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An Explanation of the Water Cycle

An Explanation of the Water Cycle
by Collin Dunn, Corvallis

Water, water, everywhere, so letís all have a drink (or so we all learned as kids, right?), but itís definitely not as easy as that these days. In honor of World Water Day (which may or may not have been today), letís sit back and enjoy an explanation of the water cycle.

Also known as the hydrologic cycle, the water cycle describes the process by which the various forms of water move about the planet in a fairly constant balance. But just because itís fairly balanced doesnít mean we have all the water we need, whenever we want it. But first, what is the cycle, really?

What is the water cycle?

Like all circular items, the water cycle has no true beginning and no end, though the water changes state from liquid to solid -- as ice and snow, for example -- and as vapor. The cycle is the process by which the water, in whatever form, goes from place to place, ocean to cloud to rainwater to river and back again through a cycle of rising air currents, precipitation, runoff and a few other processes.

How does the water cycle work?

Itís a big circle: Rising air currents take the water, as vapor, up into the atmosphere, along with water from "evapotranspiration," which is water transpired or "breathed out" from plants and evaporated from the soil. The cooler temperatures in the atmosphere cause it to condense into clouds, which float around until the fall from the sky as precipitation. Some precipitation falls as snow and can accumulate as ice caps and glaciers, where it can stay, as frozen water, for thousands of years. In warmer climates, snow melts during the warmer spring and summer months, and that water flows into streams and rivers, which eventually return it to the ocean, or into the groundwater, which eventually reach underground aquifers. Over time, the water continues flowing, some to reenter the ocean, where the water cycle renews itself. There are four basic steps that tie this all together.


Four steps in the water cycle

  • Evaporation occurs when water transforms from liquid to gas, usually as a result of the sunís warming rays. Evaporation often technically includes transpiration from plants (the vapor the "breathe" out as they grow), though together they are specifically referred to as evapotranspiration.
  • Condensation occurs as the vapor rises into the atmosphere, creating clouds and fog. Once clouds are formed, advection -- the movement of water in its various states -- through the atmosphere. Without advection, the cycle would screech to a halt, as the water would evaporate and precipitate (the next step) in the same place.
  • Precipitation occurs when the vapor that condensed comes back out of the sky as rain, snow, sleet, hail. Most of it comes back to the ground or body of water, but some of it is intercepted by plant foliage and evaporates back to the atmosphere instead of making it to the ground, in a process called "canopy interception."
  • Runoff is the process by which water moves across land and includes both surface runoff -- when water travels over land -- and channel runoff -- when it gets into streams and rivers. As is bubbles and rambles along, it can drain into the ground, evaporate into the air, run into and become stored in lakes or reservoirs, or be gathered up for human uses.


What makes the water cycle work?

Itís not a perfectly linear cycle; the same water molecules donít go through the four cycles at the same speed, or spend the same amount of time in each one. As it turns out, much more water is "in storage" -- frozen in glaciers, sitting in lakes or reservoirs, or underground aquifers -- than is actually moving through the cycle, and most of it -- 95% of the worldís water supply, actually -- is stored in our planetís oceans.

Because of global warming, the water cycle will continue to intensify during the 21st century, according to the Intergovernmental Panel on Climate Change; notably, though, this doesnít mean increased precipitation across the board. In places where itís already dry, itís going to get drier, increasing the probability of drought.

Glacial retreat is another water cycle-related consequence of a warming globe; as the temperature rises, the supply of water to glaciers from precipitation cannot keep up with the loss of water from melting and sublimation. When it rains, it pours, so to speak.

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