create new water. It constantly recycles the same water that existed on the
planet billions of years ago.
This recycling process, known as the
hydrologic or water life cycle, works like a giant plumbing system to transport
and renew our planet's water within the closed system of the earth's atmosphere. Operating since the beginning of time, the
hydrological cycle describes the movement of water as it passes through three
phases: solid, liquid and gas.
How the Cycle Works
You'll find the
hydrological cycle hard at work high in our atmosphere as well as several
kilometres below the earth's surface. The cycle operates through a number
of natural processes scientists call evaporation, transpiration and
Evaporation describes the
change from water as a liquid to water as a vapour. Water evaporated from
ocean surfaces combines with water evaporated from freshwater sources such
as rivers, lakes and streams to produce clouds.
The clouds, in turn,
produce precipitation, 78 percent of which falls back into oceans while
the remaining 22 percent falls back on land.
Transpiration, on the other
hand, is the loss of water by plants. Transpiration rates depend upon
temperature, humidity and wind speed conditions near the leaves of plants.
And since plants draw water from the soil, transpiration rates can greatly
effect soil moisture content. Soil water loss resulting from both
transpiration and evaporation is called evapotranspiration.
Where Does Precipitation Go?
When precipitation reaches
the ground, it follows two basic pathways: surface flow and infiltration.
As water soaks into the subsurface through infiltration, it moves through
the pores of the soil until the soil reaches saturation. Once infiltrated,
water continues to filter through soil or rock through vertical movement
called percolation. Percolation results in the movement of water from the
soil layer to groundwater. Underground formations that contain groundwater
are called aquifers.
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