Do
you have any information on the hydrologic cycle specific to Utah?
by Mark Milligan
What
is the hydrologic cycle?
Before I address the peculiarities of Utah, what is the hydrologic
cycle?
The hydrologic cycle is the continuous circulation of water among
the oceans, continents, and atmosphere. It can be thought of as
a machine endlessly in motion, powered by the sun's energy and assisted
by gravity.
Essentially the same water has been circulating in this machine
since the first clouds formed and the first rains fell on our earth;
very little is ever lost or gained.
The continents contain about 2.5 percent of our planet's water,
mainly in the polar ice caps and ground water. The atmosphere accounts
for only about 0.0001 percent. The oceans hold the remaining 97.5
percent of our planet's water.
About 90 percent of the water entering oceans is in the form of
precipitation - rain and snow falling directly on the oceans. Runoff
from the land accounts for the remaining 10 percent. The only significant
outlet for ocean water is evaporation via the sun's energy (heat).
On average, a molecule of water will remain in the oceans about
3,000 years before being transferred back to the atmosphere by evaporation.
Water evaporated into the atmosphere stays there an average of
only 10 days before being dropped as rain, snow, or condensation
back into the oceans or onto the land. In general, water precipitated
onto land can (1) infiltrate the ground, becoming ground water that
slowly flows to the sea, (2) flow across the surface, entering a
system of streams and lakes which eventually flows to the sea, or
(3) become glacial ice, eventually flowing to the sea.
Of course, this is a simple description of a complex system and
not all water travels completely through the cycle every time. Some
water evaporates from streams and lakes, and even glacial ice, before
reaching the sea, and plants use a relatively large amount of water
and transfer it directly back to the atmosphere by a process called
transpiration. While the system does not lose or gain water, the
distribution in various parts of the cycle over different areas
of the globe does change, causing floods and droughts.
Great Salt Lake's closed basin
Along the Wasatch Front and for most of northwestern Utah, a special
circumstance exists where the surface runoff and ground-water components
of the hydrologic cycle cannot flow to the ocean, but are limited
to Great Salt Lake's closed basin.
Storm tracks bring us summer rainfall and winter snowfall all
the way from the Pacific Ocean, but this precipitation cannot flow
back to the Pacific Ocean. Mountains and other topographic highlands
contain the water within the basin (a sub-cycle within the larger
hydrologic cycle).
For a molecule of water to leave this basin, it must be evaporated
and carried in clouds beyond the Wasatch Range, where it might fall
as rain or snow, eventually flow into the Colorado River, and, with
luck, on to the Pacific Ocean.
A Wasatch Front hydrologic sub-cycle contains many complexities
and feedback loops. Great Salt Lake, through a process called "lake
effect," can increase precipitation along the Wasatch Front. This
lake effect contributes to "the Greatest Snow on Earth" at the ski
resorts in the Cottonwood Canyons.
At least two major phenomena control lake-effect precipitation:
added moisture to the air due to evaporation from the lake's surface,
and atmospheric instability caused by the temperature contrast between
the air and lake water.
In prehistoric times, the lake effect may have played even more
of a role in the weather. A look around hillsides all across western
Utah reveals the bathtub-like rings marking the shorelines of ancient
Lake Bonneville. Lake Bonneville existed approximately 12 to 28
thousand years ago, covering much of western Utah and even parts
of Nevada and Idaho at its highest level.
This enormous surface area, up to approximately 19,800 square
miles (51,280 km2), could have contributed to greater lake-effect
precipitation. Increased precipitation, especially on the Wasatch
Front, causes increased runoff to the lake, helping to maintain
Lake Bonneville's high level, in turn increasing lake-effect precipitation,
and so on (a feedback loop).
The hydrologic cycle is complex. The precipitation component of
the Wasatch Front hydrologic sub-cycle is not the sole control of
the level of Great Salt Lake and its predecessor, Lake Bonneville.
The interplay of precipitation and evaporation largely control lake
level. When the amount of water entering the lake (precipitation,
surface water, and ground water) exceeds the amount of water leaving
the lake (evaporation), lake level rises and vice versa.
Many factors influence the evaporation portion of our hydrologic
cycle, including temperature and wind. Decreases in temperature
or wind speed decrease lake evaporation, thereby promoting lake-level
rise. Some evidence, including that of glaciers in the Wasatch,
suggests that the climate in Utah was colder, and this may have
been partially responsible for Lake Bonneville's rise.
This article barely scratches the surface of the complexities
of our hydrologic cycle and geologic consequences such as lake levels.
For example, we have not considered salinity, which can retard evaporation
and slow the fall of Great Salt Lake. Salt crystals from Great Salt
Lake and the surrounding salt flats can also become airborne and
act as natural cloud seeding to enhance precipitation.
Regardless of the specific details of our local hydrologic cycle,
our water and all of the world's water continuously and endlessly
circulates though the hydrologic cycle. On a global scale little
water has been lost or gained over eons of geologic time. It has
just been redistributed through various phases of the hydrologic
cycle over various areas of the planet. |