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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14

General Discussion of the Maps. The values shown on the annual maps are
in general agreement with those of Technical Paper No. 40. Because of the
greater attention to physiographic features, some values on these maps differ
from those shown in Technical Paper No. 40. Only the major physiographic
features were considered in analyzing the maps. There are undoubtedly some
basins that are more sheltered or exposed than a generalized topography map
would indicate, and the map values may not be representative of the precipi-
tation regimes of such typical basins.

The rainfall-frequency maps for the May to October season were prepared
in a similar fashion. Equations shown in table 5 were used to estimate
values at a 5-minute latitude-longitude grid over Colorado. These values,
together with those computed from atation data, were used to analyze the 50-
and I-percent probability maps for the 6- and 24-hour durations. Inter-
mediate maps were prepared by computing values for the other return periods
utilizing the same relation as for the annual maps

Once the four "corner" (the 2- and 100-year 6- and 24-hour maps) maps
were completed, 6- to 24-hour ratios and 100- to 2-year ratios were computed
at each of the 5-minute latitude-longitude grid intersections, and the re-
sulting maps were analyzed. These maps were used to insure that there were
no "unreasonable" ratios. The next step was the reading of values at each
grid point on the "corner" maps, and the computation of the values for the
return periods between 2 and 100 years. The relation illustrated in figure 7
of Weather Bureau Technical Paper No. 40 (1961) was used for the computations.

The 2-year 6-hour amounts computed for the period 1949-63 together with
the estimated values from the equations shown in table 4, were used in the
development of the 2-year 6-hour map. The lOo-year 6-hour map was prepared
using the equations shown in table 4 and amounts computed from the entire
record for all recording-gage stations with adequate records.

Data used for the development of the 2-year 24-hour precipitation map
are from the l5-year period 1949-63. A generalized topographic map was used
as an underlay when the map was analyzed and the orientation, slope and
other topographic features were considered subjectively in the analysis. In
development of the 100-year 24-hour precipitation map data for the entire
period of available record was used for each station. In addition to the
computed frequency values for each station the length of record and the
width of the confidence band at each station was plotted so that individual
station values could be evaluated in the analysis.

precipitation values were obtained from physiographically adjusted normal
precipitation maps prepared by the ESSA-Weather Bureau Water Supply Forecast
Unit at Salt Lake City, Utah. The elevations, distances to and heights of
barriers, and distances to generalized contours and to the confluence of the
San Juan and Colorado Rivers were determined from 1:1,000,000 World Aero-
nautical Charts. Estimated values for each grid intersection were plotted
on the appropriate map together with values computed for each station.