Earth’s Fresh Water
Fresh Water
 Oceans cover more than 70% of the Earth’s
surface.
 97% is found in the oceans.
 Fresh water makes up only 3% of the
Earth’s water.
 85% of the fresh water is frozen in the polar
icecaps.
The
Hydrologic Cycle
The Hydrologic Cycle
 Most of the fresh water on the Earth’s
surface is found in moving and in stand
water.
 Rivers, streams and springs are moving
water.
 Ponds, lakes, and swampy wetlands are
standing water.
Water Cycle
 The hydrologic cycle or water cycle is the
movement of water from the oceans and
freshwater sources to the air and land and
finally back to the oceans.
 The water cycle constantly renews the
Earth’s supply of fresh water.
Steps of the Cycle
 Evaporation
 Condensation
 Precipitation
Evaporation
 The first step involves the heat energy given
off by the sun. This energy causes water on
the surface of the Earth to change to water
vapor.
 This process is called evaporation.
Sources of Evaporation
 Enormous amounts of water evaporate from
the oceans.
 Water also evaporates from freshwater
sources and from the soil.
 Animals and plants release water vapor into
the air.
Condensation
 The second step is condensation.
 Condensation is the process by which water
vapor changes back into a liquid.
Condensation
 For condensation to occur, the water vapor
must be cooled.
 This happens when the warm air close to
the surface of the Earth rises.
 As it moves farther from the Earth, it cools.
 Cool air holds less vapor and it condenses
into droplets of water known as clouds.
Water Vapor
 When water evaporates from the ocean, it
leaves the salts behind.
 Water vapor is made of fresh water only.
Precipitation
 The third step is precipitation.
 Precipitation occurs when the water returns
to the Earth in the form of rain, snow, sleet
or hail.
Precipitation
 Precipitation occurs when the water droplets
that form in clouds become too numerous
and heavy to remain afloat in the air.
 It returns to the Earth as rain, snow, sleet or
hail.
 After it falls, some returns to the atmosphere
through evaporation and the cycle is
complete.
Groundwater
 Some of the water that falls as precipitation
may run off into land, ponds, streams, river
or oceans.
 Some may soak into the ground and
become groundwater.
 At some point, the groundwater flows
underground to the oceans.
Frozen Water
 Snow is a frozen form of water.
 The pressure of piled-up snow causes some
of the snow to change into ice and
eventually glaciers form.
 A glacier is a huge mass of moving ice and
snow.
Glaciers
 Glaciers form in very cold areas. Glaciers
hold 2% of fresh water.
 Because of the extremely cold
temperatures, the snow that falls does not
melt completely. As more snow falls it do
covers the old snow. The pressure
squeezes the snow crystals together and ice
forms. When the layer of ice become very
thick and heavy, the ice begins to move.
Valley Glaciers
 Long, narrow glaciers that move downhill
between the steep sides of mountain valley
are called valley glaciers.
 Usually valley glaciers follow channels
carved by running water. The movement of
the glacier downhill causes cracks in the ice
known as crevasses.
Valley Glaciers
 As valley glaciers slide downward, they tear
rock fragments from the mountainside.
 The rock fragments become frozen in the
glaciers and cut deep grooves in the valley
walls. Mountains located anywhere from the
equator to the poles can contain glaciers.
 Mount Rainier in Washington and Mount
Washington in New Hampshire contain
small glaciers.
Meltwater
 As the valley glacier moves, some of the ice
begins to melt, forming a stream of water,
called meltwater.
 Meltwater is usually pure water and some
cities such as Boulder, CO use it as a
source of drinking water.
 Some meltwater is used to generate
electricity although it is costly and could alter
the environment.
Continental Glaciers
 In the polar regions, snow and ice have built
up to form thick sheets, called continental
glaciers.
 Continental glaciers cover million of square
kilometers of the Earth’s surface.
 They move slowly in all directions.
 Continental glaciers are found in Greenland
(85%) and Antarctica (98%).
