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Q S _ GROUNDWATER

%Q S = × 100%

P
Precipitation soaks into the ground, or percolates slowly down-
_
160 mm

= × 100% ward, until it reaches an area, or zone, where the open spaces

914 mm
between rock and soil particles are completely filled with
= 17.5%
water. Water from such a saturated zone is called groundwater.
Q G _

%Q G = × 100% There is a tremendous amount of water stored as groundwa-
P
ter, which makes up a supply about 25 times larger than all the
_
68 mm
= × 100% surface water on Earth. Groundwater is an important source

914 mm
of freshwater for human consumption and for agriculture.
= 7.4%
Groundwater is often found within 100 m (about 330 ft) of the
surface, even in arid regions where little surface water is found.
EXAMPLE 24.2 Groundwater is the source of water for wells in addition to
being the source that keeps streams flowing during dry periods.
In watershed budget analysis, each component is expressed as a length. Water is able to percolate down to a zone of saturation be-
This enables the volume of each component to be calculated from the
watershed area ( A) by multiplying the component by the area. The cause sediments contain open spaces between the particles called
units are cubic meters. pore spaces. The more pore space a sediment has, the more water
2
If the watershed area is 12 km , what is the volume of the ground- it will hold. The total amount of pore spaces in a given sample of
3
5
water discharge (Q G ) component? (Answer: 8.2 × 10 m .) sediment is a measure of its porosity. Sand and gravel sediments,
for example, have large grains with large pore spaces between
them, so these sediments have a high porosity. In order for water
to move through a sediment, however, the pore spaces must be
Two adjacent watersheds are separated by a line called a connected. The ability of a given sample of sediment to trans-
divide. Rain that falls on one side of a divide flows into one mit water is a measure of its permeability. Sand and gravel have
watershed, and rain that falls on the other side flows into the a high permeability because the grains do not fit tightly together,
other watershed. A continental divide separates river sys- allowing water to move from one pore space to the next. Sand and
tems that drain into opposite sides of a continent. The North gravel sediments thus have a high porosity as well as a high per-
American continental divide trends northwestward through the meability. Clay sediments, on the other hand, have small, flattened
Rocky Mountains. Imagine standing over this line with a glass particles that fit tightly together. Clay thus has a low permeability,
of water in each hand, then pouring the water to the ground. and when saturated or compressed, clay becomes impermeable,
The water from one glass will eventually end up in the Atlantic meaning water cannot move through it at all (Figure 24.6).
Ocean, and the water from the other glass will end up in the
Pacific Ocean. Sometimes the Appalachian Mountains are con-
sidered to be an eastern continental divide, but water from both CONCEPTS Applied
sides of this divide ends up on the same side of the continent, in
the Atlantic Ocean. Room for Water
Water moving downhill is sometimes stopped by a depres- Compare the amount of porous space in sandstone,
sion in a watershed, a depression where water temporarily collects limestone, and other rocks. Dry samples of the rocks in a
as a standing body of freshwater. A smaller body of standing wa- warm oven until they are thoroughly dry, then record the
ter is usually called a pond, and one of much larger size is called mass of each rock. Place each in a container of boiling
a lake. water and watch until no more bubbles are coming from the
A pond or lake can occur naturally in a depression, or rocks. Discard the water and blot dry. Again find the mass
it can be created by building a dam on a stream. A natural and calculate how much water entered the porous space of
pond, a natural lake, or a pond or lake created by building each rock. Note that 1 g of water has a volume of 1 mL, or
3
1 cm . Report the volume of water that can occupy the pore
a dam is called a reservoir if it is used for (1) water storage,
space of a certain mass of each kind of rock tested.
(2) flood control, or (3) generating electricity. A reservoir can
be used for one or two of these purposes but not generally for
all three. A reservoir built for water storage, for example, is
kept as full as possible to store water. This use is incompatible The amount of groundwater available in a given location
with use for flood control, which would require a low wa- depends on a number of factors, such as the present and past
ter level in the reservoir in order to catch runoff, preventing climate, the slope of the land, and the porosity and permeability
waters from flooding the land. In addition, extensive use of of the materials beneath the surface. Generally, sand and gravel
reservoir water to generate electricity requires the release of sediments, along with solid sandstone, have the best porosity
water, which could be incompatible with water storage. The and permeability for transmitting groundwater. Other solid
water of streams, ponds, lakes, and reservoirs is collectively rocks, such as granite, can also transmit groundwater if they are
called surface water, and all serve as sources of freshwater. sufficiently fractured by joints and cracks. In any case, ground-
The management of surface water, as you can see, can present water will percolate downward until it reaches an area where
some complicated problems. pressure and other conditions have eliminated all pores, cracks,

602 CHAPTER 24 Earth’s Waters 24-6