Download The hydrologic cycle

SGCEP SCIE 1121
Environmental Science
Spring 2012
Section 20531
Steve Thompson: [email protected]
http://www.bioinfo4u.net/
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Sunday, February 19, 2012
Water, a vital resource: the
hydrologic cycle and human use
• Water is fundamental to all life!
• The Earth is flooded with water . . .
• Covering 75% of the Earth’s surface. But . . .
• 97.5% of all water is salt water (oceans, seas).
• Fresh water: contains < 0.1% (1,000 ppm) salt.
• But 67% of fresh water is bound up in ice caps
and glaciers. Therefore, . . .
• Only 0.77% of all water is in lakes, wetlands,
rivers, groundwater, biota, soil, and the
atmosphere.
• Evaporation and precipitation cycle water
through the solar-powered hydrologic cycle.
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Sunday, February 19, 2012
The Earth’s water
Check out: http://www.youtube.com/watch?v=nSENolWbyYQ
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Sunday, February 19, 2012
The hydrologic cycle
• Remember those other biogeochemical cycles.
Here we’ll look at the hydrologic (water) cycle:
the cycling of water through the Earth
• Evaporation and transpiration: water rises
to the atmosphere.
• Condensation and precipitation: water
returns to the land and oceans.
• Green water: water in vapor form.
• Blue water: water in liquid form.
• http://ga.water.usgs.gov/edu/watercycle.html
• http://en.wikipedia.org/wiki/Water_cycle
• http://www.youtube.com/watch?v=EkfM5-B61w
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The hydrologic cycle
Animation: Hydrologic Cycle
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Evaporation
• Hydrogen bonding holds water molecules (H2O)
together at most temperatures.
• Below freezing (32°F, 0°C): low kinetic energy
allows hydrogen bonding to hold molecules in
place as a solid in the form of ice.
• Above freezing but below boiling: the kinetic
energy allows hydrogen bonds to rapidly break
and reform so it exists as a liquid.
• Evaporation: as water molecules absorb energy
from the Sun (or other sources), kinetic energy
allows the molecules to become more energetic
and enter the atmosphere as . . .
• Water vapor: water molecules in the gaseous
state.
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Sunday, February 19, 2012
Condensation
• Relative humidity: the amount of water vapor in the air
as a percentage of what the air can hold at any
particular temperature.
• The amount of water vapor the air can hold varies
with the temperature:
• Cooling causes water vapor to condense to liquid
water; Condensation is the opposite of evaporation.
• Water molecules rejoin by hydrogen bonding, forming
liquid water.
• Fog and clouds: droplets forming in the atmosphere.
• Dew: droplets forming on solid surfaces.
• Aerosols: microscopic liquid or solid particles, which . . .
• Help condensation occur.
• Originate naturally from volcanoes, dust, soil, salt;
• And from human sources: sulfates, carbon, dust.
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Condensation
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Purification
• Evaporation and condensation purify water
naturally. This is the basis for distillation.
• Evaporation removes only water molecules,
not salts and other solids.
• Atmospheric water turns over every 10 days.
• Water is constantly purified.
• Evaporation and condensation are the source of
all fresh water.
• Water eventually reaches the oceans, inland
seas, lakes.
• Evaporation and transpiration (evaporation
from plants) return water to the atmosphere.
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Sunday, February 19, 2012
Precipitation
• Warm, less dense air rises.
• Adiabatic cooling: warm air gradually
cools as it rises and expands.
• Adiabatic warming: occurs as cold air
descends and is compressed by higher air
pressure.
• Earth’s precipitation ranges from near
zero to 2.5 meters (100 in.) per year.
• The distribution depends on rising and
falling air currents.
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Sunday, February 19, 2012
Air currents affect precipitation
• Rising air cools and condenses . . .
• Precipitation results.
• Descending air warms, causing evaporation . . .
• Dryness results.
• A cold front causes warm, moist air in the
area to rise.
• The cold air of the advancing front is denser.
• The rising warm air cools and condenses and
precipitation occurs.
• Global convection currents and rain shadows
cause rising and falling air currents and
affect precipitation.