Icebergs
 At the edge of the sea, continental glaciers
form overhanging cliffs. Large chunks of
ice, called icebergs, often break off from
these cliffs and drift into the sea. Some
icebergs are as large as the state of Rhode
Island.
 Icebergs pose a major hazard to ships, as in
the sinking of the Titanic in 1912.
Fresh Water in Icebergs
 Much fresh water is frozen in icebergs.
 Attempts have been made to tow icebergs
to areas that need fresh water.
 There are problems with transporting
icebergs. First the effect of icebergs on
local weather must be considered. Second,
it would be costly and time-consuming.
Last, ways are needed to stop melting.
Running Water
 Rivers and streams are important sources of
fresh water. Many towns and cities build
near rivers and streams.
 The water is used to irrigate crops, generate
electricity, drinking and other household
purposes.
 Rivers are also used for recreational
purposes.
 Rivers have been used to remove waste.
Surface Runoff
 Rain and melted snow that do not evaporate
or soak into the soil flow into rivers and
streams. The water that enters a river or
stream after a heavy rain or spring thaw is
called surface runoff.
Amount of Runoff
 The amount of surface runoff is affected by several
factors. One is the type of soil the precipitation fall
on. Some soils are more porous and soak up
more water.
 The condition of the soil is also important. Dry soil
will soak up more. Wet soil will accept less water
 Plant roots absorb water. In areas where there
are many plants, more water is absorbed.
Watershed
 A land area in which surface runoff drains
into a river or a system of rivers is called a
watershed.
 Water sheds prevents floods and water
shortages by controlling the amount of water
that flows into streams and rivers.
 Watersheds also provide a steady flow of
fresh water into the oceans.
Usefulness of Rivers
 Many rivers are sources of fresh water. The
amount of water in a river and the speed at which
the water flows affect the usefulness or a river as a
source.
 Rivers that move quickly carry more water. They
also carry a lot of soil, pebbles and sediments.
Slow moving rivers appear more clear.
 Pollution also affects the usefulness of a river.
Standing Water
 Within a watershed, some of the surface
runoff gets caught in low places. Standing
bodies of fresh water are formed there.
 Depending on their size, these bodies of
water are called ponds or lakes, which
provide fresh drinking water.
 Surface runoff keeps the ponds and lakes
from drying up. The ponds and lakes
control flooding and hold water in reserve.
Lakes
 Lakes are usually large, deep depressions
in the Earth’s crust that have filled with fresh
water. Rain , melting snow and water from
springs and rivers fill these depressions.
 A lake is sometimes formed when there is a
natural blockage of a river or stream.
 Lakes are usually found at high altitudes
where glaciers were once present.
Ponds
 Ponds are shallow depression in the Earth’s
crust that have filled with fresh water.
 They are smaller and not as deep as lakes.
 Sunlight penetrates to the bottom of a pond
so plants can be found throughout a pond.
Reservoirs
 The most frequent source of fresh water are
artificial lakes known as reservoirs.
 A reservoir is built by damming a stream or
river that runs through a low lying area.
 Reservoirs help control flooding and store
water.
 They sometimes provide irrigation for crops.
 They can also be used to generate
electricity.
Groundwater
 There is more fresh water below the surface
of the land than in all the lakes and
reservoirs on the Earth’s surface.
 Ground water moves slowly downward
through pores in the rocks and soil.
 Material through which water can move
quickly is described as permeable.
 Water cannot move quickly through
impermeable material.
Zone of Saturation
 Groundwater continues to move downward
through permeable rock and soil until it
reaches an impermeable layer. This
underground zone in which all the pores are
filled with water is called the zone of
saturation.
Zone of Aeration
 Above the water-filled zone, the ground is
not as wet. Pores in the soil and rocks are
filled mostly with air. This drier region in
which the pores are filled mostly with air is
call the zone of aeration.
Water Table
 The surface between the zone of saturation
and the zone of aeration is an important
boundary. It marks the level below which
the ground is saturated, or soaked, with
water. This level is called the water table
Depth of the Water Table
 The depth of the water table varies based
on location, climate, weather and man-made
structures.
