Class Session 23>
The purpose of this section is to introduce the basic characteristics of the hydrosphere, the water system of the earth. The hydrosphere of the earth is massive in its size and complexity, yet it is not wholly beyond comprehension. This section on the hydrosphere will help the student understand where water is found, how water flows throughout the earth, and how much freshwater is available for use by humans.
Earth is the water planet. Over 70% of the surface of the earth is covered with water. All living creatures - plants, insects, birds, animals, and humans - contain water. The water of the earth contains all the elements necessary to sustain life. Water exists in three states on the earth, liquid (water), solid (ice or snow), & gas (vapor). There are some 326 million cubic miles or .36 billion cubic kilometers of water on earth. The vast majority of the surface of the earth, 73%, is covered by water. Water is distributed on earth in the following way:
2.0% glaciers, ice caps
.5% ground water
.1% fresh water lakes
.1% salt water lakes
.04% soil root zone
.01% stream channels - include rivers, atmosphere & biosphere
Of The Water On Earth, How Much Is Available For Use By Humans?
97% - Saltwater (oceans and seas)
2% - Freshwater (polar ice caps and glacier)
1% - Freshwater ( only available to mankind)
Water circulates throughout the earth's system via enormous natural water system called the hydrologic cycle. The hydrologic cycle works in the following way:
1. Water from oceans, lesser extent continents is evaporated and transpired into atmosphere.
2. Moisture laden air is transported, often great distances by wind & convection.
3. Clouds are set in motion as the result of convectional, orographic, & frontal uplifting.
4. Condensation occurs, then precipitation which includes rain, snow, sleet & hail.
5. Precipitation that falls on ocean is finished and is ready to start anew
6. Precipitation that falls on land, makes its way either back to atmosphere or oceans - rivers
The amount of time for water to circulate through the various components of the hydrologic cycle varies and is referred as residence time. Listed below are the residence times for the various components of the hydrologic cycle.
2 weeks - l year
10s or 100s of years
Shallow ocean depth
Deep ocean depth
Up to 10,000 years
The amount of water vapor in the air at any one point in time is a minute fraction of the total amount in the system, only about 3/100ths of 1 percent of total amount in system. Yet, that amount is immense, around 380,000 cubic kilometers. The total amount of water vapor cycled through the hydrologic cycle stays nearly the same from year to year. In addition, the average annual precipitation & transpiration over the earth is equal to the amount of water evaporated. However for all the continents taken together, the amount of precipitation received exceeds the amount evaporated. Conversely over the oceans, the amount of evaporation exceeds precipitation. Since the level of the oceans is not dropping then runoff from the continents must balance the deficit of precipitation over oceans.
If most of the water on earth is in the oceans, than the oceans must be enormous. The oceans cover 71% of earth's surface and they are really one large body of water. There are three principal oceans:
Pacific - 64 million sq. miles -
14,000 feet average depth
Indian - 28 million sq. miles - 12,500 feet average depth
The seas are inward extension of
ocean and include the Mediterranean, Baltic, Bering, Caribbean, East China,
North, Black, Yellow, and
The composition of seawater is as follows: H20 plus (in parts per thousand or ppt):
Chlorine - 18.98
Sodium - 10.56
Magnesium - 1.27
Sulfur - .88
Calcium - .40
Potassium - .38
Bromine - .065
Carbon - .028
Strontium - .013
Boron - .005
In fact, every element found on land is found in the oceans. For example, it is estimated that there are 40 pounds of gold in every cubic mile of ocean water.
The two primary components in ocean or seawater are chlorine (18.98 parts per thousand) and sodium (10.56 parts per thousand). Together, they make up sodium chloride or salt. The concentration of salt in water is called salinity and the average salinity in seawater worldwide is around 3.5%, but varies from 3.3% to 3.7%. The greater the salinity the more dense water is Fresh water has a density of 62.4 lbs. per cubic foot at 39 degrees north. Ocean or salt water at the same latitude has a density of 64 lbs. per cubic foot.
Three factors affect the salinity of seawater:
because fresh water dilutes seawater;
2) the flow of rivers, because rivers are fresh water, and;
3) evaporation, because the salt in seawater is not evaporated.
Therefore, you end up with
variations on the surface, in both salinity and temperature. Salinity is around
highest around 30 degrees N and 30 degrees S, because of low precipitation,
scant stream flow, and high rate of evaporation. Salinity is lower near the
equator because there is more rain and more stream flow. For example, the
salinity off coast of
The highest salinity is found in enclosed
seas, in dry, hot regions where there is high level of evaporation and low
stream flow. For example, the Red Sea has a salinity of 4%, the Dead Sea has a
salinity of 23.8%, and the
Humans, as well as animals, cannot drink salt water. However, there is a process called desalinization, which removes salt from water. Creating freshwater through desalinization is much more expensive than using freshwater directly and so it is not used that much. As the amount of fresh water available to humans becomes scare, due to increases in demand, changing patterns of precipitation, or pollution of freshwater sources, desalinization is being used more and more around the world.
