Introduction:
Earth's water is always in movement, and the natural water cycle, also known as the hydrological cycle, describes the continuous movement of water on, above, and below the surface of the Earth. Water is always changing states between liquid, vapor, and ice, with these processes happening in the blink of an eye and over millions of years.
This Information page provides an understanding of the hydrological cycle. It describes the principal stages of the cycle, with a brief description of each stage.
This Information page provides an understanding of the hydrological cycle. It describes the principal stages of the cycle, with a brief description of each stage.
Where is all the Earth’s water?
Water is the most widespread substance to be found in the
natural environment and it is the source of all life on earth. Water covers 70%
of the earth’s surface but it is difficult to comprehend the total amount of
water when we only see a small portion of it. The distribution of water
throughout the earth is not uniform. Some places have far more rainfall than
others.
Notice how of the world's total water supply of about 332.5 million cubic miles of water, over 96 percent is saline. And, of the total freshwater, over 68 percent is locked up in ice and glaciers. Another 30 percent of freshwater is in the ground. Fresh surface-water sources, such as rivers and lakes, only constitute about 22,300 cubic miles (93,100 cubic kilometers), which is about 1/150th of one percent of total water. Yet, rivers and lakes are the sources of most of the water people use everyday.
The stages of the cycle are:
- Evaporation
- Transpiration
- Condensation
- Precipitation
- Groundwater
- Runoff
Water is transferred from the surface to the atmosphere through evaporation, the process by which water changes from a liquid to a gas. The sun’s heat provides energy to evaporate water from the earth’s surface. Land, lakes, rivers and oceans send up a steady stream of water vapour and plants also lose water to the air (transpiration).
Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation.
The release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen.
The transported water vapour eventually condenses, forming tiny droplets in clouds.
The primary mechanism for transporting water from the atmosphere to the surface of the earth is precipitation.
When the clouds meet cool air over land, precipitation, in the form of rain, sleet or snow, is triggered and water returns to the land (or sea). A proportion of atmospheric precipitation evaporates.
Some of the precipitation soaks into the ground and this is the main source of the formation of the waters found on land - rivers, lakes, groundwater and glaciers.
Some of the underground water is trapped between rock or clay layers - this is called groundwater. Water that infiltrates the soil flows downward until it encounters impermeable rock and then travels laterally. The locations where water moves laterally are called ‘aquifers’. Groundwater returns to the surface through these aquifers, which empty into lakes, rivers and the oceans.
Under special circumstances, groundwater can even flow upward in artesian wells. The flow of groundwater is much slower than run-off with speeds usually measured in centimeters per day, meters per year or even centimeters per year.
Most of the water which returns to land flows downhill as run-off. Some of it penetrates and charges groundwater while the rest, as river flow, returns to the oceans where it evaporates. As the amount of groundwater increases or decreases, the water table rises or falls accordingly. When the entire area below the ground is saturated, flooding occurs because all subsequent precipitation is forced to remain on the surface.
Different surfaces hold different amounts of water and absorb water at different rates. As a surface becomes less permeable, an increasing amount of water remains on the surface, creating a greater potential for flooding. Flooding is very common during winter and early spring because frozen ground has no permeability, causing most rainwater and melt-water to become run-off.
References:
- http://www.euwfd.com/html/hydrological_cycle.html
- http://en.wikipedia.org/wiki/Water_cycle
- http://ga.water.usgs.gov/edu/watercycle.html
- http://www3.geosc.psu.edu/~dmb53/DaveSTELLA/Water/global%20water/global_water.htm
