Water Resources - DOC

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					Water Resources

Objectives
Review the hydrologic cycle
Describe the important sources of water and the major ways we use it
Appreciate the causes and consequences of water shortages around the world
Debate the merits of proposals to increase water supplies and manage
  demands

Hydrologic cycle
Describes the circulation of water as it:
   Evaporates from land, water, and organisms
      Plants lose water through transpiration
   Enters the atmosphere
   Condenses and precipitates back to the earth’s surfaces
   Moves underground by infiltration or overland runoff into rivers, lakes and
     seas

Hydrologic cycle
Solar energy drives the hydrologic cycle by evaporating surface water
   Evaporation - Changing liquid to a vapor below its boiling point
   Only water evaporates - salt and contaminants stay behind
Hydrologic cycle
Humidity - Amount of water vapor in the air
Warm air can hold more water than cold air
   Saturation point
      When a volume of air contains as much water vapor as it can hold at a
        given temperature

Hydrologic cycle
Condensation is when air exceeds saturation point and water becomes liquid
  or ice again
   Condensation Nuclei - Tiny particles that facilitate condensation
       Smoke, dust, sea salt, spores, volcanic ash

Hydrologic cycle
Oceans account for 86% of total evaporation
   Ninety percent of water evaporated from the ocean falls back on ocean as
     rain
      Remaining 10% is carried by prevailing winds over continents
Hydrologic cycle
When water falls on land, some is incorporated into biological tissues and a
  large share seeps into the ground to be stored
All water makes its way back downhill to oceans
Water is distributed unevenly on earth

Regions of plenty and regions of deficit
Principal factors that control global water deficits and surpluses
  Global atmospheric circulation
  Prevailing winds
  Topography
  Human activity
Global atmospheric circulation
Creates areas low rainfall about 20° to 40° north and south of the equator
Prevailing winds
Areas far from oceans - in a windward direction are usually relatively dry
Topography
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
   Mt Waialeale windward side receives 12 m/year, while the leeward side
    receives 46 cm

Human activity
Removal of plants causes desertification
  Plants recycle moisture, produce rain, and slow water flow to streams
Oceans
Together, oceans contain more than 97% of all liquid water in the world
Average residence time of water in the ocean is about 3,000 years
  Residence time is the amount of time a water molecule spends in the
    ocean before entering the hydrologic cycle again
Glaciers, ice and snow
   2.4% of world’s water is fresh
   90% in glaciers, ice caps, and snowfields
      Now, Antarctic glaciers contain nearly 85% of all ice in the world
Groundwater
Second largest reservoir of fresh water
   Infiltration - Process of water percolating through the soil and into fractures
     and permeable rocks

Groundwater
Aquifers - Porous layers of sand, gravel, or rock lying below the water table
   Artesian - Pressurized aquifer intersects the surface
     Water flows without pumping

Groundwater
Recharge Zones - Area where water infiltrates into an aquifer
   Recharge rate is often very slow
   Presently, groundwater is being removed faster than it can be replenished
    in many areas

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
Lakes contain 100 times as much water as all rivers and streams combined
Great Lakes
20% of the world's supply of surface water
95% of the North American supply of surface freshwater
Rivers and streams
Precipitation that does not evaporate or infiltrate into the ground runs off the
  surface, back toward the sea
   16 largest rivers in the world carry nearly half of all surface runoff on earth
       Amazon carries 10 times the volume of Mississippi River

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
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

Water-Rich and Water-Poor Countries
Water availability usually measured in terms of renewable water per capita
  Highest per capita generally found in countries with moist climates and low
    population densities

Drought cycles
Every continent has regions of scarce rainfall due to topographic effects or
  wind currents
   Water shortages have most severe effect in semi-arid zones where
     moisture availability is the critical factor in plant and animal distributions
      U.S. seems to have 30 year drought cycle
      Climatic changes such as global warming may alter cycles

Droughts
Land use exacerbates the effects of drought
   Dust bowl of 1930’s
The dust bowl
Occurred in the 1930s in the Great Plains
Overgrazing and prolonged drought left the ground bare
1934 winds produced dust storms that stripped about 9 million acres of topsoil
Dust clouds reached as far as Washington DC
Quantities of water used
Human water use has been increasing about twice as fast as population
  growth over the past century
Americans use 1,300 gallons per person per day, while Haitians use 8 gallons
  per person per day
Quantities of water used
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
Domestic water use
Worldwide, domestic water use accounts for about one-fifth of water
  withdrawals

American water use

A precious resource
 Currently, 45 countries, most in Africa or Middle East, are considered to have
  serious water stress, and cannot meet the minimum essential water
  requirements of their citizens
    More than two-thirds of world’s households have to retrieve water from
     outside the home
      Sanitation levels decline when water is expensive

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
       On a broader scale, heavy pumping can deplete an aquifer
          Ogallala Aquifer, which once held more water than all the freshwater
            lakes, streams, and rivers on earth, has fallen dramatically

Depleting groundwater
Withdrawing large amounts of groundwater in a small area causes porous
  formations to collapse, resulting in subsidence
   Sinkholes form when an underground channel or cavern collapses
   Saltwater intrusion can occur along coastlines where overuse of freshwater
     reservoirs draws the water table low enough to allow saltwater to intrude

Increasing water supplies
Seeding clouds
   Condensation nuclei
Towing Icebergs
   Cost
Desalination
   Three to four times more expensive than most other sources
Increasing water supplies
Dams, reservoirs, canals and aqueducts
   Common to trap excess water in areas of excess and transfer it to areas of
     deficit
   Environmental costs
      Upsets natural balance of water systems
   Ecosystem losses
      Loss of wildlife habitat
          Reservoir size
          Water quality

Dams, reservoirs, canals and aqueducts
  Displacement of people
     Three Gorges Dams in China is forcing relocation of over a million
       people
  Evaporation, leakage, siltation
     Evaporative losses from Lake Mead and Lake Powell on the Colorado
       River is about 2 billion m3 per year
     Dams slow water flow, allowing silt (nutrients) to drop out
  Loss of free-flowing rivers
Watershed management
Watershed - All the land drained by a stream or river
Watershed management
Retaining vegetation and ground cover helps retard rainwater and lessens
 downstream flooding
Retaining crop residue on fields reduces flooding
Minimizing plowing and forest cutting on steep slopes protects watersheds
Domestic conservation
Estimates suggest many societies could save as much as half of current
  domestic water usage without great sacrifice or serious change in lifestyle
   Low-volume shower heads
   Efficient dishwashers and washing machines
   Landscape choices
   Waterless or low-volume toilets

				
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posted:10/3/2012
language:English
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