WMOD00593

• Contract/Permit #: 14-06-D-7052
• Title: Ecological Impacts of Snowpack Augmentation in the San Juan Mountains, Colorado
• Date: 3/1/1976
• State: CO
• Weather Modification - Doc Type: Report

Clearly, to obtain more information for this period
will require a study over a larger area encompassing
more stations; with our present state of knowledge
such conclusions as have been drawn should be con-
sidered tentative. However, an examination of the
temperature record, which appears more complete, may
shed more light on the problem, in view of the
generally inverse relationship noted between 20th
century precipitation and temperature records
(see section on "Analysis of temperature data" in
this report).

Precipitation size-frequency analysis

As mentioned previously, an interesting aspect of
climatic variation in the area is the question of
precipitation size-frequencies changing over time.
An analysis of the trends in size-frequency dis-
tributions has been performed using daily precipi-
tation data for Durango (1896-1970) and Silverton
(1906-1970). Figure 15 shows the change in frequency

120
110
100
90
80
;;-70
;60
&50
..
'::40
30
20
18961900 10

-x

30

40

50

60 1970

20

Figure 15. Frequency of precipitation-size class
0.05-1.25 cm in Durango. Annual.

of precipitation events resulting in daily totals
of less than 12.5 mm (.5 inches) at Durango. Pre-
cipitation events of less than 12.5 mm (.5 inches)
were markedly below normal around the turn of the
century, corresponding to the major dry period at
this time, followed by a period of above average
frequency over the subsequent 10 years. Since the
early 1920's, however, no progressive changes and
no major periods of above-or below-average frequency
of occurrence have appeared.

For precipitation size classes greater than 12.5 mm
(.5 inches), frequencies were on the whole above
average prior to the late 1920's or early 1930's
(particularly in winter months) and below average
for the subsequent 20 years (Figures 16 and 17).
It would appear that the major precipitation minima
and maxima in the record are not the result of a
change in the frequency of anyone precipitation size
class, but rather. the integrated result of changes
in the frequency of precipitation events of all
sizes. However, it is clear that small changes in
the frequency of large precipitation events can
greatly influence the overall annual totals of
precipitation.

ANALYSIS OF TEMPERATURE DATA, SAN 'JUAN AREA

Running Mean Analysis

Long-term temperature records for southern Colorado
are less common than long-term precipitation
records. However, the temperature record for the
Fort Massachusetts/Fort Garland data appears to have

54

7
6
>>5
o
c: 4
CII
~ 3
tT
CII 2
U:1

o
1890 1900 10

20

30

40

50

60 1970

Figure 16.
19
18
17
16
15
14
13
>>12
gll
CII
&10
~9
I.L.
8
7
6
5
4
3
2
1900 10
Figure 17.

Frequency of precipitation-size
classes > 2.5 cm in Durango. Annual.

- --X

20

30

40

50

60

1970

Frequency of precipitation-size
classes 1-2.5 cm in Silverton. Annual.

been more carefully kept than the precipitation
record and thus a valuable analysis of the data for
the latter half of the 19th century is possible.
For the period 1889-1900, virtually no records are
available for the area. For this reason, 19th
century stations operative in adjacent states were
used to supplement the data available in Colorado
(e.g. Fort Union, New Mexico, Figure 18).

Mean annual temperatures .rose markedly during the
late 1850's and 1860's to a high around 1867.
Temperatures then fell slowly to 1890, and possibly
over the subsequent 10 years (here the record is
absent), with a continuing decline to approximately
1930. Again, seasonal values shed more light on
the fluctuations.

An examination of the seasonal temperature record
by means of 9 year weighted running means
shows that individual peaks and troughs in the
records do not always correspond. Indeed in some
cases one station may show a marked peak in the
record while another may show a major trough
(Figure 19). Such variations are probably partly
related to the topographic characteristics of each