Chapter 7Water: Hydrologic Cycle

and Human Use

Copyright © 2008 Pearson Prentice Hall, Inc.

EnvironmentalScienceTenth Edition

Richard T. Wright

Water: A Vital Resource

• 71% of Earth’s surface is covered with water– 97.5% of this volume is salt water of the oceans

and seas– The remaining 2.5% is fresh water – water with a

salt concentration of less than 0.1%.• 1.97% of fresh water is frozen in ice caps and glaciers• 0.5% is located as groundwater• 0.03% is surface water (lakes, rivers, streams) or water

vapor in our atmosphere.

Water: A Vital Resource

Hydrologic Cycle

• Earth’s water cycle, also called the hydrologic cycle consists of – Water rising to the atmosphere through

evaporation and transpiration• Transpiration – loss of water vapor as it moves from the

soil through green plants and exits through leaf stomata.

– Water returning to thee land and oceans through condensation and precipitation

The Water Cycle

Properties of Water

• Weak attraction known as hydrogen bonding tends to hold water molecules together– Polar molecule – one end is slightly positive, one

end is slightly negative• Below 32oF (0oC) – water freezes; 212oF,

(100oC) water boils.

Polarity in H2O: The Water Molecule

O

H H+ +

--

covalent bond

Penny Activity

• How many drops of water do you think will fit on a penny? – Write down your hypothesis: _______________– Write down actual number: ________________– Draw what your penny looked like :

– Why do you think this occurred?

Natural Filter

• The Hydrologic cycle naturally purifies water– When water in an ocean or lake evaporates, only

the water molecules leave the surface; the dissolved slats and other solids remain behind in solution

– When water vapor condenses again, it is purified water – except for the pollutants and other aerosols it make pick up from the air• Aerosol – microscopic liquid or solid particles

originated from land and water surfaces.

Ground Water

• As precipitation hits the ground, it may either soak into the ground (infiltration) or run off the surface.– The amount that soaks in compared with the amount

that runs off is called the infiltration-runoff ratio.• Runoff flows over the surface of the ground into

streams and rivers, which make their way to the ocean or inland seas.– All the land that contributes water to a particular

stream or river is referred to as the watershed

Ground Water

• Has Two Options– May be held in soil – capillary water• Will return to the atmosphere by evaporation through

the soil, or through transpiration through plants• EVAPOTRANSPIRATION

– Percolation• Water that is not held in soil is called gravitational

water because it trickles down through pores or cracks until it reaches an impervious layer of rock or clay

Ground Water

• Once gravitational water reaches the impervious rock layer it accumulates– Rock formations that contain groundwater are

called aquifers• Confined aquifers – has impervious rock layer above

water (artesian aquifer)• Unconfined aquifer – upper layer coincides with the

water table

• Recharge zone– Area above an aquifer that replenishes the water

Natural Filter

• As water percolates through the soil, debris and bacteria from the surface are filtered out

• May also dissolve and leach out certain minerals– In most cases, the minerals that leach into

groundwater are harmless• Exceptions – sulfide, arsenic

Closure

• How much of our Earth’s surface is covered with water?

• How much of that water is salty?

Warm-up

• Discuss how freshwater is distributed among our earth

Human Impact on the Hydrologic Cycle

• A large share of the environmental problems we face today stem from direct or indirect impacts on the water cycle.

• These impacts can be classified into four categories– Changes to Earth’s surface– Floods – Changes to Earth’s climate– Atmospheric pollution– Withdrawals for human use

Human Impacts on the Hydrological Cycle

Water Uses and Sources

• Nonconsumptive vs. Consumptive– Nonconsumptive• Most of the water used in homes and industries • Used for washing and flushing away unwanted

materials, or for cooling water in electric power generation– The water will be contaminated with wastes and will remain

available for human use if adequate treatment is used.

Water Uses and Sources

• Nonconsumptive vs. Consumptive– Consumptive • Example – irrigation• The applied water does not return to the water

resource– It is allowed to percolate into the ground or return to the

atmosphere through evapotranspiration

Water Uses Worldwide

• Largest use of water is for irrigation (agriculture)– 70%

• Second is for industry– 20%

• Third is for direct human use (municipal)– 10%

• Water use will differ depending on natural precipitation and the degree to which the region is developed

Trends in Population and Freshwater Withdrawals, U.S.

Regional Usage of Water

Water in the Developing World

Municipal Water Useand Treatment

What is your community’swater source? (field trip to well 11!)

Dams & Reservoirs

Pros• Ensure year-round supply of

water in areas that have seasonal precipitation

• Reservoirs control flow of water

• Generation of hydropower• Provide flood control• Provide recreational

activities on reservoirs.

Cons• Heavy deposition of

sediment upstream• Disrupts ecosystems• Adversely affects fish

population• Disrupts flow of river

downstream• Dam failures

Glen Canyon Dam

Upper Klamath Lake Before Diversion

Upper Klamath Lake After Diversion

Warm Up

• What is the difference between consumptive and non-consumptive uses of water?

• List the three main uses for water world wide. Circle the one that takes the most water

Consequences of Overdrawing Groundwater

• Falling water tables• Diminishing surface waters (wetlands)• Land subsidence • Saltwater intrusion

Ogallala Aquifer

• Within seven-state High Plans region of the United States– Supplies region with irrigation water

• More than 3/4ths of the groundwater on earth is stored in this aquifer

• Takes centuries to recharge aquifer– Considered a nonrenewable aquifer because

water is being drawn faster than can be recharged

Center Pivot Irrigation10,000 gallons/minute

Aerial View of Center Pivot Irrigation Sites

Aquifer Exploitation

• Ground water use exceeds aquifer recharge

• Many remaining aquifers are heavily polluted

Land Subsidence

• Over ages, groundwater has leached cavities in the ground

• When water fills the spaces, it helps supports overlying rock and soil

• If water table drops, the land sinks

Sinkhole

Saltwater Intrusion

• In coastal regions, springs of out-flowing ground water may lie under the ocean

• As long as the water table is high, there is enough pressure in the aquifer so that fresh water will flow into the ocean

• If water table lowers, pressure is lost and the ocean will flow backwards into the aquifer

Saltwater Intrusion

Water Stewardship: Public-Policy Challenges

• Obtaining more water– More dams (Three Gorges Dam)– Tapping more groundwater– Desalting seawater

• Using less water• Public-policy challenges

Dams

• Three Gorges Dam– Largest in world– Controls deadly floods– Displaced 1.2 million people including farms,

cities, homes, and factories– 370 mile-long reservoir

Dam Construction