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 it's fairly balanced doesn't 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?
It's 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 sun's 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.
How does the water cycle work?
It's not a perfectly linear cycle; the same water molecules don't 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 world's water supply, actually -- is stored in our planet's 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 doesn't mean increased precipitation across the board. In places where it's already dry, it's 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.
For further water cycle-relating reading in TreeHugger, check out Dumb Question Dept.: If Earth is a Closed System and We're Running Out of Water, Where's it All Going?, Water Running Out in Atlanta and our How to Green Your Water guide. Elsewhere the USGS has a good intro and Wikipedia covers the basics as well.
Quench your thirst for more green knowledge with our Green Basics column, which appears Thursdays here at TreeHugger.