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Global precipitation patterns
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Convection currents
• Convection currents occur because the Sun
heats the Earth most intensely over and
near the equator (perpendicular rays).
• Heated air expands, rises, and cools;
• Condensation and precipitation occur.
• The equator’s constant heat causes this
process to repeat over and over . . .
• Supporting tropical rain forests.
• The now dry air “spills over” north and south
of the equator . . .
• Descending over subtropical areas and
creating deserts.
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Sunday, February 19, 2012
Global air circulation: Hadley cells
• Hadley cell: the
global system
composed of rising
and falling
convection air
currents.
Animation: Hadley Cells
• Trade winds: the
Earth’s rotation
deflects winds
east and west.
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Sunday, February 19, 2012
Rain shadow
• Moisture-laden trade winds encounter mountain
ranges, and . . .
• The deflected air rises and cools and
precipitation occurs on the windward side of the
mountains.
• Air crossing the mountains warms and picks up
moisture. Therefore, . . .
• Deserts occur on the leeward sides of mountains.
• This is called a rain shadow: the dry region
downwind of a mountain range.
• Causes the severest deserts in the world.
• For example, Death Valley, east of the Sierra
Nevada mountains in California.
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Rain shadow
And temperatures
are cooler the higher
up you go.
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Groundwater
• Precipitation can either soak into the ground
(infiltration) or run off the surface.
• Infiltration-runoff ratio: the amount of
water that soaks into the ground compared
with the amount that runs off.
• Runoff flows into streams and rivers,
eventually reaching oceans or inland seas.
• Watershed: all the land area contributing to
a stream or river.
• Surface waters: ponds, lakes, streams,
rivers, etc. on the Earth’s surface.
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Sunday, February 19, 2012
Infiltrating water has
t wo alternatives.
• Capillary water: held in the soil, according
to the soil’s capacity. It . . .
• Returns to the atmosphere by
evaporation or transpiration (green
water flow).
• Evapotranspiration: the combination of
evaporation and transpiration.
• Gravitational water: is not held in the soil.
• Percolation (blue water flow): trickling of
water through pores or cracks in the soil.
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Sunday, February 19, 2012
Groundwater and the water table
• Groundwater: water accumulated in the
earth.
• It lies on top of an impervious layer of
rock or clay.
• Water table: the upper surface of
groundwater.
• Gravitational water becomes groundwater
when it reaches the water table.
• Wells dug below the water table allow
groundwater to seep into the well.
• The groundwater fills the well to the level
of the water table.
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Sunday, February 19, 2012
Recharge
• Groundwater seeps laterally as it seeks its lowest
level.
• Aquifers: layers of porous material through which
groundwater moves.
• It is hard to determine the location of aquifers.
• Layers of porous rock are found bet ween layers
of imper vious rock.
• The entire formation may be fractured and
folded.
• Recharge area: the area where water enters an
aquifer.
• May be miles away from where water leaves
the aquifer.
• Aquifers hold 99% of all liquid water.
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Sunday, February 19, 2012
Underground purification
• As water percolates through the soil, debris and
bacteria are filtered out.
• Water may dissolve or leach out minerals.
• However, some minerals can be dangerous
(arsenic, sulfides, etc.).
• Drawn by gravity, groundwater moves through
aquifers until it finds an opening to the surface.
• Seep: water flows out in a wide area. Versus . . .
• Spring: water flows from a small opening.
• Seeps and springs feed streams, lakes, and rivers.
• The Floridan Aquifer is one of the most productive
in the world.
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Sunday, February 19, 2012
Loops, pools, and fluxes
• The hydrologic cycle has four physical
processes: evaporation, condensation,
precipitation, and gravitational flow.
• It also has three major loops.
• Evapotranspiration loop: green water that
evaporates and returns by precipitation.
• This is the main source for ecosystems and
natural agriculture.
• It is held as capillary water. And . . .
• It returns to the atmosphere by
evapotranspiration.
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Sunday, February 19, 2012
Other loops of the hydrologic cycle
• Surface runoff loop: blue water across the
ground’s surface . . .
• Becomes part of the surface water system.
