The water cycle describes the existence and movement of water on, in, and above the Earth. Earth`s water is always in movement and is always changing states, from liquid to vapor to ice and back again. The water cycle has been working for billions of years and all life on Earth depends on it continuing to work- meaning that the water you drink today could possibly have been used by a dinosaur millions of years ago! The water cycle thus provides the scale for continuing life on the Earth.
The water cycle is a teleological cycle, with various stages but no particular starting point. Let us consider the major water source, the oceans, and the sun, which drives the water cycle, heating the ocean water.
Evaporation is the primary pathway that water changes is phase and moves from the liquid state back into the water cycle as atmospheric water vapor. Nearly 90% of the moisture in the atmosphere via evaporation occurs through oceans, seas, lakes, and rivers while 10% is contributed by plant transpiration or Evapotranspiration wherein water is lost to the atmosphere from the ground surface and transpiration of groundwater by plants through their leaves. During a growing season, a leaf will transpire many times more water than its own weight. A large oak tree can transpire 151,000 liters per year.
Heat (energy) is necessary for all types of evaporation to occur. Energy is used to break the bonds that hold water molecules together, which is why water easily evaporates at the boiling point but evaporates much more slowly at the freezing point. Net evaporation occurs when the rate of evaporation exceeds the rate of condensation. The process of evaporation, in fact, removes heat from the environment, which is why water evaporating from your skin cools you.
Another process of phasal transition is sublimation, one most often used to describe the process of snow and ice changing into water vapor without first melting into water. Sublimation is a common way for snow to disappear in certain climates. Such a process occurs more readily when certain weather conditions are present, such as low relative humidity and dry winds. It also occurs more at higher altitudes, where the air pressure is less than at lower altitudes. Energy, such as strong sunlight, is also needed.
The atmosphere is full of water. The water cycle is all about storing water and moving water on, in, and above the Earth. Although the atmosphere may not be a great storehouse of water, it is the superhighway used to move water around the globe. There is always water in the atmosphere. Clouds are, of course, the most visible manifestation of atmospheric water, but even clear air contains water—water in particles that are too small to be seen.
One estimate of the volume of water in the atmosphere at any one time is about 3,100 cubic miles (mi3) or 12,900 cubic kilometers (km3), around 0.001 percent of the total Earth`s water volume. Condensation is the process in which water vapor in the air is changed into liquid water. Condensation is crucial to the water cycle because it is responsible for the formation of clouds. These clouds may produce precipitation, which is the primary route for water to return to the Earth`s surface within the water cycle. Condensation is the opposite of evaporation.
Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail. It is the primary connection in the water cycle that provides for the delivery of atmospheric water to the Earth. Most precipitation falls as rain.
The clouds floating overhead contain water vapor and cloud droplets, which are small drops of condensed water. However these are way too small to fall as precipitation, but they are large enough to form visible clouds. Water is continually evaporating and condensing in the sky. If you look closely at a cloud you can see some parts disappearing (evaporating) while other parts are growing (condensation). Most of the condensed water in clouds does not fall as precipitation because their fall speed is not large enough to overcome updrafts which support the clouds.
For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. Water droplets may grow as a result of additional condensation of water vapor when the particles collide. If enough collisions occur to produce a droplet with a fall velocity which exceeds the cloud updraft speed, then it will fall out of the cloud as precipitation. This is not a trivial task since millions of cloud droplets are required to produce a single raindrop.
A more efficient mechanism (known as the Bergeron-Findeisen process) for producing a precipitation-sized drop is through a process which leads to the rapid growth of ice crystals at the expense of the water vapor present in a cloud. These crystals may fall as snow, or melt and fall as rain.
Many people probably have an overly-simplified idea that precipitation falls on the land, flows overland (runoff), and runs into rivers, which then empty into the oceans. That is "overly simplified" because rivers also gain and lose water to the ground. Still, it is true that much of the water in rivers comes directly from runoff from the land surface, which is defined as surface runoff.
When rain hits saturated or impervious ground it begins to flow downhill. It is easy to see if it flows down your driveway to the curb and into a storm sewer, but it is harder to notice it flowing overland in a natural setting. During a heavy rain you might notice small rivulets of water flowing downhill. Water will flow along channels as it moves into larger creeks, streams, and rivers. Runoff flowing over bare soil deposits sediment into rivers, which is not good for water quality.
As with all aspects of the water cycle, the interaction between precipitation and surface runoff varies according to time and geography. Similar storms occurring in the Amazon jungle and in the desert Southwest of the United States will produce different surface-runoff effects. Surface runoff is affected by both meteorological factors and the physical geology and topography of the land. Only about a third of the precipitation that falls over land runs off into streams and rivers and is returned to the oceans. The other two-thirds is evaporated, transpired, or soaks into ground water. Surface runoff can also be diverted by humans for their own uses.
