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A basic understanding of the hydrolytic cycle

Lake Michigan

Lake Michigan

The greatest average yearly precipitation in the world of 460 inches (1,168 cm) occurs at Mt. Waialeale, Hawaii.

The lowest average annual precipitation of 1.63 inches ( 4.1 cm) in the United States occurred over a 42-year period in Death Valley, California.

The longest dry period with no precipitation in the United States occurred during a 767-day period from October 3, 1912 to November 8, 1914 at Bagdad, California.

These are all examples of extremes in the hydrolytic cycle of the world. “Lytic” as a suffix means relating to a specified kind of decomposition, in this case of water. Simply put, rain comes down, evaporates, and becomes rain clouds. But of course the cycle is much more than that, as water changes state from solid to liquid to gas and back.

The hydrolytic cycle covers:

  • Evaporation
  • Condensation
  • Precipitation
  • Interception
  • Infiltration
  • Percolation
  • Transpiration
  • Runoff
  • Storage

Most of the evaporation occurs from the surface of the oceans. At this point, water changes from a liquid to a gas. Once in the atmosphere, water is carried on air molecules and in collections we recognize as clouds. Here the clouds are affected first by winds – meteorologists and aviators use the formations caused by the winds to predict weather and flying conditions. About 91% of this evaporated water returns to the ocean by precipitation. The remaining 9% gets carried over to landmasses, where the climate changes would cause the precipitation. The actual amount of evaporation depends of solar radiation, temperature, vapor pressure, wind and atmospheric pressure. Evaporation also occurs off land surfaces, human activity, heated buildings, soil and rocks.

Eye of the Storm Hurricane Elena

Eye of the Storm Hurricane Elena

The water which lands on land masses may infiltrate water runoff areas, be absorbed by plant and tree roots, or condense out of the atmosphere on cool objects such as a glass of iced tea or grass cooled overnight (dew). Here, water changes from a gaseous state to a liquid.

Precipitation occurs when a drop becomes a critical size and starts to fall. While falling, it causes a turbulence which causes other droplets to combine and fall as well. Of course, wind can push droplets together to have the same effect. If the air is cold and there are updrafts, these drops go up and down, resulting in snow or sleet, changing the state from liquid to solid.

Water that reaches soil can infiltrate the soil and sometimes reach streams. Precipitation on mountains will also run off into streams. Called percolation, this runoff can collect in underground reservoirs called water tables. . When stream levels drop, the water table refills them; when streams overflow, they will drain some of the water into the soil. Water also seeks a level area, accumulating in lakes and streams. Melting icebergs release water to rivers and streams as well as oceans.

Or it may be intercepted by lakes or vegetation. Water which lands on vegetation is absorbed, used for transport of minerals, then transpired back out into the atmosphere.

Rainbow over Bavaria, Germany

Rainbow over Bavaria, Germany

Water seeks equilibrium – going where it is less dense from more-dense areas. But a lot of water retention has to do with the climate. Water-soaked soil will evaporate the excess. Yet desert surfaces may obstruct water seepage, while the hot air evaporates it faster than it can be absorbed. In a rainforest, precipitation occurs faster than transpiration, leaving a water-soaked atmosphere and ground. Precipitation exceeds transpiration over continents, while evaporation exceeds precipitation over ocean reservoirs.

Eventually the water runs into the oceans, to be evaporated again.

The cycle is slowed down in moments of retention. While evaporated water is in the atmosphere for no more than two weeks, it may be retained in cells for a longer time, and remaining in deep groundwater or ocean basins for decades or more. And frozen water at the poles may last for eons.

Because of the irregularities in water retention and seasonal changes in climate, the hydrolytic cycle is not completely regular. However, by monitoring the conditions that effect the hydrolytic cycle, meteorologists can make educated predictions for the near future. And if an area is going through a dangerously dry spell, we can “seed” the thin gaseous water in the air to cause precipitation that would not normally occur. And all of this from only nine percent of the ocean’s evaporated water.


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