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Chapter 17 Water: Resources and Pollution
"Water will be more important than oil this century.”
- Former U.N. Secretary General Boutros Boutros Gali
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Ch 17 Outline
• Hydrologic Cycle• Major Water Compartments• Water Availability and Use
Types of Water Use• Freshwater Shortages• Dams and Diversions• Increasing Water Supplies
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Objectives
• 17.1 Explain why water is a precious resource and why shortages
occur.• 17.2 Compare major water compartments.• 17.3 Summarize water availability and use.• 17.4 Investigate freshwater shortages.• 17.5 Illustrate the benefits and problems of
dams and diversions.• 17.6 Understand how we might increase
water supplies.
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Why is Water Important?
• Necessary for all life to exist.• Next to antibiotics, single biggest increase in
human life span is due to having access to clean water.
• Diseases/toxins transmitted by contaminated water: Intestinal disease (cholera/dysentery) Arsenic poisoning Mercury poisoning
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Average Annual Precipitation
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Hydrologic Cycle
• The water on earth is continually recycled.• The hydrologic cycle describes this circulation:
1. Water evaporates from wet land, lakes, or oceans and transpires from plants as they dry up.
2. Enters the atmosphere, which is much colder, condenses and falls as precipitation.
3. Moves underground by infiltration or runs off into rivers, lakes, or the ocean.
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Hydrologic Cycle
• Solar energy drives the hydrologic cycle by evaporating surface water. Evaporation - changing liquid to a vapor below
its boiling point Sublimation - changing water between solid and
gaseous states without ever becoming liquid
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Hydrologic Cycle
• Humidity - amount of water vapor in the air Saturation point - when a volume of air contains
as much water vapor as it can hold at a given temperature
Relative humidity - amount of water vapor in the air expressed as a percentage of the maximum amount that can be held at that temperature
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Hydrologic Cycle
• Dew point - temperature at which condensation occurs for a given amount of water vapor Condensation nuclei - tiny particles that facilitate
condensation- Smoke, dust, sea salt, spores
A cloud is an mixture of condensed water vapor in droplets or ice crystals. When droplets become large enough, gravity overcomes air currents and precipitation occurs.
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Regions of Plenty and Regions of Deficit
• Three principal factors control global water deficits and surpluses:
Global atmospheric circulation Proximity to water sources Topography
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Regions of Plenty and Regions of Deficit
• Mountains act as cloud formers and rain catchers. Air sweeps up the windward side of a mountain,
pressure decreases, and the air cools.- Eventually saturation point is reached, and
moisture in the air condenses. Rain falls on the mountaintop.
Cool, dry air descends and warms, absorbing moisture from other sources (Rain shadow).
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Rain Shadow – Hawaiian style
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All water
Oceans andsaline lakes
97.4%
Earth’s Water Budget
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All water
Oceans andsaline lakes
97.4%
Earth’s Water Budget
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All water Fresh water Readily accessible fresh water
Oceans andsaline lakes
97.4%
Fresh water2.6%
Groundwater0.592%
Ice capsand glaciers
1.984%
Soilmoisture0.005%
Biota0.0001%
Rivers0.0001%
Atmosphericwater vapor
0.001%
0.014%
Lakes 0.007%
Earth’s Water Budget
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Major Water Compartments
• 1. Oceans Together, oceans contain more than 97% of all
liquid water in the world.- Contain 90% of world’s living biomass- Moderate earth’s temperature
Gulf Stream carries 100X more water than all rivers on earth.
Average residence time of water in the ocean is about 3,000 years
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Major Water Compartments
• 2. Glaciers, Ice, and Snow 2.4% of world’s water is classified as fresh.
- 90% is frozen in glaciers, ice caps, and snowfields
As recently as 18,000 years ago, one-third of continental landmass was covered by glacial ice sheets.
Now, Antarctic glaciers contain nearly 85% of all ice in the world.
Greenland, together with ice floating around the North Pole, is another 10%.
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Major Water Compartments
• 3. Groundwater Second largest reservoir of fresh water
- Infiltration - process of water percolating through the soil and into fractures and permeable rocks
Zone of aeration - upper soil layers that hold both air and water
Zone of saturation - lower soil layers where all spaces are filled with water
Water table - top of zone of saturation
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Infiltration
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Groundwater
• Aquifers - Porous layers of sand, gravel, or rock lying below the water table. Aquifers are separate from groundwater;
they are usually isolated by layers of rock. Artesian Well – Directly tapes into an
aquifer as a source of water.
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Groundwater
• The recharge zone is an area where water infiltrates and refills an aquifer. Recharge rate is often very slow,
depending on the size of the recharge zone and how often it rains there.
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Major Water Compartments
• Rivers and Streams Precipitation that does not evaporate or infiltrate
into the ground runs off the surface, back toward the sea.
