WSP00036

• File Number: 8200.750
• Description: San Juan River General
• State: CO
• Basin: San Juan/Dolores
• Water Division: 7
• Date: 3/1/1975
• Author: BOR
• Title: San Juan Ecology Project - Interim Progress Report - March 1975
• Water Supply Pro - Doc Type: Report/Study

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0015'73

4. EFFECTS OF SNOWPACK VARIATION ON THE FOREST ECOSYSTEMS

4.1 INTRODUCTION (Dr. Harold Steinhoff, Team Leader)

Changes of biomass of trees, forage, elk, and small
mammals in relation to snowpack variation, continue
as the primary concern of research efforts in the for-
est ecosystems of the San Juans. Often indirect ap-
proaches must be used, such as the study of phenology
to determine when growth begins, the measurement of
moisture stress as an indicator of growth rates, or
the change in winter distribution of elk. Details
of the research approach and findings to date for each
of the ecosystem components mentioned above are found
in the project reports which follow. The general
rationale for study and the nature of the spruce-fir
and aspen ecosystems which are of primary concern are
described in the first Interim Progress Report of
December, 1971. Data on characteristics of intensive
study sites, and summarized climate variables of the
general area are found in the second Interim Progress
Report of March 1973.

4.1.1 Study Areas

Description of study areas and representative maps of
plot location are found in the second Interim Report.

4.1.2 Microclimatic Data and Analvses,

The year-week has been chosen as the compilation
interval for all climatic data in the forest ecosys-
tems. Daily data are available but are too detailed
for most ecologic uses. MOnthly summaries obscure
too much. Temperature sums above OOC are accumlated
beginning each January 1. ~recipitation is accumlated
each October 1.

A complete data set was needed for the cumulative
temperatures and precipitation analyses. Therefore
missing data were recovered by prediction from the
most comparable weather station. Correlations
of year-weekly precipitation among the six intensive
study sites, six nearby Western Scientific Services
sites, and four adjacent U.S. Weather Service sites
are shown in Table 1.

Multiple stepwise regressions last year revealed that
usually only the ane best correlated station was
necessary to produce a prediction equation. Little
improvement was noted in adding multiple stations.
Therefore prediction equations were developed for
each intensive site, using the stations which correl-
ated be~with it, (Table 2).

Figure 1 represents an attempt to picture the yearly
climates of Durango graphically. Deviations from the
mean of monthly precipitation are plotted along the
x-axis and of monthly mean temperature along the
y-axis. The resultant patterns shows how each month
related to the long-term mean in the combination of
temperature and precipitation. Winter months such
as those in 1972-73 all fall in the lower right
quadrant, an indication of high precipitation and low
temperature. The weather in 1970-71 was consistently
closer to "normal", with all months clustered fairly
close to the intersection of the mean precipitation
and temperature line.

4.1.3 Sununary

1972-73 was characterized by unusually heavy rains
which saturated the soil in October just before
freeze-up, and by snow depths 150 to 200% of normal.
Temperatures followed an average regime. Snowmelt
was delayed by from two to six weeks by the deeper
snowpacks.

Table 1. Correlation of weekly precipitation among 17 weather station sites near Durango and Wolf Creek,
June 1971 to December 1973.

Intensive Study Sites
Top Little Bear Middle Wolf Creek

Western Sei. Servo
Missionary Rid.lLEl. W"lf
Lime Kroeger -Wallace Sk

U.S. Weather Service
~:fiI~ay W.Hiway Dur- Silver-Ta- Val- Pagosa

Park Fork Mesa Park Lake ~ ton comaDam ~
M35 M32S M32A M29 W31 W33 JiiI KGI- KG 2 KP2 KP3 K03 Drno Sltn Tcma Val asS
KG 3 Dam
M35 1.00 0.93 0.90 0.77 0.76 0.88 0.94 0.91' 0.85 0.8 0.84 0.72 0.78 0.76 0.85 0.77 0.78
M32S 1.00 0.91 0.72 0.74 0.90 0.94 0.95 0.78 0.78 0.78 0.78 0.78 0.83 0.89 0.7 0.76
H32A 1.00 0.70 0.72 0.90 0.90 0.97 0.74 0.77 0.76 0.83 0.82 0.86 0.89 0.78 0.76
M29 1.00 0.90 0.68 0.68 0.74 0.92 0.89 0.92 0.70 0.71 0.66 0.74 0.76' 0.83
W31 1.00 0.74 0.73 0.78 0.91 0.93 0.92 0.70 0.69 0.71 0.78 0.75i 0.85
W33 1.00 0.91 0.89 0.80 0.80 0.79 0.68 0.79 0.75 0.84 0.72' 0.77
JHI 1.00 0.90 0.78 0.78 0.78 0.72 0.75 0.80 0.85 0.75' 0.76
KGI 1.00 0.79 0.79 0.79 0.84 0.83 0.88 0.91 0.84 0.81
KG' &3 1.00 0.96 0.95 0.67 0.66 0.65 0.78 0.78 0.87
KP2 1.00 0.94 0.74 0.70 0.69 0.83 0.83: 0.93
KP3 1.00 0.70 0.68 0.68 0.82 0.80 0.87
K03 1.00 0.73 0.84 0.86 0.80 0.74
Drn 0 1.00 0.78 0.80 0.72 0.72
Sltn 1.00 0.85 0.74 0.76
Toms 1.00 0.85; 0.84
ValDm , , 1.00.0.81
Pa S I 1.00

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