chapter 7 water: hydrologic cycle and human use copyright © 2008 pearson prentice hall, inc....
TRANSCRIPT
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