The water cycle: an explanation
Water, water, everywhere, so let's all have a drink (or so we all learned as
kids, right?), but it's definitely not as easy as that these days. In honor of
World Water Day (which may or may not have been today), let's 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 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
What makes 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.