The hydrosphere in physical geography describes the combined mass of water found on, under,
and over the surface of a planet. Igor Shiklomanov, the man selected by the
United Nations to do its world inventory of water resources, estimated that there are
1386 million cubic kilometres of water on earth. This includes water in liquid and frozen
forms in groundwaters, glaciers, oceans, lakes and streams. Saline water account for 97.5%
of this amount. Fresh water accounts for only 2.5%. Of this fresh water 68.7% is in the
"form of ice and permanent snow cover in the Arctic, the Antarctic, and in the mountainous
regions. Next, 29.9% exists as fresh groundwaters. Only 0.26% of the total amount of fresh waters
on the Earth are concentrated in lakes, reservoirs and river systems where they are most easily
accessible for our economic needs and absolutely vital for water ecosystems." The total mass
of the Earth's hydrosphere is about 1.4 × 1018 tonnes, which is about 0.023% of the Earth's
total mass. About 20 × 1012 tonnes of this is in the Earth's atmosphere. Approximately
75% of the Earth's surface, an area of some 361 million square kilometers, is covered
by ocean. The average salinity of the Earth's oceans is about 35 grams of salt per kilogram
of sea water
Water cycle
The hydrological cycle transfers water from one state or reservoir to another. Reservoirs
include atmospheric moisture oceans, rivers, lakes, groundwater, subterranean aquifers,
polar icecaps and saturated soil. Solar energy, in the form of heat and light, and gravity
cause the transfer from one state to another over periods from hours to thousands of years.
Most evaporation comes from the oceans and is returned to the earth as snow or rain.
Sublimation refers to evaporation from snow and ice. Transpiration refers to the expiration
of water through the minute pores or stomata of trees. Evapotranspiration is the term used
by hydrologists in reference to the three processes together, transpiration, sublimation
and evaporation. In a award-winning book titled 'Water'by-
[Marq de Villiers] described the hydrosphere as a closed system in which water exists.
The hydrosphere is intricate, complex, interdependent, all-pervading and stable and "seems purpose-built
for regulating life." De Villiers claimed that, "On earth, the total amount of water
has almost certainly not changed since geological times: what we had then we still have. Water
can be polluted, abused, and misused but it is neither created nor destroyed, it only
migrates. There is no evidence that water vapour escapes into space."
"Every year the turnover of water on Earth involves 577,000 km3 of water. This is water
that evaporates from the oceanic surface and from land. The same amount of water falls
as atmospheric precipitation, 458,000 km3 on the ocean and 119,000 km3 on land. The
difference between precipitation and evaporation from the land surface represents the total
runoff of the Earth's rivers and direct groundwater runoff to the ocean. These are the principal
sources of fresh water to support life necessities and man's economic activities."
Water is a basic necessity of life. Since 2/3 of the Earth is covered by water, the
Earth is also called the blue planet and the watery planet. Hydrosphere plays an important
role in the existence of the atmosphere in its present form. Oceans are important in
this regard. When the Earth was formed it had only a very thin atmosphere rich in hydrogen
and helium similar to the present atmosphere of Mercury. Later the gases hydrogen and helium
were expelled from the atmosphere. The gases and water vapor released at the time of the
cooling of the Earth helped in the formation of the atmosphere into its present form. Various
types of gases and water vapor released by the volcanoes also reached the atmosphere.
When the Earth cooled due to continuous rains the water vapor also condensed and fell as
rain. As carbon dioxide in the atmosphere started to dissolve in rain water, atmospheric
temperature decreased considerably. This helped the water vapor to condense fast and fall
as rain. It is this rain water that filled the depressions on the Earth's surface to
result in the formations of the oceans. It is estimated that these oceans formed about
4000 million years ago. It is in the oceans that the first life forms took shape. They
did not breath oxygen. Later, when cyanobacteria evolved, the process of conversion of carbon
dioxide into food and oxygen began. As a result of the Earth's atmospheric composition became
distinctly different from that of the other planets. It is this atmosphere of the Earth
that is the basis for the survival of life on the Earth.
Recharging reservoirs According to Igor A. Shiklomanov, it takes
2500 years for the complete recharge and replenishment of oceanic waters, 10,000 years for permafrost
and ice, 1500 years for deep groundwater and mountainous glaciers, 17 years in lakes and
16 days in rivers. Specific fresh water availability
"Specific water availability is the residual per capita quantity of fresh water." Fresh
water resources are unevenly distributed in terms of space and time and can go from floods
to water shortages within months in the same area. In 1998 76% of the total population
has a specific water availability of less than 5.0 thousand m3 per year per capita.
Already by 1998, 35% of the global population suffered "very low or catastrophically low
water supplies" and Shiklomanov predicted that the situation would deteriorate in the
twenty-first century with "most of the Earth's population will be living under the conditions
of low or catastrophically low water supply" by 2025.
See also Biosphere
Cryosphere Earth's atmosphere
Lithosphere Ocean
Pedosphere Table of global climate system components
Water cycle Notes
References
External links Ground Water - USGS