- Best measure of water volume carried by a river is discharge (cfs).
The amount of water that passes a fixed point in a given amount of time
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Major Water Compartments
• Lakes and Ponds Ponds are generally considered small bodies of
water shallow enough for rooted plants to grow over most of the bottom.
Lakes are inland depressions that hold standing fresh water year-round.
- Both ponds and lakes will eventually fill with sediment, or be emptied by an outlet stream.
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Major Water Compartments
• Wetlands Play a vital role in hydrologic cycle
- Lush plant growth stabilizes soil and retards surface runoff, allowing more aquifer infiltration.
Disturbance reduces natural water-absorbing capacity, resulting in floods and erosion in wet periods, and less water flow the rest of the year.
Half of U.S. wetlands are gone. Can hold excess water during flooding.
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Major Water Compartments
• The Atmosphere Among the smallest water reservoirs
- Contains < 0.001% of total water supply- Has most rapid turnover rate- Provides mechanism for distributing fresh
water over landmasses and replenishing terrestrial reservoirs
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Water Availability and Use
• Renewable Water Supplies Made up of surface runoff plus infiltration into
accessible freshwater aquifers- About two-thirds of water carried in rivers and
streams annually occurs in seasonal floods too large or violent to be stored effectively for human use.
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Drought Cycles
• Every continent has regions of scarce rainfall due to topographic effects or wind currents. Water shortages have most severe effect in
semiarid zones where moisture availability is the critical factor in plant and animal distributions.
- U.S. seems to have 30 year drought cycle. Dust Bowl in 1930s
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Drought Cycles
• Much of Western U.S. is still plagued by drought and overexploitation of limited supply of water.
• El Nino plays an important role in determining when North America has drought.
• Global warming may make droughts more frequent and severe.
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Water Consumption
• Withdrawal - total amount of water taken from a source
• Consumption - fraction of withdrawn water made unavailable for other purposes (not returned to its source) Degradation - Change in water quality due to
contamination making it unsuitable for desired use. Much water that is not consumed is nevertheless polluted.
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Water Use is Increasing
• Many societies have always treated water as an inexhaustible resource. Natural cleansing and renewing functions of
hydrologic cycle do not work properly if systems are overloaded or damaged
Renewal of water takes time Rate at which we are now using water makes
conservation necessary
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Quantities of Water Used
• Human water use has been increasing about twice as fast as population growth over the past century, but impact varies with location. Canada withdraws less than 1% of its renewable
supply per year. In Israel, groundwater and surface water
withdrawals equal more than 100% of the renewable supply. Obviously, this is not sustainable.
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Agricultural Water Use
• Water use is divided into agriculture, domestic use and industrial use.
• Worldwide, agriculture claims about two-thirds of total water withdrawal and 85% of consumption. Aral Sea, once the fourth largest inland body of
water in world, has been drained. Lake Chad in northern Africa went from 400,000
sq. km to less than 1,000 sq. km.
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Exhibit A
• During the late 1940s, Joseph Stalin proposed a plan in the Soviet Union to prevent a repeat of drought-caused famine in 1947 that killed nearly a million people.
• A wide variety of ideas were implemented during the next two decades.
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• A network of irrigation canals were built to divert water from two rivers into a desert region, to encourage the cultivation of crops such as rice, wheat, and cotton.
• The rivers normally fed into the Aral Sea.
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The Aral Sea
• In the 1950s, the Aral sea was one of the four largest lakes in the world, with an area of 26,300 square miles.
• The entire lake was part of the U.S.S.R., in the countries that today are Kazakhstan and Uzbekistan.
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Irrigation Canals
• The irrigation canals were poorly built and uncovered, allowing about half of the water to evaporate or leak into the surrounding soil.
• The Aral sea, deprived of its main water supply, began to shrink in 1961.
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Aral Sea
Satellite imagery of the
Aral Sea shrinkage from
1961-2011.
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Water Use
• Consumption – Water that is withdrawn and no longer available for use because it has evaporated, been consumed by animals or plants, or discharged to a different location.
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Water Use in Agriculture
• Irrigation can be inefficient. Flood or furrow irrigation
- Half of water can be lost through evaporation.- Flood irrigation used to remove salts from
field, but salt contaminates streams Sprinklers have high evaporation. Drip irrigation releases water near roots,
conserving water.
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Quantities of Water Used
• Water use has been increasing twice as fast as population growth over past century. Worldwide, agriculture claims about 70%
of total water withdrawal.- In many developing countries,
agricultural water use is extremely inefficient and highly consumptive.
Worldwide, industry accounts for about 25% of all water use.
- Cooling water for power plants is single largest industrial use.
