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

Table 1. Hours of observation and methods of computing averages

Observations Time Averages computed from Dates
U.S. Army Surgeons 1847-1891
sunrise, 0900, 1500, 2100 local time ~(sunrise+0900+1500+21) to Dee 1854
0700, 1400, 2100 local time 1/3(0700+1400+2100) Jan 1855-Jun 1888
0700, 2000, min, max 75th meridian ~(min+max), civil day of 75,th Jul 1888-Dec 1891
time meridian
U.S. Signal Service 1870-1891
0700, 1400, 2100, min, max local time ~(0700+1400+2100+2100) Jan 1871-1880
0700, 1500, 2300, min, max 75th meridian 1/8(0700+1500+2300) Jul 1880-Jun 1887
time
0700, 1500, 2200, min, max 75th meridian 1/3(0700+1500+2200) Jul 1887-Jun 1888
time
0800, 2000, min, max 75th meridian ~(min+max), civil day of 75th Jul 1888-Dec 1891
time meridian
U.S. Weather Bureau 1891-Present
0800, 2000, min, max 75th meridian ~(min+max), civil day of 75th Jan 1891-Dec 1904
time meridian
min, max local standard ~(min+max), civil day Jan 1905-
time

1 +-----..
2------------+

34 ~ 4 . 4 ~
44 ~ +----+ 4 .
5. ~
I , I I I I I I I
1890 1900 1910 1920 1930 1940 1950 1960 1970
.. Schulman (1954) - Northern Arizona
2. Sellers (1960) - Arizona and western New Mexico

3. Thomas (1959) - Southwestern Utah, Son Juan
Mountains and Upper Colorado
River Basin

4. Thomas (1959) - Southern and central California
("Pacific border" areas)

5. Keen (1937) - Western Oregon

Figure 4. Comparison of drought periods noted in
the western U.S. by a selected number of
authors.

Fritts (1965) has computed ten-year relative depar-
tures of tree ring indices for overlapping ten-year
periods from 1951 to 1920. These are mapped for the
western states of the U.S.; areas of positive de-
parture are considered to represent "moist" periods
(high precipitation and low temperatures) whereas
areas of negative departure are considered to repre-
sent "dry" periods (low precipitation and high
temperatures). The relative departures 1855-1935
have been abstracted from Fritts' maps for south-
western Colorado and plotted in Figure 5 for the
mid-point of each ten-year period. The data
indicate that on the basis of tree growth, there
was a trend towards low precipitation and high

~ +0.8
::>
~+O,6
0.
~ +0.4
~ +0.2
'0 0
Q;
a:: -0.2

IIMoist"

(High precipitation and
low temperatures)

~
~ -0.4

:ii-O.6
I-
-0.8
18556065 70 75 8085 90 9519000510 15 20 25 30 1935
. Mid-point of Ten Year Period

"Dry"

(Low precipitation and
high temperatures)

Figure 5. Ten year relative departures of tree
ring indices, San Juan area, southwest
Colorado (after Fritts 1965).

temperatures ("dry") from 1855 until approximately
1900 when there was a marked change towards high
precipitation and low temperatures. This
culminated around 1915.

One of the most interesting studies of historical
climatic change in the Southwest is that of
Leopold (1951). Leopold examines precipitation
size-frequency classes for long-term stations in
New Mexico and finds the frequency of small rains
(less than 12.7 mID (0.5 inches)) increased con-
siderably from the turn of the century to the 1930's
or later. Thus, not only have annual and seasonal
totals changed over time but the character of
precipitation events has altered, a fact altogether
obscured in the monthly totals. The change in

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