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OVERVIEW

Throughout history, humans have diverted rivers and reshaped the land to ensure a supply of freshwater. There is
evidence, for example, that ancient civilizations along the Nile River diverted water for storage and irrigation some
5,000 years ago. The ancient Greeks and Romans built systems of aqueducts to divert streams to their cities some
2,000 years ago. Some of these aqueducts are still standing today. More recent water diversion activities were
responsible for the name of Phoenix, Arizona. Phoenix was named after a mythical bird that arose from its ashes after
being consumed by fire. The city was given this name because it is built on a system of canals that were first designed
and constructed by ancient Native Americans, then abandoned hundreds of years before settlers reconstructed the
ancient canal system (Figure 24.1). Water is, and always has been, an essential resource. Where water is in short
supply, humans have historically turned to extensive diversion and supply projects to meet their needs.
Precipitation is the basic source of the water supply found today in streams, lakes, and beneath Earth’s surface.
Much of the precipitation that falls on the land, however, evaporates back into the atmosphere before it has a chance
to become a part of this supply. The water that does not evaporate mostly moves directly to rivers and streams,
flowing back to the ocean, but some soaks into the land. The evaporation of water, condensation of water vapor, and
the precipitation-making processes were introduced in chapter 23 as important weather elements. They are also part
of the generalized hydrologic cycle of evaporation from the ocean, transport through the atmosphere by moving air
masses, precipitation on the land, and movement of water back to the ocean. Only part of this cycle was considered
previously, however, and this was the part from evaporation through precipitation. This chapter is concerned with the
other parts of the hydrologic cycle, that is, what happens to the water that falls on the land and makes it back to the
ocean. It begins with a discussion of how water is distributed on Earth and a more detailed look at the hydrologic
cycle. Then the travels of water across and into the land will be considered as streams, wells, springs, and other
sources of usable water are discussed as limited resources. The tracing of the hydrologic cycle will be completed as
the water finally makes it back to the ocean. This last part of the cycle will consider the nature of the ocean floor,
the properties of seawater, and how waves and currents are generated. The water is now ready to evaporate, starting
another one of Earth’s never-ending cycles.

called freshwater. About two-thirds of Earth’s freshwater supply
24.1 WATER ON EARTH
is locked up in the ice caps of Greenland and the Antarctic and
Some water is tied up in chemical bonds deep in Earth’s inte- in glaciers. This leaves less than 1 percent of all the water found
rior, but free water is the most abundant chemical compound on Earth as available freshwater. There is a generally abundant
near the surface. Water is five or six times more abundant supply, however, because the freshwater supply is continually
than the most abundant mineral in the outer 6 km (about replenished by the hydrologic cycle.
4 mi) of Earth, so it should be no surprise that water covers Evaporation of water from the ocean is an important pro-
about 70 percent of the surface. On average, about 98 per- cess of the hydrologic cycle because (1) water vapor leaves
cent of this water exists in the liquid state in depressions on the dissolved salts behind, forming precipitation that is fresh-
the surface and in sediments. Of the remainder, about 2 per- water, and (2) the gaseous water vapor is easily transported
cent exists in the solid state as snow and ice on the surface in in the atmosphere from one part of Earth to another. Over a
colder locations. Only a fraction of a percent exists as a vari- year, this natural desalination process produces and transports
able amount of water vapor in the atmosphere at a given time. enough freshwater to cover the entire Earth with a layer about
Water is continually moving back and forth between these 85 cm (about 33 in) deep. Precipitation is not evenly distrib-
“reservoirs,” but the percentage found in each is essentially uted like this, of course, and some places receive much more,
constant. while other places receive almost none. Considering global
As shown in Figure 24.2, over 97 percent of Earth’s water averages, more water is evaporated from the ocean than re-
is stored in the oceans. This water contains a relatively high turns directly to it by precipitation. On the other hand, more
level of dissolved salts, which make ocean water unfit for hu- water is precip itated over the land than evaporates from the
man consumption and for most agricultural purposes. All other land back to the atmosphere. The net amount evaporated and
water, which is fit for human consumption and agriculture, is precipitated over the land and over the ocean is balanced by

598 CHAPTER 24 Earth’s Waters 24-2