THE WATER CYCLE
The water cycle is the most familiar of all biogeochemical cycles. All life
depends directly on the presence of water, since the bodies of most organisms
consist mainly of this substance. Water is the source of hydrogen ions, whose
movements generate ATP in organisms, and for that reason alone it is indispensable
to their func-tioning-
The oceans cover three fourths of the earth's surface. From their surface,
water evaporates into the atmosphere, a process powered by energy from the
sun. Approximately 90% of the water that reaches the atmosphere comes from
plants via transpiration. Most of it falls directly into the oceans, but some
falls onto the land, where it passes into surface and subsurface bodies of
fresh water. Only about 2% of all the water on earth is fixed in any form-frozen,
held in the soil, or incorporated into the bodies of organisms. All of the
rest is free water, circulating between the atmosphere and the earth. Regardless
of where this water is held temporarily, it eventually returns to the atmosphere
and the oceans,
Organisms live or die on the basis of their ability to capture some of this
water and incorporate it into their bodies. Plants take up water from the
earth in a continuous stream; crop plants require about 1000 kilograms of
water to produce one kilogram of food, and the relationships in natural communities
are similar. Animals obtain water directly or from the plants or other animals
they eat. The amount of free water present at a particular place often determines
the nature and abundance of the living organisms present there.
Much less obvious than the surface waters, which we see in streams, lakes,
ant ponds, is the groundwater, which occurs in aquifers-permeable, saturated,
underground layers of rock, sand, and gravel. In many areas, groundwater is
the most important reservoir of water; it amounts to more than 96% of all
fresh water in the United States. The upper, unconfined portion of the groundwater
constitutes theater table, which flows into streams and is partly accessible
to plants; the lower confined layers are generally out of reach, although
they can be "mined" by humans,
Groundwater flows much more slowly than surface water, anywhere from a fen
millimeters to as much as a meter or so per day. In the United States, groundwalei
provides about 25% of the water used for all purposes and provides about 50%
of the population with drinking water. Rural areas tend to depend on groundwater
almost exclusively, and its use is growing at about twice the rate of surface
water use. In the Great Plains of the central United States, the depletion
of the Ogallala Aquifer seriously threatens the potential for agriculture;
similar problems are appearing throughout the drier portions of the globe,
Because of the greater rate at which groundwater is being used, and also because
it flows so slowly, the increasing chemical pollution of groundwater is a
very serious problem. It is estimated that about 2% of the groundwater in
the United States is already polluted, and the situation is worsening. Pesticides,
herbicides, and fertilizers have become a serious problem, and another key
source of groundwater pollution consists of the roughly 200,000 surface pits,
ponds, and lagoons that are actively used for the disposal of chemical wastes
in the United States alone. Recharging the aquifers is an important strategy
for conservation, but it depends on the purity of the water there initially.
Because of the large volume of water, its slow rate of turnover, and its inaccessibility,
removing pollutants from aquifers is virtually impossible.
Some 96% of the fresh water in the United States consists of groundwater.
This groundwater, which already provides 25% of all the water used in this
country, will be used even more extensively in the future, even though problems
of pollution are increasing.
