Geology Reference
In-Depth Information
Figure 12.2 The Danube River at Novi Sad, Serbia The Danube is one of Europe's important
waterways used for commerce and recreation.
paving in urban areas increases surface runoff to waterways,
and other human activities such as building dams and im-
pounding reservoirs also alter the dynamics of stream and
river systems. Natural changes, too, affect the complex inter-
acting parts of stream and river systems.
Regardless of its source, water vapor rises into the
atmosphere where the complex processes of cloud forma-
tion and condensation take place. About 80% of the world's
precipitation falls directly back into the oceans, in which
case the hydrologic cycle is a three-step process of evapo-
ration, condensation, and precipitation. For the 20% of all
precipitation that falls on land, the hydrologic cycle is more
complex, involving evaporation, condensation, movement
of water vapor from the oceans to land, precipitation, and
runoff. Some precipitation evaporates as it falls and reenters
the cycle, but about 36,000 km 3 of the precipitation that falls
on land returns to the oceans by runoff, the surface fl ow in
streams and rivers.
Not all precipitation returns directly to the oceans.
Some is temporarily stored in lakes and swamps, snowfi elds
and glaciers, or seeps below the surface where it enters the
groundwater system (see Chapter 13). Water might remain
in some of these reservoirs for thousands of years, but even-
tually glaciers melt, lakes and groundwater feed streams and
rivers, and this water returns to the oceans. Even the water
used by plants evaporates, a process known as transpiration ,
and returns to the atmosphere. In short, all water derived
from the oceans eventually makes it back to the oceans and
can thus begin the hydrologic cycle again.
The Hydrologic Cycle
The connection between precipitation and clouds is
obvious, but where does the moisture for rain and snow
come from in the first place? In the previous section, we
noted that 97.2% of all water on Earth is in the oceans,
so you might immediately suspect that the oceans are the
ultimate source of precipitation. In fact, water is continu-
ously recycled from the oceans, through the atmosphere,
to the continents, and back to the oceans. This hydro-
logic cycle, as it is called (
Figure 12.3), is powered by
solar radiation and is possible because water changes eas-
ily from liquid to gas (water vapor) under surface condi-
tions. About 85% of all water entering the atmosphere
comes from a layer about 1 m thick that evaporates from
the oceans each year. The remaining 15% comes from
water on land, but this water originally came from the
oceans as well.
 
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