Ocean currents are the movement of water in response to varying water density. The density of water is determined by the amount of water, salty water is denser, and temperature, warm water expands and so is less dense.
Glaciers are thick ice masses that originate on land from accumulation, compaction, and recrystallization of snow. There are two types of glaciers, continental glaciers, which are found in high latitudes and alpine glaciers, which are found in high altitudes. Some have suggested a third type, which can be found in non-frost free refrigerators, but we will not bother with these. Continental and alpine glaciers together make up 10% of the land surface of the earth.
There are two massive continental
glaciers, Greenland and
While the oceans contain most of the water on earth, some 97.2%, ocean water is salt water which humans, animals, and plants are not too crazy about. Humans, animals, and plants like freshwater and about 2.8% of the water on earth is fresh. Glaciers contain a little over 70% of the freshwater on the planet.
By far, most of the ice is in the
Together, the big ice chunk
The next largest storage basin for water is in the ground. Groundwater makes up about .5% of all water. The U.S. Geologic Survey estimates that the quantity of water in the upper 800 meters (2600 feet) of the continental coast is about 3000 times greater that the volume of water in all rivers at any one time, an amount some 20 times the amount of water contained in all lakes and rivers.
Groundwater replenishment is one component of the hydrologic cycle. As rain falls, some of it evaporates, some runs off streams and rivers, and the rest goes into the ground. Several factors determine how much water will infiltrate into the ground, including the steepness of slope, the nature of surface material, the intensity of rainfall, and the type and amount of vegetation. Of the water that soaks into ground:
1) Some does not travel far -
molecular attraction holds it as a film on surface of soil particles -
eventually evaporates into atmosphere
2) What stays is used by plants as a source of moisture between rains belt of soil moisture (soil root zone)
3) water that is not held in this belt of soil moisture penetrates downward until it reaches a zone where all of the open spaces in sediment and rock are completely filled with water.
The water in this saturated zone is known as ground water.
The upper limit of this zone of saturation is the water table. Another way to think about it is that the water table is the point, below which, all the space between rocks is filled with water.
Groundwater comes to the surface through one of three mechanisms:
1. Spring - occurs when
the water table intersects the surface, which results in a natural outflow of
2. Well - artificial opening bored below the level of the water table into which is inserted a pipe with a pump at the top
3. Artesian well - occurs when ground water rises under pressure above the level of the aquifer due to the presence of an impermeable layer, which hinders the downward movement of water. This impermeable layer is called an aquiclude.
II. What is an aquifer?
An aquifer is a permeable rock stratum that holds and transmits ground water freely.
III. What is a hot spring?
Hot spring results from water below the surface meeting heated rock below the surface. Water temperatures increase with increasing depth below surface 1 degree Fahrenheit for every 100 feet in depth. As the water warms, it expands and rises to the surface and either bubbles continuously or intermittently.
IV. What is a geyser?
Geysers are the result of unique geologic subsurface features in which water seeps into the ground into narrow or restricted subterranean openings. The water temperature increases as the water is exposed to heated rocks. As the water continues to accumulate in this narrow subterranean reservoir, it becomes heated and some of the water becomes steam. The accumulation of steam at the bottom of the reservoir and water higher up in the reservoir results in a great surge of water upward that results in both water and steam being erupted from the surface. The steam and water being ejected results in a loss of pressure and temperature in the column at which point the process begins again.
V. Where Else Is Water Found?
Besides the oceans, glaciers, and in the ground, the rest of the earth's water, less than half a percent, is found on the surface of continents. Of the water on continents, some 90% is found in lakes, of which there are two kinds, fresh water & salt water. There is slightly more freshwater than salt water in lakes, about 55% freshwater & 45% saltwater. Lakes result when water is left over in excess of that which is evaporated or infiltrates the ground. Lakes can be fed either by water from the surface, from groundwater seepage and springs, or both.
VI. How About Rivers?
Only a small proportion of the water of the earth is found in rivers & streams, yet rivers and streams are enormously important to humans and the biosphere. Streams and rivers serve as channels for runoff of precipitation, melting ice, snow, and ground water, off the continents and into the oceans. Water flows off the continents in response to the gravity of the earth and this flow is often highly variable.
VII. What is a Watershed?
A watershed represents the area of land around a stream or river from which water flows. The watershed is the drainage area for a stream or river. Watersheds are particularly important because anything that comes in contact with the surface of a particular watershed, such as rain or chemicals, often flows into a specific river or stream.