Water Systems

*The hydrosphere makes up about 70% of the Earth’s surface.  However, that water does not just sit around, inert.  The location and state of water on the earth is constantly changing in a fairly regular pattern:  this is called the water cycle.

*The sun drives the majority of the water cycle.  It does so by heating exposed water (in oceans, lakes, or rivers) so that it turns into a gas.  This is called evaporation.  As water evaporates, the vapour gathers in the air.

*Depending on the condition of the air, the amount of water it can hold varies.  Warm air can hold more molecules of water vapour than cold air, which is why the summer is (or can be) so much more humid than the winter. 

*Air eventually cools, either by rising too high, by circulating to latitudes farther from the equator, or simply by experiencing the cooling effect of darkness at night.  As the air grows cooler, it approaches the dew point, the point at which a given volume of air becomes too cold to contain all the water vapour it holds. 

*As moist air approaches the dew point, the water molecules begin to turn from vapour into liquid form.  This is known as condensation.  This forms clouds or may become visible as fog (which is why fog is seen most often in cool, damp areas).

*When moist air reaches its dew point, it may deposit some of its moisture as dew, or, if it falls far below the dew point, the water that condenses as clouds will be released as precipitation—the name for water in any form that falls to the ground as a liquid.  Depending on the temperature, this may be rain, snow, or sleet.

*When air that is rich in moisture passes over mountains (due to wind or to the rotation of the earth), it cools off as it rises over them.  As the air gets cooler, clouds form, and eventually rain falls on the mountains.  This means that mountains typically see more rain on the windward side (usually the west side) than the leeward side.  The side that gets less rain is said to be in a rain shadow.  This is also known as the orographic effect. 

*In some cases, particularly with very high mountains, the orographic effect creates a desert in the rain shadow area and particularly wet and (usually) fertile areas on the rainy side of the mountains.  In the Appalachian Mountains this is not a big deal, but in other parts of the world it significantly affects local climates.

*Eventually, the water that returns to the Earth through precipitation evaporates again, and the water cycle continues.  The amount of water in the global cycle stays more or less constant, although it can be changed somewhat as water becomes locked up in glaciers and ice caps, or released from them.

*Water is also transferred from the Earth to the atmosphere through transpiration, the release of water vapour from trees and plants.  In some cases, this carries a significant amount of water inland across vast jungles, particularly the rain forests.

*97% of Earth’s water is in our four oceans:  the Pacific, the Atlantic, the Indian, and the Arctic.  The Pacific Ocean (largest of the four) covers more of the Earth’s surface than all the continents combined.

*Oceans tend to be subdivided into smaller regions, particularly seas (areas largely, but not entirely, separated from the main body of the ocean by land), and by gulfs and bays (areas partly surrounded by land).

*About 3% of the hydrosphere is fresh water, but most of that is locked up in glaciers; over ½ of 1% of our fresh water is underground.  Less than ½ of 1% of the Earth’s potable water is available in lakes, streams, and rivers.

*Because so little fresh water exists, many places with limited access to fresh water, especially in the Middle East, have experimented with desalination of ocean water.  One desalination treatment boils the water to separate the water from the salt, and then condenses the water fresh. Overall, this is expensive and inefficient, but it is used in some particularly dry area.

*Streams and rivers begin in glacial run-off or in springs.  They may flow together to form larger rivers, but they almost always flow into the ocean.  A few flow into inland lakes, however.  Any place a river flows into another body of water is called the river’s mouth (although if a river simply flows into another river, it may be called a confluence).

*In any given part of a continent, water will flow towards one major body of water.  This is because continents are divided by the highest points, and water flows away from these points.  For example, the United States has its Eastern Continental Divide at the crest of the Appalachian Mountains.  Water on the east of the Appalachians flows into the Atlantic Ocean.  Water on the west flows into the Gulf of Mexico.  The Rocky Mountains form the western Continental Divide.

*All the rivers that flow into a particular river are known as its watershed.

*Rivers can be useful for drinking water, for irrigation, for fishing, for drawing boundaries, and, if large enough, for trade.

*People also get useful water form groundwater.  This is water that lies below the Earth’s surface.  Most comes from rain and melted snow that soaks deep into the Earth until it hits a layer of rock it cannot seep through.  A layer of more porous rock that can hold a lot of groundwater is called an aquifer.  Some of these contain water that has accumulated over centuries of millennia. 

*Groundwater and aquifers are very important sources of water when they are tapped for wells, especially in areas that do not have a great deal rainfall today.  However, some aquifers, particularly in the American west, are being drained by man faster than they are being replenished by nature.

This page last updated 17 August, 2005.