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<Lakes and Reservoirs - Similarities, Differences and Importance>
Why are Lakes and Reservoirs Important?
Water is an inescapable necessity for all life on Earth. In pictures taken by
astronauts circling the Earth, we see a globe covered mostly by a rich, blue
color, signifying the presence of huge quantities of water – approximately 1.4
billion cubic kilometers of it! In fact, the absence of water produces a very
dramatic landscape - the most striking feature cited by Astronaut Jay Apt while
orbiting the Earth in the space shuttle was a belt of desert extending nearly
unbroken from northwest Africa to China.
If we consider human water needs, however, this picture is deceiving. This is
because more than 97 percent of the Earth’s water is in its oceans, leaving
only about 2.5 percent of it on land. Furthermore, about 70 percent of this
water is frozen in polar icecaps or glaciers. Most of the rest lies deep
underground and beyond easy human reach, and another sizable fraction is saline.
This means, therefore, that all human and other life depends on less than one
percent of the total quantity of water on this planet.
Even this relatively small fraction, however, is thought to be adequate to
meet human freshwater needs, if it were evenly distributed around the world.
Unfortunately, nature does not distribute its water resources evenly with regard
to either location or time of need. Some areas of the world have a rich supply
of fresh water, while others are arid or semi-arid areas with limited supplies.
Further, in some areas, nature supplies much of the water at times that do not
coincide with human water needs. If there is no mechanism to store this water,
much is wasted, or causes serious flooding, with resultant loss of life and
Adequate supplies of clean, safe fresh water are fundamental for human
survival and well being. That most human civilizations of the world emerged in
or near river valleys provides ample evidence of human needs for water.
Unfortunately, waterborne diseases and aquatic vectors continue to be the
largest single cause of human illness and death around the world.
Fresh water also is a basic requirement for the economic development of
nations and regions. Economic development in most arid and semi-arid regions is
limited. Humans also find it convenient to dispose of unwanted wastes and
byproducts of economic development activities in watercourses.
Fresh Water and the Hydrologic Cycle
Nature supplies fresh water for human needs and the maintenance of the
natural environment and ecosystems with a continuous recycling and renewal
process of evaporation, precipitation and runoff: the hydrologic cycle (Fig. 1).
Fig.1: Hydrologic Cycle
This cycle provides a continuous supply of fresh water in the form of
precipitation and snowfall. After falling onto land, precipitation flows over it
in a journey determined by gravity and the Earth’s contours, most eventually
making its way back to the ocean. Some terminates in inland drainage basins,
e.g. the Caspian and Aral Seas.
From the human perspective, the runoff or drainage of water from the land
surface is the most important source of fresh water. It also is the primary
mechanism by which water on the land surface flows into large downstream rivers
and lakes, and eventually into the oceans. On a global scale, water runoff is
generally concentrated in the temperate climatic zone and equatorial region,
which also contains many developed countries. In contrast, many developing
countries are located in the arid, semi-arid, tropical or sub-tropical regions
of the world. The volume of runoff from rivers that drain these regions is
unevenly distributed. The runoff in the Amazon River, for example, comprises
about 80 percent of the average water runoff each year from the entire South
American continent. The runoff from the Congo and Zaire Rivers comprise about 30
percent of the annual average water runoff from the continent of Africa. The
water runoff from the Ganges-Brahmaputra and Mekong drainage basins comprises a
high proportion of the total annual water runoff from Asia.
Fresh water may spend some time in lakes and reservoirs during the course of
its journey to the oceans. Referred to by some as “pearls on a river”, lakes
and reservoirs can have a significant effect on the quantity and the quality of
the fresh water that eventually reaches the oceans. It is the intention of this
volume to highlight lakes and reservoirs, and illustrate how their similarities
and differences can affect human use of this water.