WSP07478

• File Number: 8272.600.60
• Description: Colorado River Basin Salinity Control Program - Basin Member State Info - Utah
• Basin: Colorado Mainstem
• Water Division: 5
• Date: 3/1/1982
• Title: Salt Uptake in Natural Channels Traversing Mancos Shales in the Price River Basin - Utah
• Water Supply Pro - Doc Type: Report/Study

terns characteristic of the hydrologic region
from the few events recorded annually at any
one gaged site in this arid climate. There-
fore the use of storms generated from region-
al data as being characteristic of and
equally likely to occur anywhere in the Price
River valley was judged superior to use of a
measured data sequence at a specific site.
Regional storm generation requires the
development of probability distributions for
principal storm pattern characteristics.
These probability distributions also provide
a potential for generating storm events of a
preselected frequency.

The five factors used in developing
these probability distributions were time of
year, probability of a storm occurring,
amount of precipitation, storm duration, and
precipitation distribution during the storm.
Time-of-year variability was handled by
developing separate distributions for the
other four variables for each month (April
through Octoher) and combining consecutive
months with like distributions where pos-
sible. These four variables were specifi-
cally handled as follows:

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1. For each month, the number of days
having measurable precipitation was deter-
mined and plotted as shown for June in Figure
5.4. A line fit by the Gumbel distribution

1)
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is shown plotted through these points.
Regressions were run for the number of days
of precipitation in a given month on the
number of days in the preceding month, but
low correlations led to dropping the number
of rainy days in the preceding month as a
significant variable.

2. Also for each month. the depths of
precipitation on days with storms were
plotted as shown for May in Figure 5.5. A
.line f it with a log-normal distribution is
shown.

3. Since storID duration varies with
storID depth, the storms were divided into
five depth ranges and durations were sepa-
rately plotted by range as shown in Figure
5.6.

4. A characteristic storm hyetograph
shape was developed from recording precipita-
tion gages in the Price and nearby Green
River Basins with the results shown in Figure
5.7. Use of this shape neglects the pos-
sibility of more than one storm occurring in
the same day.

The plotted information for these four
distributions for the corresponding mohth
provided the data used in Subroutine RAiN to

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.1

5

20

60

PROBABILITY (percent)

90

95

98

99

Figure 5.4. Gumbel distribution of days with precipitation in June. Weather data were taken
from U. S. Weather Bureau station records in the Price and San Rafael River Basins.

49