The Earth's Water

WATER DISTRIBUTION

H2O

Everything is made of atoms.
Atoms join together to form molecules.
two hydrogen (H) atoms and one oxygen (O) atom.

The Earth's Water
returns as precipitation. Locally however, if more fresh water is consumed through human activities than is naturally restored, this may result in reduced fresh water availability from surface and underground sources and can cause serious damage to surrounding and associated environments.

Causes of limited fresh water
There are many causes of the apparent decrease in our fresh water supply. Principal amongst these is the increase in population through increasing life expectancy, the increase in per capita water use and the desire of many people to live in warm climates that have naturally low levels of fresh water resources.
 
Climate change is also likely to change the availability and distribution of fresh water across the planet:
"If global warming continues to melt glaciers in the polar regions, as expected, the supply of fresh water may actually decrease.
First, fresh water from the melting glaciers will mingle with salt water in the oceans and become too salty to drink. 

Agriculture
Changing landscape for the use of agriculture has a great effect on the flow of fresh water. Changes in landscape by the removal of trees and soils changes the flow of fresh water in the local environment and also affects the cycle of fresh water. As a result more fresh water is stored in the soil which benefits agriculture. However, since agriculture is the human activity that consumes the most fresh water, this can put a severe strain on local freshwater resources resulting in the destruction of local ecosystems. In Australia, over-abstraction of fresh water for intensive irrigation activities has caused 33% of the land area to be at risk of salination.[15] With regards to agriculture, the World Bank targets food production and water management as an increasingly global issue that will foster debate.
 
Limited resource
Fresh water is a renewable and variable, but finite natural resource. Fresh water can only be replenished through the process of the water cycle, in which water from seas, lakes, forests, land, rivers, and reservoirs evaporates, forms clouds, and

Water fountain found in a small Swiss village.
They are used as a drinking water source for people and cattle. Almost every Alpine village has such a water source.
An important concern for hydrological ecosystems is securing minimum streamflow, especially preserving and restoring instream water allocations. Fresh water is an important natural resource necessary for the survival of all ecosystems. 
Pollution from human activity, including oil spills, also presents a problem for freshwater resources. The largest petroleum spill that has ever occurred in fresh water was caused by a Royal Dutch Shell tank ship in Magdalena, Argentina, on 15 January 1999, polluting the environment, drinkable water, plants and animals. Fresh and unpolluted water accounts for 0.003% of total water available globally.[14]

Aquatic organisms
Fresh water creates a hypotonic environment for aquatic organisms. This is problematic for some organisms with pervious skins or with gill membranes, whose cell membranes may burst if excess water is not excreted. Some protists accomplish this using contractile vacuoles, while freshwater fish excrete excess water via thekidney. Although most aquatic organisms have a limited ability to regulate their osmotic balance and therefore can only live within a narrow range of salinity,diadromous fish have the ability to migrate between fresh water and saline water bodies. During these migrations they undergo changes to adapt to the surroundings of the changed salinities; these processes are hormonally controlled. The eel (Anguilla anguilla) uses the hormone prolactin,[10] while in salmon (Salmosalar) the hormone cortisol plays a key role during this process.[11]

Many sea birds have special glands at the base of the bill through which excess salt is excreted. Similarly the marine iguanas on the Galápagos Islands excrete excess salt through a nasal gland and they sneeze out a very salty excretion.

Out of all the water on Earth, salt water in oceans, seas and saline groundwater make up about 97% of it. Only 2.5–2.75% is fresh water, including 1.75–2% frozen in glaciers, ice and snow, 0.7–0.8% as fresh groundwater and soil moisture, and less than 0.01% of it as surface water in lakes, swamps and rivers.[3][4] Freshwater lakes contain about 87% of this fresh surface water, including 29% in the African Great Lakes, 20% in Lake Baikal in Russia, 21% in the North American Great Lakes, and 14% in other lakes. Swamps have most of the balance with only a small amount in rivers, most notably the Amazon River. The atmosphere contains 0.04% water. In areas with no fresh water on the ground surface, fresh water derived from precipitation may, because of its lower density, overlie saline ground water in lenses or layers. Most of the world's fresh water is frozen in ice sheets. Many areas suffer from lack of distribution of fresh water, such as deserts.

Sources
The source of almost all fresh water is precipitation from the atmosphere,
in the form of mist, rain and snow.
 
Fresh water falling as mist, rain or snow contains materials dissolved from the atmosphere and material from the sea and land over which the rain bearing clouds have traveled.
 
Industrialized areas rain is typically acidic because of dissolved oxides of sulfur andnitrogen formed from burning of fossil fuels in cars, factories, trains and aircraft and from the atmospheric emissions of industry. In some cases this acid rain results in pollution of lakes and rivers.

Desert areas, or areas with impoverished or dusty soils, rain-bearing winds can pick up sand and dust and this can be deposited elsewhere in precipitation and causing the freshwater flow to be measurably contaminated both by insoluble solids but also by the soluble components of those soils. 

Second, the increased ocean volume will cause sea levels to rise, contaminating freshwater sources along coastal regions with seawater”. Water pollution and subsequent eutrophication also reduces the availability of fresh water.

For example, Canada has 20% of the world's fresh water supply, while India has only 10% of the world's fresh water supply, even though India's population is more than 30 times larger than that of Canada. A reason for the uneven distribution of fresh water supply may be the differences in climate.

For example, in some countries in Africa, the frequent lack of rain has led to insufficient water supply for irrigation. This has affected agriculture and has led to a shortage of food for the people.

The surface of a freshwater lake.
freshwater habitats are divided into lentic systems,
stillwaters- including ponds, lakes, swamps and mires;
lotic systems- which are running water;
 groundwater- which flows in rocks and aquifers. 
in addition, a zone which bridges between groundwater and lotic systems, 
hyporheic zone- which underlies many larger rivers and can contain substantially more water than is seen in the open channel. It may also be in direct contact with the underlying underground water. The majority of fresh water is in icecaps

Fresh water systems

ice sheets, ice caps, glaciers, icebergs, bogs, ponds,lakes, rivers and streams and underground as groundwater in aquifers and underground streams. 

Systems
97% of water is saltwater.
2% of the water on earth is on glacier ice.
Less than 1% of all the water on earth is fresh water .

by Vanessa Ayog