diagram showing distribution of earth's water

As we know, about 70% of Earth is covered by water. 97% of all the Earth's water is contained within the oceans as salt water, while the remaining 3.5% is freshwater lakes and frozen water locked up in glaciers and the polar ice caps. Of that fresh water, almost all of it takes the form of ice: 69% .

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The Oceans

Map of the world's oceans

As the kindergarteners in my daughter's class reminded me many years ago, there is really only one ocean. But generally, geographers and oceanographers divide that ocean into four: the Pacific, Atlantic, Indian and Arctic Oceans.

The Pacific Ocean is twice as large as any other body of water on Earth. Five of the seven continents are on its fringes. It occupies about 1/3 of the surface of the planet, more than all the world's land surfaces combined. It contains the greatest average depth of any ocean (14,040 ft), the deepest known oceanic trenches (the Marianas Trench, Challenger Deep (36,200 ft), and 46% of Earth's surface water.

The Atlantic Ocean is half the east/west dimension of the Pacific. It averages 11 962 feet in depth and contains about 23% of the Earth's surface water.

The Indian Ocean covers about 14% of the Earth’s surface and contains about 20% of the Earth’s Surface Water. It is slightly deeper than the Atlantic with an average depth of 12, 274 feet. 9/10's of its area is south of the equator and the Indian Ocean is the warmest ocean in the world.

The Arctic is by far the smallest and shallowest of the four. It covers about 4% of the Earth’s surface and has an average depth is only about 3,406. Ice on the Arctic Ocean has been decreasing since 1980 and may be ice free for the first time in human history by 2040.

Ice

Second only to the world's oceans as a storage reservoir for moisture on Earth is ice. Although tiny in comparison to the oceans, the moisture content of ice on Earth is more than twice as large as the combined total of all the other types of water storage on our planet. Almost 70% of the freshwater on Earth is tied up as ice. Approximately 10 % of the land surface of Earth. It is estimated that there is enough water locked up in ice to feed all the rivers of the world at their present flow for nearly 900 years.

Ice on land includes alpine glaciers, ice sheets, and ice caps. The vast majority of land ice is found in the ice caps of Antarctica and Greenland. The largest concentrations of oceanic ice are the ice pack that covers most of the Arctic Ocean and the ice pack that fringes most of the Antarctic continent. Both ice packs grow in winter and decrease in summer. Over the last decade, because of increasing temperatures, 3000 square miles of Antarctic ice shelves have disintegrated. The Arctic ice sheet is also declining rapidly due to warming seas. September is the month that the Arctic sea ice is at is minimum. September Arctic sea ice is now declining at a rate of 13.1 percent per decade.

A relatively small portion of the world's ice occurs beneath the land surface as ground ice or permafrost. This type of ice occurs only in areas where the temperature is continuously below freezing so is only found in high latitude and high altitude locations. It is widespread in northern Canada, Alaska, and Siberia in in small patches in some high mountain areas.

Surface Water

Lakes

Surface water (other than oceans) only accounts for about 0.3% of the water on Earth. This includes lakes, swamps, marshes and rivers. Lakes are the largest part of this storage with 87% of the non-frozen surface water on the continents. The largest lake in the world in terms of surface area is the Caspian Sea. Like many of the largest lakes on Earth it is saline. In fact 40% of the water in the world's lake is salty. Lake Baykal (or Baikal) in Siberia is the world's largest lake by volume, containing more water than the combined contents of the all five of the Great Lakes. It is also the world's deepest lake (5,715 ft).

Lakes are distributed very unevenly over Earth's surface. They are very common in regions that were glaciated because glaciation creates basin for water to occupy and disturbs existing drainage systems. There are large lakes in eastern and central Africa created by the African Rift zone where two tectonic plates are pulling apart from each other.

Aral sea through time

One lake worth of a special mention is the Aral Sea (see above). In 1960, it was the fourth largest lake on Earth. Today it doesn't make it into the top 10. The lake straddles Uzbekistan and Kazakistan, once part of the former Soviet Union. It is fed by the Amu Darya and Syr Darya rivers. In 1960 there were 24 native species of fish and a very healthy fishing industry accounting for more than 10% of the Soviet Union's annual catch. Today just 4 species survive and the fishing industry has been decimated.

The Soviet Union had few areas that it could grow cotton and had to import all of its cotton. As early as the 1920's, the Soviets decided to irrigate this portion of Central Asia to grow cotton. By the 1960s, the Soviet government converted large acreages of pastures or untilled lands in what are now Uzbekistan, Kazakhstan, Turkmenistan, and elsewhere in Central Asia into irrigated farmlands to grow cotton by diverting the waters of the Amu Darya, Syr Darya, and their tributaries. The amount of water from those rivers that reached the Aral Sea dropped dramatically. By the late 1980s the lake had lost more than half the volume.

