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Average annual precipitation is important in forestry, agriculture, urban and regional planning, and water resources. An understanding of average precipitation received regionally in Wyoming can aid in understanding potential runoff in streams and moisture contents in soils, as well as in knowing areas where drought is a common problem (Plate 7A).
Mean annual precipitation varies widely across Wyoming depending largely on orographic (mountain) influences. The wettest parts of the state which receive more than 60 inches of precipitation per year (dark blue) are in the high mountains above about 10,000 feet elevation, such as in the Medicine Bow, Absaroka, Teton, and Washakie mountains, and parts of the Yellowstone Plateau and Salt River Range in the west. Surrounding these at lower elevations, are areas (light blue) that receive 40 to 60 inches of precipitation, covering large portions of the Absaroka, Washakie, the Teton, and Gros Ventre Mountains, Wind River mountains, and the Salt River Range.
Areas mapped in dark green receive between 20 and 40 inches of precipitation annually, including large areas of the Yellowstone Plateau and around Jackson Hole, as well as parts of the Medicine Bow Mountains and the Sierra Madre. Many foothill areas along the flanks of major mountains receive 16 to 20 inches of precipitation annually (light green). Areas shown in yellow, generally at lower elevations surrounding the mountains and in the basins, receive only 12 to 16 inches. At lower elevations, areas mapped in orange receive a meager 8 to 12 inches. Areas shown in red receive less than 8 inches and are true (but cold) deserts. Within the last category are the Washakie (Red Desert) and Bighorn basins. The driest portions of Wyoming are the basins in the central part of the state, including the Bighorn, Washakie, Green River, Wind River, Laramie, and similar small basins or adjacent areas.
The high mountains of the western part of the state that reach above timberline receive more precipitation than comparable areas of the east, such as the Bighorn Mountains which receive only slightly more than 30 inches per year. This indicates that moisture in these high elevations has a westerly source. High western mountains acquire much of their moisture from east-moving storm systems, whereas high mountains in the east receive only remnant moisture from these storms. The Medicine Bow Mountains and Sierra Madre in the southeast, however, are anomalous in this respect. Their highest parts receive more than 60 inches of precipitation per year, but much of this moisture results from strong westerly air flow characteristic of the southern part of the state in winter which builds large orographic clouds over the range, causing heavy snowfall. In contrast, mountains of the north and west do not frequently experience the strong westerly winds common to the south.
Mountain areas that do not extend above timberline generally receive less moisture (dark and light green) than those which reach above timberline (dark and light blue). In the former are the Laramie Range from Casper to the Colorado border in the southeast, the foothills areas of the Black Hills in the northeast, much of the Bighorn Mountains, the foothill areas of the Uintas in the southwest, and sections of the Wind River Mountains.
Low elevations are generally the driest and lie within rainshadow positions where surrounding mountains block or deplete much moisture from storms. Descending winds from the mountains related to east-moving storms are heated in the rainshadows, resulting in a drying effect with little release of rain or snow in the low elevations.
The greatest mean precipitation in the highest
elevations generally occurs in the west and the greatest mean precipitation
in the low elevations occurs in the east, covering large areas of the Powder
River Basin and the Black Hills and the High Plains east of the Laramie
Range near Cheyenne. Moisture in the east comes from the monsoonal flow
of moisture from the Gulf of Mexico in summer and from intensification
of east-moving storms over the Great Plains in spring, frequently resulting
in blizzards. Both the Laramie Range and Bighorn Mountains, in addition
to the Casper arch that connects these two uplifts, commonly act to confine
moisture from the Gulf of Mexico to the east of these uplifts. In summer,
monsoonal flow from the Gulf of Mexico aids in the development of intense
thunderstorms east of these Liplifts, with lower frequency and intensity
to the west. The thunderstorms are commonly accompanied by hail and sometimes
tornadoes. Similarly, east-moving springstorms over the Rockies can intensify
once they reach east of the Bighorn Mountains and Laramie Range where they
gain latent energy from moisture fed into them out of the Gulf of Mexico.
Therefore, severe storms are common in eastern Wyoming and less common
in the west. In contrast to the western part of the state which relies
heavily on a westerly source for much of its moisture, the eastern part
of Wyoming depends greatly on moisture from the Gulf of Mexico. Consequently,
the driest basins of Wyoming, those under the strongest rainshadow influence,
are west of the Laramie Range, Casper Arch, and the Bighorn Mountains.
The largest concentration of glaciers in the American Rocky Mountains occurs in the Wind River Range of Wyoming, covering an area of more than 17 square miles. However, the present-day ice cover is a small remnant of Pleistocene glaciation (Plate 7B). Gannett and Dinwoody glaciers are two of the larger glaciers. Between 1958 and 1983, however, they lost an area-weighted thickness of 61 and 77 feet, respectively. This amounts to a combined loss in water equivalent of nearly 99,000 acre-feet, or 12.3 percent of the annual June-October flow to Dinwoody Creek. Approximately the same volume remains in Dinwoody Glacier in 1989 as was lost in the period 1958-1983. If the glaciers continue to downwaste, the implications for water supplies will be far-reaching. Irrigators on the Wind River Indian Reservation and further downstream will have to plan for a reduction in late summer runoff when glacier melt augments snowmelt. This effect will be especially severe in years of low snowmelt when glacier melt assumes added importance.
Plate 7A was digitized from Mariner (1986) and is published with permission from the University of Nebraska Press. Plate 7B was digitized from Roberts (1989) and is published with her permission.
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