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

r-'-
,\ ;'
" ,
I \ ,I
/ \ /-"\ I
*\ .................... "''// \/ \ /
I I V
* I'*/V*
I ~ ~ 1
* \ *-' V
* *-*/

30

25

g20
>-

~
~15

III
CIl
~IO
c:

5

1856 58 60 62 64 66

68 70 72 74

Figure 2.

75

65
55 ~
c
CIl
45 >-

~
CIl
35 a.

25 E
u

15

5

76 78

80 82 84 86 88

90 1892

Nineteenth century annual precipitation data for Fort Massachusetts (crossed line), Fort Garland
(solid line), and Fort Lewis (dashed and dotted line). Starred lines indicate some months missing
or data considered unreliable. Hermosa (dotted line).

/ ~-~
/---------------- --.. "'" ",.-------..........

...... - ,-----..-----;......"'..--' ,------
- -- -/ ,...----.----,,'...-..-...... "

;/.--..~~~~~-~~~:'_...
-.-.-./
"".
.........-.
,1'......-.....-.

.I
.-.-/

1.0
r
0.9
0.8
0.7
0.6
+0.5
0.4
0.3
0.2
0.1

1914-38

1922-46

1926-50

Figure 3. Annual correlations with Durango - 25 year periods (1914-38) to (1944-68).

1934-58

1918-42

Tree ring evidence also suggests that contemporary
anomalous periods of opposite sign tend to occur in
the northern Rocky Mountain, states compared to the
southern Rocky Mountain states. Schulman (1954),
for example, notes that the dry period at the turn
of the century in the southwestern U.S.

is less severe in the Missouri Basin and in
fact is replaced still further north by what
appears to be the greatest maximum in three
centuries, at latitude 510N in Banff National
Park.

Keen (1937) notes poor growth of annual increments
associated with low precipitation between 1914 and
1933 in eastern Oregon, whereas precipitation was
generally above normal in the Southwest for much of
the same period. This inverse association between
precipitation in the northern and southern Rocky
Mountain states, noted also by Sellers (1968), is
presumably connected with the relative position of
storm tracks across the western U.S. and further
investigation of this may prove instructive (see
section "Dendroclimatology", below).

-""'-----------""'-

r-'-'''"'\
/ \
-.-.............----.-.. '\._..~

-Fort Lewis
-.- Telluride

---Ignacio
..m_ Silverton

1930-54

1938-62 1942-66 1944
-68

Thomas (1959) recognizes three drought periods in
the southwestern U.S. over the previou~ 70 years
(Figure 4). From run-off data for the area of
southwestern Utah, the San Juan Mountains, and
the Upper Colorado River Basin, he notes dry periods
from 1893-1904, 1931-1940, and 1943-1956. However,
he also suggests that "Pacific border" areas, in
particular southern and central California,
experienced droughts in 1894-1904, 1924-1934, and
1946-1957. The latter periods fit San Juan
precipitation data most closely.

Sellers (1968) in his study of precipitation trends
in Arizona and western New Mexico found a definite
downward trend in precipitation post-1905, averaging
approximately 2.54 cm (one inch) every 30 years.
This was due almost entirely to a decrease in
winter precipitation. Using running mean analysis,
a 25 percent drop in mean annual precipitation was
noted between 1921-1926 and again in 1954-1959.
Seller's figures are largely supported by Von
Eschen's study (1958) of four long-term climatologi-
cal stations in New Mexico. This study indicates
at all stations since approximately 1915, a downward
trend in annual precipitation associated with a
rising temperature trend, again most marked in the
winter months.

45