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Water Withdrawal
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Typical Household Water Use in U.S.
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FRESHWATER SHORTAGES
• Areas that consume more water than typically falls as precipitation are considered to have water stress.
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Freshwater Shortages
• U.N. estimates a billion people lack access to safe drinking water. 2.6 billion lack acceptable sanitation.
• At least 45 countries, mostly in Africa and the Middle East, are considered to have serious water stress.
• Water shortages could lead to wars as population grows and climate change dries up some areas. An underlying cause of the Darfur genocide is water scarcity.
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Freshwater Shortages
• Privatization of public water supply in Bolivia sparked a revolution that overthrew the government in 2000.
• Multinational corporations are moving to take control of water supplies in many countries.
• Global warming may make water shortages much worse in many parts of the world.
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A Precious Resource
• Currently, 45 countries cannot meet the minimum essential water requirements of their citizens.
• Will increase to 60 countries by 2050.
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Dams and Diversions
• Before 1900 there were 250 high dams in the world; today there are more than 45,000.
• In the U.S. dams are built by Army Corps of Engineers and Bureau of Reclamation Provide cheap hydroelectric power Jobs Reduce flooding Allow farming on lands that would otherwise be
too dry
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Dams and Diversions
• On the downside, dams Drown free flowing rivers Submerge farmlands and towns Block fish migration e.g. salmon Change aquatic habitats for native species Can sometimes fail, causing catastrophe
- Johnstown flood (city just east of Pittsburgh, PA) killed 2,200 people when dam broke.
- Dam failure in China killed 230,000.
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Dams and Diversions
• Tide of public opinion is turning• Army Corps of Engineers announced in 1998 that it
would no longer be building large dams and would be removing some older dams to restore natural habitat
• Sediment carried by rivers eventually fills up dams• Lakes behind dams lose huge amounts of water
through evaporation and seepage• Downriver habitats lose nutrients and the beaches
disappear as sediment is no longer available
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Dams and Diversions
• Diversion projects can dry up rivers Yellow River in China is dry 226 days per year
due to diversions Colorado River in the U.S. is so depleted that
most of the year no water reaches the mouth of the river in the Sea of Cortez
Mono Lake has been depleted to send water to Los Angeles. Salinity of water doubled, killing the brine shrimp that fed huge flocks of migratory birds
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Mono Lake in California• Diversion of water from the lake to Los Angeles has
shrunk the lake by 1/3, exposing these towers where calcium- rich springs once entered the lake.
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Depleting Groundwater
• Groundwater is the source of nearly 40% of fresh water in the U.S. On a local level, withdrawing water faster than it
can be replenished leads to a cone of depression in the water table.
Heavy pumping can deplete an aquifer. Ogallala Aquifer
Underlies 8 states between Texas and North Dakota
Wells have dried up and whole towns are being abandoned
Will take thousands of years to refill
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The Ogallala Aquifer
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Depleting Groundwater
• Withdrawing large amounts of groundwater in a small area causes porous formations to collapse, resulting in subsidence (settling). Sinkholes form when an underground channel or
cavern collapses. Results in permanent loss of aquifer.
Saltwater intrusion can occur along coastlines where overuse of freshwater reservoirs draws the water table low enough to allow saltwater to intrude.
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Saltwater Intrusion
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Increasing Water Supplies
• Seeding Clouds Condensation nuclei
• Desalination - removing salt from ocean water or brackish water to get fresh water Most common methods are distillation and
reverse osmosis.- Three to four times more expensive than most
other sources
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Domestic Conservation• Estimates suggest we could save as much as half
of domestic water usage without change in lifestyle Largest domestic use is toilet flushing
- Can use low volume toilets or waterless composting
- Anaerobic digesters use bacteria to produce methane gas from waste
Significant amounts of water can be reclaimed and recycled.
Purified sewage effluent San Diego pumps water from sewage
plant directly into drinking reservoir
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Price Mechanisms and Water Policy
• Through most of U.S. history, water policies have generally worked against conservation. In well-watered eastern states, water policy was
based on riparian use rights. In drier western regions where water is often a
limiting resource, water law is based primarily on prior appropriation rights.
- Fosters “Use it or Lose it” policies, where if you conserve you lose your rights to the water
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Price Mechanisms and Water Policy
• In most federal reclamation projects, customers were only charged for immediate costs of water delivery. Dam and distribution system costs were
subsidized. Underpriced water in some areas amounted to a
subsidy of $500,000 per farm per year.• Growing recognition that water is a precious and
finite resource has changed policies and encouraged conservation across the U.S.
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Price Mechanisms and Water Policy
• Charging a higher proportion of real costs to users of public water projects has helped encourage conservation.
• Conservation has been successful. U.S. today uses 10% less water than in 1980 but has 37 million more people.
