|An Explanation of the Water Cycle
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
- 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
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.
(« Go Back)