Geoscience Reference
In-Depth Information
that moisture falls in the basin. the annual precipita-
tion at Laramie is only about 11 inches, most of which
falls in April and May. the mountains and basins to the
west tend to receive a higher proportion of the annual
precipitation in the winter, usually more than half. As a
result, sagebrush steppe is more common in the basins
to the west (see chapter 7).
even with low precipitation, the Laramie Basin
was hospitable to both indigenous tribes and euro-
Americans because of snowfall in the adjacent moun-
tains, which fed the Big and Little Laramie rivers in
addition to numerous springs on the east side of the
basin. Low-lying areas had numerous lakes and ponds,
such as in the vicinity of Lake Hattie and in the Sand
dant. notably, creeks flowing into the basin from the
drier Laramie Range are ephemeral and provide little
surface water.
90
4.0
3.2
66
2.4
43
1.6
32
19
0.8
-5
0.0
JFM
AMJJASOND
Fig. 17.6. Average monthly precipitation (bars) and average
high and low temperatures (curves) at the airport in Laramie
(1981-2010). comparable graphs from other weather stations
can be seen at
http://www.usclimatedata.com.
the Laramie Basin is also windy at times, with a
mean annual velocity estimated at about 12 miles/
hour. For comparison, mean annual wind velocities
for Missoula and Denver are about 6 and 9 miles/hour,
respectively. Snow drifting commonly occurs in lee-
ward locations (see also chapter 3), which favors the
development of shrublands dominated by big sage-
brush. Grasslands are found on surfaces that usually
are blown free of snow.
the basin is semi-arid for two primary reasons.
First, evapotranspiration is more rapid at high eleva-
tions, especially when the wind is blowing. Second,
annual precipitation is quite low because of mountain
ranges to the east and west that cause rainshadow on
both sides. Several times each season it is common
to see storm clouds on the horizons that never reach
the basin. those on the east are typically produced by
upslope storms, created when a low-pressure cell passes
through southern colorado or northern new Mexico.
At such times, moist air from the Gulf of Mexico cir-
culates counterclockwise onto the east slope of the
Rocky Mountains. As the air rises, the air cools and
condensation occurs—the upslope conditions men-
tioned by weather forecasters. the precipitation that
falls is mostly on the east side of the mountains. to the
west of the basin, the air rising over the Medicine Bow
Mountains generally flows from the Pacific ocean and,
having already passed over several mountain ranges,
is comparatively dry. thus, only a small proportion of
Vegetation Patterns
Forests and woodlands are conspicuous as one crosses
mountain passes and descends into the Laramie Basin
from the east, south, and west, whereas shrublands are
more likely to be seen when entering at a lower eleva-
tion from the north. However, mixed-grass prairie cov-
ers about two-thirds of the basin floor (fig. 17.7). Plant
cover is sparse on windswept knolls, consisting of little
more than a few cushion plants and arid-land sedges
(threadleaf sedge and needleleaf sedge). in more favor-
able environments, the vegetation is dense, dominated
by blue grama, western wheatgrass, Sandberg bluegrass,
junegrass, needle-and-thread grass, fringed sagewort,
sand lily, Hood's phlox, and many other species (figs.
17.8-17.10). Big sagebrush, mostly Wyoming big sage-
brush, usually grows where snow accumulates on the
leeward side of ridges. Less than 10 percent of the basin
has sagebrush-dominated shrublands, and much of that
Mountain-mahogany grows on exposed bedrock, and
some mountain big sagebrush is found where the soil
is deeper and more snow accumulates (see chapter 7).
in general, plant growth and species composition vary
largely as a function of wind exposure; the potential
for snow drifting; and the depth, salinity, and water-
holding capacity of the soil.
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