Geoscience Reference
In-Depth Information
Fig. 3.4. Average annual pre-
cipitation, 1981-2010. From
Wyoming Water Resources
Data System, prepared with
PRiSM climate Group data,
oregon State University
cartography by christopher
nicholson.
slopes and basins are drier than might be expected,
because descending air masses become warmer, which
increases the potential for evaporation instead of pre-
cipitation. this is the well-known rainshadow effect.
Most Wyoming basins and plains are in a rainshadow,
with the relatively dry conditions ameliorated during
years when more than average amounts of moist air
from the Gulf of Mexico reach the plains and moun-
the Gulf of california (the Arizona Monsoon) causes
added summer precipitation in southern Wyoming, a
phenomenon that could account for the presence of
Gambel oak and the extensive groves of aspen on the
west slopes of the Sierra Madre (see chapter 10), where
they grow much as they do in parts of colorado and
Utah. Some years have high amounts of precipitation,
generally caused by flows of such air masses from the
south and southeast. other years are extremely dry,
occurring when these air masses are diverted to the
east. As with rain in the lowlands, snowfall in the
mountains varies greatly from one year to the next.
this variation in Wyoming is not well correlated with
el niño and La niña years, as seems to be true farther
to the south and north.
6
the form, duration, and intensity of precipitation
are as important as the amount. Rain that occurs dur-
ing warm periods evaporates quickly, sometimes even
before it wets the rooting zone of the soil. Heavier
rainstorms can cause infiltration down to 4 inches
or more, but plant water stress soon develops if sig-
nificant rainfall events are separated by more than
a week or two, as they usually are. in contrast, snow
commonly accumulates during winter. Some snow
will be sublimated, but there is the potential for
deeper infiltration into the soil when the snow melts,
at a time when less evaporation occurs because of
lower temperature. Such deep percolation and storage
enable plant growth well into the summer and seem
important for the survival of big sagebrush in most
intermountain basins. not surprisingly, big sagebrush
Search WWH ::
Custom Search