Exam 4.5

• Hydrologic Cycle


  Water evaporates from moist surfaces, falls as rain or snow, passes through living organisms and returns to the oceans

  
• 500,000 km3 of water evaporates each year from the world's oceans and enters the hydrologic cycle
• Over 90% of this water rains back into the oceans
• Some water is carried over land where it renews freshwater systems
• Plants


  • Play a major role in the hydrologic cycle as they pump water from the soil and release it into the atmosphere

  
• Solar energy drives the hydrologic cycle
• Average Annual Precipitation


  Rain falls unevenly across the earth's surface

  
• Factors controlling global water deficits and surpluses

  • Global atmospheric circulation
  • Proximity to water sources
  • Topography

  
• Mountains act as cloud formers and rain catchers

  1. Air sweeps up the windward side of a mountain, pressure decreases, and the air cools

  2. Eventually saturation point is reached, and moisture in the air condenses

  3. Rain falls on the mountaintop

  4. Rain Shadow - Cool, dry air descends from the mountaintop down the other side of the mountain (leeward side) creating dry areas with very little precipitation

  
• At Mount Waialeale in Hawaii, rain on the east side of mountain is >20 times that on the west side
• Oceans


  • Hold 97% of all liquid water on the earth
  • 90% of the earth's biomass is found in the oceans
  • Play a major role in moderating earth's climate
  • Ocean currents moderate the climate by redistributing warm and cold water around the earth
• Frozen Water


  • 2.4% of world's water is classified as fresh
  • 90% in glaciers, ice caps, and snowfields
  • Climate change is shrinking glaciers and snowfields

  
• 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

  
• Aquifers


  • Porous layers of sand, gravel, or rock lying below the water table
  • Artesian - Pressurized aquifer intersects the surface (water flows without pumping)
  • 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
• Rivers and Streams


  • Precipitation that does not evaporate or infiltrate into the ground runs off the surface, back toward the sea
  • Discharge - Best measure of water volume carried by a river, the amount of water that passes a fixed point in a given amount of time, usually expressed as cubic feet per second (CFS)

  
• 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 with depths ranging from a few meters to >1500 meters
  • Both ponds and lakes will eventually fill with sediment, or be emptied by an outlet stream
• 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 primarily due to agricultural drainage
• 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

  
• 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
  • Readily accessible, renewable supplies are only about 400,000 gal /person/year
• The world's major river basins differ in their degree of freshwater-scarcity stress
• Water Stress


  Occurs when human and ecosystem needs exceed the renewable water supplies, resulting in competition
• Periodic droughts create severe regional water shortages in semiarid zones where moisture availability is the critical factor in plant and animal distributions
• The effects on water supplies may be the most serious consequence of global climate change
• Groundwater overdrafts are occurring in the United States

  
• Easily available fresh water is scarce
• 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, Libya and Yemen groundwater and surface water withdrawals equal more than 100% of the renewable supply, which is not sustainable
  • U.S. uses 20% of renewable water/yr
• Water Withdrawal


  Total amount of water removed from a water body
• Water Consumption


  Loss of water due to evaporation, absorption, or contamination
• Agriculture


  • Worldwide, agriculture claims about 70% of total water withdrawal and 85% of consumption
  • The Aral Sea is a tragic example of this, as it has lost 90% of its volume since 1975 due to water diversion for irrigation of rice and cotton crops
• Domestic and Industrial Water Use

  • Worldwide, domestic water use accounts for about 6% of water withdrawals and only about 10% of consumption
  • Industry accounts for 20% of global freshwater withdrawals, ranging from 5% to 70% in various locations based upon the amount of industry
• Typical household water use in the U.S.
• U.N. estimates that a billion people lack access to safe drinking water, and 2.6 billion lack acceptable sanitation resulting in millions of water-related illnesses and deaths
• By 2025, the U.N. estimates that 2/3 of the word's population will be living in water-stressed countries
• Groundwater depletion


  • Groundwater is the source of nearly 40% of fresh water in the U.S.
  • 50% of Americans (95% in rural areas) depend on groundwater for drinking and domestic uses
  • In many places in the U.S., groundwater is being withdrawn faster than it is replenished, which can deplete entire aquifers
• Diversions


  • Transfer and store water for redistribution
  • China faces a massive water crisis and is planning the South-Water-North diversion project to move water from the Yangtze River to northern China, at an estimated cost of 400 billion yuan (about US $62 billion)
• Dams


  • Provide inexpensive hydroelectric power
  • Provide jobs
  • Reduce flooding
  • Allow farming on lands that would otherwise be too dry
• In many parts of the world, severe droughts are already resulting in depleted rivers, empty reservoirs, and water shortages for millions of people
• South Australia is suffering from droughts, water shortages, and wildfires
• China is facing a massive water crisis
• Prolonged drought in Syria and Yemen is threatening food supply