• Groundwater loop: blue water that infiltrates,
percolates, and joins groundwater . . .
• Moves through aquifers and exits through
seeps, springs, and wells to rejoin surface
water.
• These t wo loops are the focus for human
water resource management.
• The hydrologic cycles “fluxes”: the exchange of
water among land, atmosphere, and oceans.
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Yearly water balance in the
hydrologic cycle
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Freshwater is a renewable resource.
• Streams, rivers, ponds, lakes, swamps, estuaries,
groundwater, bays, oceans, and atmosphere all
contain water. Therefore, . . .
• They represent ecosystem capital (goods and
services).
• Water is used for drinking, industries, irrigation,
energy, transportation, recreation, waste
processing, and habitats. Moreover, . . .
• Water modifies the climate. Plus . . .
• Humans have constructed huge infrastructures to
control water. These include things like . . .
• Dams, canals, reservoirs, aqueducts, sewer systems;
• Water treatment plants, water towers, pipelines,
irrigation projects; and . . .
• Desalinization plants.
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Sunday, February 19, 2012
Water in developed and developing nations
• Developed countries benefit from controlling water:
• Controlling diseases, building cities in deserts,
irrigation, electricity, etc.
• Water in developing countries is costly or inaccessible:
• They lack access to safe drinking water and
sanitation.
• People die from waterborne diseases.
• However, because of infrastructure built to control
water . . .
• Seas and rivers are being lost.
• Millions of people and entire ecosystems have been
displaced to make room for reservoirs. And . . .
• Tensions increase for access to water.
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Sunday, February 19, 2012
Human impact and future challenges
• Increasing population will challenge water management
infrastructure in many ways:
• Increasing agricultural output; and . . .
• Cities and industries compete with agriculture.
• Climate change also affects the hydrologic cycle.
• E.g. rainfall variability will cause floods and droughts.
• There are t wo ways to consider water issues:
• Quantity and Quality.
• Many environmental problems stem from direct or
indirect impacts on the water cycle.
• Four categories of impacts:
• Changes to Earth’s surface;
• Changes to Earth’s climate;
• Atmospheric pollution; and
• Withdrawals for human use.
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Sunday, February 19, 2012
Changes to the Earth’s surface
• In natural systems, vegetation intercepts precipitation.
• Water infiltrates porous topsoil, filtering out debris.
• Evapotranspiration sustains ecosystems and recycles
water.
• Recharged groundwater reservoirs release water
through springs and seeps into streams and rivers.
• In cleared forests and overgrazed land, plants do not
intercept rainfall.
• Water shifts from infiltration and recharge into
runoff causing a sudden influx of water into rivers
and streams . . .
• Causing floods, pollutants from erosion, and less
evapotranspiration and groundwater recharge.
• Resulting in dry, barren, lifeless streambeds.
• Wetlands also store and release water.
and
polluted
water
ways.
• Destruction leads to flooding
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Sunday, February 19, 2012
Climate change
• There is unmistakable evidence that the Earth is
warming.
• Increasing greenhouse gases are changing the
water cycle.
• Evaporation increases with a warmer climate.
• A wetter atmosphere means more and heavier
precipitation and floods. And . . .
• More hurricanes and droughts.
• Water-stressed areas (e.g., East Africa) will get
less water.
• Global warming may be speeding up the water
cycle.
• This affects precipitation, evapotranspiration,
groundwater recharge, runoff, snowmelt, etc.
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Sunday, February 19, 2012
Atmospheric pollution
• Aerosol particles form nuclei, enabling water to condense
into droplets so more clouds form.
• Anthropogenic particles are increasing . . .
• From sulfates, carbon (soot), dust. They . . .
• Form a brownish haze associated with industrial
areas, tropical burning, and dust storms.
• Solar radiation is reduced, producing a cooling effect.
• They promote smaller droplets, which . . .
• Suppresses rainfall, even though clouds form.
• Atmospheric cleansing is also suppressed, such that . . .
• The aerosols remain in the air longer, further increasing
drier conditions. Dust, smoke, and aerosols increase.
• Aerosols work differently from greenhouse gases:
• Aerosols have more local (versus global) impacts.