The salt and mineral content of the lake rose drastically because of lower water levels, making the water unfit for drinking and killing off the once-abundant supplies of sturgeon, carp, barbel, roach, and other fishes in the lake. The fishing industry along the Aral Sea collapsed. The ports of the Aral Sea in the northeast and Mŭynoq in the south were now far from the lake’s shore and were abandoned. A partial depopulation of the areas along the lake’s former shoreline ensued. The contraction of the Aral Sea also made the local climate noticeably harsher, with more-extreme winter and summer temperatures. Winds blowing across the exposed seabed produced dust storms that buffeted the region with a toxic dust contaminated with salt, fertilizer, and pesticides. As a result, the areas’s inhabitants have suffered health problems at unusually high rates—from throat cancers to anemia and kidney diseases—and infant mortality in the region has been among the highest in the world.

Rivers

Rivers account for only 2% of the freshwater on the planet but they are of huge importance to human beings. They provide drinking water, irrigation water, transportation and they are the single most powerful erosional force on our planet. The longest river on the planet is the Nile River, flowing 4,130 miles across North Africa. It is followed by the Amazon, Missouri-Mississippi, the Yangtze and the Ob-Irtysh.

Three Gorges Dam Yangtze River

Another, perhaps more important, measure of a river is the volume of water it carries. Here the Amazon is the clear winner carrying 20% of all the water carried by a river on the planet Earth. It carries 4.5 times more water than the second largest river by volume, the Congo (or Zaire). The discharge of the Amazon is three times as great as the total combined discharge of all the rivers of the U.S. The five largest rivers by volume are the Amazon, the Congo, the Yangtze, the Brahmaputra and the Ganges. The Yangtze makes both lists - length and volume. The Three Gorges Dam on the Yangtze was completed this century and is the largest hydroelectric dam in the World. Please read about the pros and cons of building this dam.

Ground Water

The total amount of water below ground on Earth is about 2.5 times that contained in Lakes and streams. Groundwater is much more evenly distributed than surface water. More than half the world's underground water is found with in about half a mile of the surface. Below that depth, the amount of water decreases and becomes less consistent.

Almost all ground water comes originally from precipitation that percolates directly into the soil or else seeps downward from lakes and streams. Groundwater is stored in and moves slowly through, moderately to highly permeable rocks call aquifers. An aquifer is a body of saturated rock through which water can easily move. Aquifers must be both permeable and porous and include such rock types as sandstone, conglomerate, fractured limestone and unconsolidated sand and gravel. Fractured volcanic rocks such as basalts also make good aquifers.Water moves in aquifers at different rates - some less than an inch per day, some several hundred feet per day. A "rapid flow" would be 40 to 50 feet per day. Impermeable rocks or soils such as clay or slate, are called aquicludes and they prevent the accumulation of groundwater.

Aquifer diagram

Below what you and I think of as soil (or the zone of aeration), there is a zone in which all pore space in the soil and cracks in the rocks are fully saturated with water. This is the Zone of Saturation. The water stored in this zone is groundwater. The top of the saturated zone is referred to as the water table. The water table roughly follows the topography of the surface above it. Where the water table intersects the Earth's surface, water flows out in a lake, a swamp, a marsh or a permanent stream. Water rises and falls in the aquifer with rainfall and drought.

Cone of depression and drawdown When ever a well is dug and water is drawn from the well it creates a cone of depression. The drawdown locally drops the level of the water table. The amount that that water table has dropped is called the drawdown. If many wells are withdrawing water faster than it can be replenished naturally (recharged), the water table may drop significantly over a large area.

Groundwater is a huge benefit to humans. Most of us in Sonoma County get all or part of our water from groundwater. Nevertheless, humans are a major threat to groundwater resources. The accumulation of groundwater is often slow. Its use by humans can be incredibly fast. In many parts of the southwestern U.S., the groundwater recharge rate is 0.2 inches per year but its not uncommon for a farmer to pump out 30 inches per year. This annual rate is equal to 150 years of natural recharge!

There are many consequences to taking more water each year than can be recharged. The most obvious is that you will eventually simply take it all. The Ogallala Aquifer is the largest aquifer in the U.S. It lies beneath portions of South Dakota, Wyoming, Nebraska, Kansas, Colorado, Oklahoma, Texas and New Mexico. Water has been accumulating in this aquifer for 30,000 years. Farms began withdrawing water in the 1930s. Wells that once hit water at 50 feet have had to be deepened to 150 to 250 as the water table has dropped. And at least 170,000 wells have run dry.

Map of Ogallala aquifer

The San Joaquin Valley is one of the most productive agricultural lands in the U.S. Farmers have been using groundwater since the 1920's. Over time, over-pumping has caused the aquifers to compact and collapse. In some places the surface has dropped 28 feet through subsidence caused by over-pumping the aquifer. This portion of the aquifer can no longer recharge and is lost.

Land subsidence in the San Joaquin Valley

Other human impacts to groundwater include saltwater intrusion and pollution. The boundary between fresh groundwater and saltwater is referred to as the freshwater/saltwater interface. Fresh groundwater discharging to the coast prevents saltwater from moving into the aquifer. If too much freshwater is pumped from the aquifer, then saltwater can migrate into the freshwater by a process referred to as “saltwater intrusion.” If a pumping well is close to the freshwater/saltwater interface, saltwater could enter the well and contaminate the water supply, too.

Diagram Salt water intrusion

Pollution comes in many forms. Leaking gas tanks, land fills, hazardous waste storage, surface contaminates, septic systems all can and do pollute groundwater regularly.

Diagram sources of pollution groundwater