WMOD00277

• Contract/Permit #: 14-06-D-6467
• Title: An Operational Adaptation Program for the Colorado River Basin
• Prepared By: Lewis O. Grant, Chappell, Crow, Mielke Jr., Rasmussen, Shobe, Stockwell, Wykstra
• Date: 10/1/1969
• State: CO
• Country: United States
• Weather Modification - Doc Type: Report

area. These are both stations that
are frequently downwind of the
mountain barrier during substantial
snowfall episodes. At an upwind
station at Durango (Figure 32), the
pattern is considerably altered with
a diurnal maximum precipitation
occurring during mid-morning a:Cld a
diurnal minimum during the late
afternoon or early evening. Despite
this change in pattern it can still be
noted from Figure 32, that the
diurnal maxin1lffi is abcut twice that
occurring during the minimum. The
diurnal pattern of precipitation i:3
shifted to even later in the day for
the mountain stations in the San
Juan area where a late afternoon
maximum can be observed,
Figure 33 shows this effect for
Silverton, Telluride, and Mesa
Verde. The maxima are also clea.r
for these stations and represents a
value some one-and-one-half to
twice as g rea t as that experienced
at hours of lowest precipitation
frequency.

hours with precipitation was not
measured, primarily due to clock
stoppage. After an adjustment for
this missing data is made, it is
estimated that the Berthoud Pass
area receives about 600 hours of
precipitation of . 01 inch per hour or
greater. It could be expected that
the hourly precipitation would be
.05 inch per hour or less on 591 of
these hours, .06 to .10 inch per
hour on 8 hours and greater than
. 10 inch per hour on 1 hour. The
number of hours when there is
precipitation in the area is con-
siderably greater than when there is
precipitation at a single station. It
is, therefore, estimated that there
are greater than 700 to 800 hours of
precipitation at intensities greater
than .01 inch per hour in the Upper
Basin of the Colorado River. More
than 96% of these can be expected to
be at an intensity of less than. 05 inch
per hour.

In the 20-year period November
1948 to April 1968, Telluride in the
San Juan Mountains had an average
of 248 hours per year when precipi-
tation at intensities of .05 inch per
hour or less was recorded, 13, hours
when the intensity was. 06 to .10 inch
per hour and 3 hours when the
intensity was greater than . 10 inch
per hour. This gives a total of 264
hours when distinct hourly precipita-
tion amounts were recorded. Many
hours of data were lost, however,
due to various causes, clock
stoppage and gage bridging being two
of the most important., The average
winter precipitation from cumulative
daily readings at Telluride during
the 1931-60 interval was 11. 82 inches.
The average precipitation given by
the incomplete hourly record,
however, was only 6.08 inches. If
a proportionate adjustment is made
to the number of hours of precipita-
tion in accordance with the percent
of total hourly precipitation recorded
to the total daily precipitation '
measured it can be estimated that
there is an average of 514 hours of
precipitation each winter at
Telluride. It could be expected that
the precipitation would be .05 inch
per hour or less for 484 hours,
.05 to . 10 inch during 25 hours, and
greater than. 10 inch on 5 hours.
Some stations, Wolf Creek Pass
for example, receive more than
twice as much precipitation on the
average during a winter season as
Telluride (24.87 inche-s to 11. 82
inches). Furthermore, the num bel' of

,
Figure 34 shows that the relati'~e
percent of daily snowfall occurring
at the respect.ive hours of the day
follows the same general diurnal
trend observed for the frequency of
snowfall. There is a broad creSi; of
larger total amounts between 0100 and
0600 at Wagonwheel Gap 3N. At
Ouray two peaks can be identified
with noon being the lowest hour of the
day.

It is clear that the diurnal varia.tion
of precipitation should be considered
in experimental design and in the
conduct of operations in the San Juan
area as well as in the other portions
of the Colorado River Basin. The
variation in the time of the diurnal
peaks and troughs in relation to
terrain needs to have special con-
sideration in the case of the San ~ruan
area. It is expected that studies in
progress will increasingly define the
precipitation process'es in these
basins through an improved under-
standing of the diurnal variation of
precipitation.

(3) Number of hours of precipitation
The hourly precipiation gage at
Berthoud Pass in the Upper Basin of
the Colorado River recorded an
average of 503 hours of precipitation,
November - April, 1965-1968. This
consisted on the average of 495 hours
at . 05 inch per hour or less, 7
hours at from. 06 to . 10 inch per
hour and 1 hour at greater than
. 10 inch. Some 10% to 20% of th~

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