• They do not accumulate — they have a lifetime of days.
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Sunday, February 19, 2012
Water management
• Humans use 27% of all accessible freshwater
runoff.
• Global withdrawal will increase 10% each
decade. Fortunately, . . .
• Americans use less water than in 1980.
• Nonconsumptive uses of water: water may become
contaminated, but is still available to humans.
• Used in homes, industries, and electric power
production.
• Consumptive uses of water: the applied water does
not return to the water resource.
• It is gone from human control. For example, . . .
• Water for irrigation.
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U.S. trends in population and
freshwater withdrawals
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Sunday, February 19, 2012
Uses of water
• Worldwide, the largest use is for irrigation,
• Then industry and direct human use.
• Use varies by region, depending on:
• Natural precipitation, and the . . .
• Degree of development.
• Most increases in withdrawal are due to
increases in agriculture.
• Irrigation accounts for 65% of freshwater
consumption in the U.S.
• See http://www.youtube.com/watch?
v=XGgYTcPzexE
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Sunday, February 19, 2012
Sources of water
• 37% of domestic water comes from
groundwater sources.
• 63% comes from surface water (rivers, lakes,
reservoirs).
• Rural people in developing nations get water
wherever they can:
• Wells, rivers, lakes, rainwater.
• Women often have to walk long distances to
get water.
• Water in developing nations is often polluted
with waste:
• 1.1 billion people use polluted water;
• 1.6 million (mostly children) die each year.
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Sunday, February 19, 2012
Dams and the environment
• Over 45,000 large dams (over 50 feet) exist
worldwide.
• Reser voirs store billions of gallons, covering more
than 120 million acres.
• Only 31% of annual runoff is available for
withdrawal, because the rivers are too remote,
and . . .
• Navigation, flood control, hydropower need water.
• Large dams have enormous social impacts.
• They have displaced 40 million people. And have . . .
• Prevented access to goods and services of the
buried ecosystems.
• Although, dams do help prevent devastating floods.
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Sunday, February 19, 2012
Dams have enormous impacts
• Valuable freshwater habitats (waterfalls, rapids,
fish runs) are lost.
• The water way below the diversion is often
deprived of water.
• Fish and other aquatic organisms are directly
impacted. And all wildlife is adversely affected
(e.g., food chains).
• Wetlands dry up and waterfowl die.
• Fish (e.g., salmon) cannot swim upstream to
spawn or downstream to return to the ocean.
• Even with fish ladders to help them pass the
dams, juvenile salmon suffer 95% mortality
going to sea.
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Sunday, February 19, 2012
Dams gone
• People are beginning to recognize the unacceptable
costs of dams. Yeah!
• 714 dams have been removed in the U.S.
• But removal is not easy.
• There are legal complexities to dam removal.
• Existing uses conflict with expected advantages
(establishing fish, recreational and aesthetic uses).
• There are also practical problems to dam removal.
• Built up sediment in the reservoir will flood
downstream and can be contaminated.
• But often removing a dam provides major benefits,
such as restoring fish runs and providing
recreational uses of the resulting river.
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Sunday, February 19, 2012
Impacts on estuaries
• Estuaries are very productive ecosystems in bays
and rivers . . .
• Where fresh river water mixes with seawater.
• Rich breeding grounds for birds, fish, shellfish.
• Decreased fresh water increases the water’s
salinity.
• In the San Francisco Bay, over 60% of freshwater
has been diverted for irrigation and municipal use.
• This has devastated the bay.
• Fish populations have disappeared or been
drastically reduced.
• Tidal wetlands have been reduced by 92%.
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Sunday, February 19, 2012
Some aquifers are nonrenewable
• Nonrenewable groundwater: more than 75% of
aquifers have recharge rates of centuries.
• Renewable groundwater is replenished by
percolation.
• It is vulnerable to variations in precipitation.
• We are tapping large, but not unlimited, natural
reser voirs.
• Sustainability depends on balancing
withdrawal rates with recharge rates.
• Most groundwater in arid regions has no
recharge.
• It must be considered nonrenewable, like oil.
