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Ocean water (salty)
97.6%

Freshwater
2.4%

FIGURE 24.1 This is one of the water canals of the present-
day system in Phoenix, Arizona. These canals were reconstructed
from a system that was built by Native Americans, then abandoned.
Phoenix is named after a mythical bird that was consumed by fire
and then arose from its ashes. Locked up
in ice
78.5%

Water vapor in
the return of water to the ocean by streams and rivers. This cycle In sediments the atmosphere
.004%
of evaporation, precipitation, and return of water to the ocean (underground)
20.6%
summarizes all aspects of the hydrologic cycle ( Figure 24.3).
This water returning on and under the land is the source of
freshwater.

Rivers, streams,
FRESHWATER and lakes
.8%
The basic source of freshwater is precipitation, but not all
precipitation ends up as part of the freshwater supply. Liquid FIGURE 24.2 Estimates of the distribution of all the water
water is always evaporating, even as it falls. In arid climates, found on Earth’s surface.
rain sometimes evaporates completely before reaching the
surface, even from a fully developed thunderstorm. Evapo-
ration continues from the water that does reach the surface. melting snow, but otherwise, most of the flow comes from
Puddles and standing water on the hard surface of city park- groundwater that seeps into the stream channel. This explains
ing lots and streets, for example, gradually evaporate back to how a permanent stream is able to continue flowing when
the atmosphere after a rain and the surface is soon dry. There it is not being fed by runoff or melting snow (Figure 24.4).
are many factors that determine how much of a particular Where or when the source of groundwater is in low supply, a
rainfall evaporates, but in general, more than two-thirds of stream may flow only part of the time, and it is designated as
the rain eventually returns to the atmosphere. The remain- an intermittent stream.
ing amount either (1) flows downhill across the surface of the The amount of a rainfall that becomes runoff or ground-
land toward a lower place or (2) soaks into the ground. Water water depends on a number of factors, including (1) the type of
moving across the surface is called runoff. Runoff begins as soil on the surface, (2) how dry the soil is, (3) the amount and
rain accumulates in thin sheets of water that move across the type of vegetation, (4) the steepness of the slope, and (5) whether
surface of the land. These sheets collect into a small body of the rainfall is a long, gentle one or a cloudburst. Different com-
running water called a stream. A stream is defined as any body binations of these factors can result in from 5 percent to al-
of water that is moving across the land, from one so small that most 100 percent of a rainfall event running off, with the rest
you could step across it to the widest river. Water that soaks evaporating or soaking into the ground. On the average, how-
into the ground moves down to a saturated zone, and it is now ever, about 70 percent of all precipitation evaporates back into
called groundwater. Groundwater moves through sediments the atmosphere, about 30 percent becomes runoff, and less than
and rocks beneath the surface, slowly moving in a downhill 1 percent soaks into the ground. (For a worked example on this
direction. Streams carry the runoff of a recent rainfall or material, see the chapter 24 resources at www.mhhe.com/tillery.)

24-3 CHAPTER 24 Earth’s Waters 599