Geoscience Reference
In-Depth Information
Rivers,
streams
0.0001%
Atmosphere
0.001%
Groundwater
0.63%
Soil water
0.005%
Freshwater
lakes,
0.009%
Oceans
97.2%
Ice sheets,
glaciers
2.15%
Salt lakes,
inland seas
0.008%
Figure 7.3 Geographic distribution of water within the hydrosphere.
Most water is contained within the oceans, with
proportionately smaller amounts stored in polar ice sheets and as fresh surface and soil water.
liter of water, it is the most important to us because it includes the
water available for plants, animals, and human use. The right-hand
pie chart in Figure 7.3 shows the geographic distribution of this part
of Earth's water realm. Of this small amount, most (0.009% of the
total volume) is stored in freshwater lakes. Another 0.008% is con-
tained within salty lakes and inland seas. Soil water, which is held
at shallow depths within reach of plant roots, comprises 0.005%.
Of the remaining 0.002% of the total water volume, the atmosphere
and streams contain 0.001% and 0.0001%, respectively.
As you probably know on some level, water moves from
one place to another within the hydrosphere. You have no doubt
seen water move from the atmosphere to the ground as rain to
form street puddles. And then, over the course of several warm
days, the water evaporates from the street as it moves back to
the atmosphere. Water not only moves back and forth from the
air to the ground, but also to and from places on and within
the ground. Taken together, this movement of water between
the various storage locations (or reservoirs) constitutes the
hydrologic cycle
.
Overall, the hydrologic cycle is balanced in the sense
that Earth has a finite amount of water, and the amount
evaporated equals the amount precipitated on a global scale.
However, a variety of local and regional imbalances occur.
To see how the hydrologic cycle fits into the context of the
global water balance, examine Figure 7.4, which shows the
flow of water on Earth in thousands of cubic kilometers
(km
3
) per year. With the notion of a global water budget in
mind, consider that water flows to the land and ocean sur-
faces are positive inputs, whereas those leaving are negative.
As you examine these geographic patterns, note the positive
balance (
+
36) on landmasses. This surplus reflects the fact
that land gains 36,000 km
3
more water through precipitation
than it loses through evaporation. In contrast, oceans have a
negative balance (
-
36). This negative balance exists because
oceans lose 36,000 km
3
more water by evaporation than they
gain through precipitation. Obviously, if there were no di-
rect connection between the land and the oceans, the land-
masses would gradually become submerged by water and the
oceans would empty. This reversal does not occur because
36,000 km
3
of water run off the land per year and return to
the oceans.
Hydrologic cycle
A model that illustrates the way that water
is stored and moves on Earth from one reservoir to another.
VISUAL CONCEPT CHECK
7.1
The hydrologic cycle has many different components. This
image shows a typical pond in New England. Explain how
this pond is part of the hydrologic cycle. What are two
ways in which water could fill the pond? What happens to
the water once it becomes stored within the pond? How
does water return to the atmosphere from the pond?