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Sunday, February 19, 2012
Impacts of falling water tables
• The simplest indication that groundwater
withdrawals exceed recharge rates is . . .
• Falling water tables. It is . . .
• Common throughout the world.
• It causes decreased crop production. And . . .
• Diminishing surface water . . .
• Dries up wetlands, springs and seeps,
streams, rivers.
• Excessive groundwater removal creates
the same results as diverting surface
water.
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Sunday, February 19, 2012
Falling water tables
• Groundwater fills spaces in the ground helping to
support overlying rock and soil. Dropping water tables
removes this support.
• Land subsidence: gradual settling of the land.
• Land may sink 10–15 cm (6–12 in.) per year. Causes . . .
• Building foundations, roadways, and pipes to crack.
• And results in flooding in coastal areas.
• And a sinkhole is another kind of land subsidence.
• It results when an underground cavern is drained of
its supporting groundwater and suddenly collapses.
• Sinkholes can be 300 feet across and 150 feet deep!
• They are particularly severe in the southeastern U.S.
• 4,000 sinkholes have formed in Alabama alone
• They have ‘consumed’ buildings, livestock, sections of
highways.
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Sunday, February 19, 2012
Therefore, using less water
• People in developing nations use 1 gallon/
person/day for all their needs, including cooking
and washing.
• An average person in the U.S. uses 380 liters
(100 gal) per day.
• Indirect uses (irrigation) increases use to
4,900 liters (1,000 gal) per person per day!
• Don’t think how much water we need and where
we get it. Rather, we need to . . .
• Think how much water is available and how
can we best use it.
• The rate of water use has dropped because of
conservation.
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Sunday, February 19, 2012
Conserving water: agriculture
• Agriculture is the largest user of fresh water.
• 40% of crops are grown in irrigated soil.
• Current flood or center-pivot irrigation
wastes huge amounts of water. From . . .
• Evaporation, percolation, and runoff.
• The surge flow method uses computers to
control the release of water.
• The drip irrigation method uses pipes with
holes to drip water at the base of each plant.
• This wastes less water, retards salinization,
and increases yields.
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Sunday, February 19, 2012
Why don’t farmers switch to
drip irrigation?
• 99% of world irrigation is still by traditional
flood or center-pivot methods. Because . . .
• It is cheaper to use traditional methods than
switch.
• The U.S. government pays $400/acre in subsidies.
• But collects only 10% of the costs in water fees.
• Farmers pay almost nothing for water!
• It makes financial sense to use the cheapest
(but most wasteful) system.
• Reducing subsidies would encourage water
conser vation through using more efficient
irrigation technologies.
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Sunday, February 19, 2012
Municipal systems
• Each person in a modern home uses 100 gallons/
day, mostly for washing and removing wastes
(flushing toilets, taking showers, doing laundry).
• It will be expensive and impossible to get more
water through traditional reser voirs and wells.
• The only practical alternative is to save water.
• Little things make a big difference, e.g. . . .
• New toilets use 1.6 gallons or less. Low-flow
showers and faucet repairs save more too.
• Xeriscaping: landscape plants that don’t require
watering.
• Gray water can be used to water lawns and
flush toilets.
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Sunday, February 19, 2012
Public policy challenges
• Humans use 25% of total terrestrial evaporation;
• 56% of accessible precipitation runoff.
• Many facets of water use are unsustainable.
• Public policy must strike a balance bet ween
water needs.
• Water wars: conflicts bet ween using water for
drinking and using it for agriculture. E.g. . . .
• In southern California, Colorado River irrigation
water is being diverted to San Diego.
• The Salton Sea is shrinking, spreading salt,
decreasing wildlife habitat, and destroying the
fishery. The Colorado doesn’t even make it to
the Gulf of California!
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Sunday, February 19, 2012
Key issues in water policy
• Water efficiency must be the primary strategy
for meeting future needs.
• Water subsidies must be reduced or eliminated.
• Polluters must be charged according to their
effluents.
• Watershed management must be integrated into
water pricing.
• Dams must be operated to maintain river flow
that simulates natural flow regimes.
• The U.S. must provide international development
aid.
• Much more research and monitoring is needed.
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Sunday, February 19, 2012