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.
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.
Glad You Asked article, Survey Notes, v. 31 no. 2, April 1999