not used even though only slight changes in elevation or distance might have
been involved.
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