Water Table Location
 In general, the water table is not very deep
near large bodies of water.
 In high areas nears hills or mountains, the
water table may be deep within the ground.
 In low-lying areas such as valleys with
swamps and marshes, the water table may
be close to the surface.
Water Table and Climate
 The depth of the water table varies with the
climate of an area.
 It will be deep in dry areas such as deserts.
 It will be near the surface in low-lying forest
areas.
 In very moist climates, the water table may
come right to the surface and form a
swamp, lake or spring.
Water Table and Weather
 Even in the same area, the water table may
change.
 Heavy rains and snow may make the water
table rise.
 If there is a long dry period, the water table
will fall.
Water Table and Man
 The depth of the water table will also
change if wells are overused or if many
wells are located in a small area. Wells are
holes drilled to bring the water table to bring
water to the surface.
Problems with Water Table
 In areas where the water table is high, it
may flood foundations for buildings.
 It is easier and less expensive to dig wells in
areas where the water table is high.
Aquifers
 As groundwater moves through a permeable
rock layer, it often reaches an impermeable
rock layer or the water table. At this point,
the groundwater may move sideways
through a layer of rock or sediment that
allows it to pass freely. Such a layer is
called aquifer. Aquifers are usually layers of
sandstone, gravel, sand or cracked
limestone.
Aquifers
 Because rocks form in layers, sometimes a
layer of permeable rock may become
trapped between two layer of impermeable
rock. Sandstone trapped between two
layers of shale is an example.
 An aquifer is a source of groundwater that is
very vulnerable to pollution.
Artesian Wells
 In some places where the underground rock
layers slope, an aquifer carries water from a
higher altitude to a lower altitude. If the
aquifer is trapped between two layers of
impermeable rock, pressure may build up at
the lower altitude. A well drilled in to the
aquifer at this point will provide water
without pumping. It is called an artesian
well.
Caverns
 In some areas, the underlying rock is
limestone. As groundwater moves down
through the soil, it combines with carbon
dioxide to form carbonic acid that can
dissolve limestone. When water enters the
cracks in limestone, the carbonic acid
causes the cracks to become wider until
large underground caverns with passage
are formed.
Stalactites and Stalagmites
 These underground caverns have long
stone icicles hanging from the ceiling
(stalactites) and built up from the floors
(stalagmites.)
 Stalactites and stalagmites are formed when
dissolved substances in groundwater are
deposited.
Composition of Water
 A water molecule is the smallest particle that
has all the properties of water.
 A water molecule forms when two atoms of
hydrogen and one of oxygen combine.
 In a water molecule the atom of oxygen has
a slight negative charge. Each atom of
hydrogen has a slight positive charge.
 These charged ends give a water molecule
the property known as polarity.
Water as a Solvent
 It is the property of polarity that gives water
molecules that makes water solvent.
 A solvent is a substance in which another
substance dissolves. The dissolving
process produces a solution.
 A solution contains two or more substances
mixed on the molecular level.
 Water is known as the universal solvent.
Hardness of Water
 The taste, odor and appearance of water
varies from area to areas. The differences
depend on the amounts and types of
materials dissolved in the water.
 Hard water contains larges amount of
dissolved minerals, especially calcium and
magnesium.
 Soft water does not contain these minerals.
Soft Water
 Some water is softened naturally as it
passes through and reacts with rock
formations that contain certain minerals.
 These minerals remove the calcium and
magnesium from the water, making it soft.
Quality of Water
 Water is necessary to all life on Earth. It is
therefore necessary to maintain the quality
of our water.
 In nature, water is usually filtered as it
passes through sand and soil. Because so
many different substances can dissolve in
water, it is becoming more and more
polluted.
Pollution of Water
 Water pollution limits the amounts and kinds
of wild life that can live in water. It also
affects drinking water supplies. It destroys
recreational areas. Nitrates and phosphates
from fertilizers pollute groundwater. They
must be removed before the water can be
used for drinking or swimming.
 Federal laws prevent industries from
dumping certain chemicals wastes.