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  1. CHAPTER 10 Water: Hydrologic Cycle and Human Use

  2. Water: a vital resource • Water is fundamental to life • Covers 75% of the Earth’s surface • 97.5% of all water is salt water (oceans, seas) • Fresh water: contains < 0.1% (1,000 ppm) salt • 67% of fresh water is bound up in ice caps and glaciers • Only 0.77% of all water is in lakes, wetlands, rivers, groundwater, biota, soil, and the atmosphere

  3. Freshwater is a renewable resource • Streams, rivers, ponds, lakes, swamps, estuaries, groundwater, bays, oceans, and atmosphere contain water • Water is used for drinking, industries, irrigation, energy, transportation, recreation, waste processing, habitats • Water modifies the climate • Humans have constructed huge infrastructures to control water • Dams, canals, reservoirs, aqueducts, sewer systems • Treatment plants, water towers, pipelines, irrigation • Desalinization plants

  4. Hydrologic cycle • 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

  5. Evaporation • Evaporation: as water molecules absorb energy from the Sun, kinetic energy allows molecules to enter the atmosphere (liquid to gas) • Water vapor: water molecules in the gaseous state

  6. Water vapor is a greenhouse gas • Water vapor contributes two-thirds of the warming of all greenhouse gases • Humidity: amount of water vapor in the air • Relative humidity: the amount of water vapor as a percent of what the air can hold at a particular temperature • The amount of water vapor the air can hold varies with the temperature • Cooling causes water vapor to condense to liquid water

  7. Condensation • 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 vegetation • Aerosols: microscopic liquid or solid particles • Help condensation occur • Originate naturally from volcanoes, dust, soil, salt • Human sources: sulfates, carbon, dust

  8. Purification • Evaporation purifies water naturally; condensation pollutes with aerosols • Evaporation removes only water molecules, not salts and other solids • Atmospheric water turns over every 10 days • Water eventually reaches the oceans, inland seas, lakes

  9. 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

  10. Convection currents • Convection currents occur because the Sun heats the Earth most intensely over and near the equator • Heated air expands, rises, and cools • Condensation and precipitation occur • The equator’s constant heat causes this process to repeat • Supporting tropical rain forests • The now dry air “spills over” north and south of the equator • Descending over subtropical areas and creating deserts • Hadley cell: the system composed of rising and falling air • Trade winds: Earth’s rotation deflects winds east and west

  11. Rain shadow • Moisture-laden trade winds encounter mountain ranges • 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 • Deserts occur on the leeward sides of mountains • 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

  12. 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

  13. Infiltrating water has two alternatives • Capillary water: held in the soil, according to the soil’s capacity • 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 • Filter process!

  14. Underground purification • As water percolates through the soil, debris and bacteria are filtered out • Water may dissolve or leach out minerals • Some minerals can be dangerous (arsenic, sulfide, etc.) • Drawn by gravity, groundwater moves through aquifers until it finds an opening to the surface • Seep: water flows out in a wide area • Spring: water flows from a small opening • Seeps and springs feed streams, lakes, and rivers

  15. 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

  16. Aquifers & Recharge Area • Aquifers: deep lakes surrounded by layers of impervious material • It is hard to determine the location of aquifers • Layers of porous rock are found between layers of impervious 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 liquidFRESH water

  17. Human impacts on the hydrologic cycle • 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 • Withdrawals for human use

  18. Changes to the surface of the Earth • In natural systems, vegetation intercepts precipitation • Water infiltrates into porous topsoil, filtering out debris • Evapotranspiration sustains ecosystems and recycles water • Recharged groundwater reservoirs release water through springs and seeps into streams and rivers • 1) In cleared forests and overgrazed land, plants do not intercept rainfall • A)Water shifts from infiltration and recharge into runoff

  19. Cleared land affects the hydrologic cycle • Removing vegetation causes a sudden influx of water into rivers and streams • B) Causing floods, pollutants from erosion, and less evapotranspiration and groundwater recharge • C) Resulting in dry, barren, lifeless streambeds • D) Wetlands also store and release water • Destruction leads to flooding and polluted waterways • In 2008, Iowa had massive flooding due to filling wetlands and converting tallgrass prairies to plowed fields

  20. 3) Climate change • There is unmistakable evidence that Earth is warming • Evaporation increases with a warmer climate • A) A wetter atmosphere means more and heavier precipitation and floods • B) More hurricanes and droughts • C) Water-stressed areas (e.g., East Africa) will get less water • D) Global warming may be speeding up the water cycle Affecting precipitation, evapotranspiration, groundwater recharge, runoff, snowmelt, etc.

  21. 4) Atmospheric pollution • A) Aerosol particles form nuclei, enabling water to condense into droplets • More clouds form • B) Anthropogenic particles are increasing • From sulfates, carbon (soot), dust • Form a brownish haze associated with industrial areas, tropical burning, and dust storms • C) Solar radiation is reduced • Therefore, aerosols have a cooling effect

  22. Aerosols affect the water cycle • D)They promote smaller droplets • E) They suppress rainfall, even though clouds form • FYI: Aerosols have more local (vs. global) impacts • They do not accumulate—they have a lifetime of days • SO… we can get rid of them quickly!

  23. Uses of water • Worldwide, the largest use is for irrigation • Then industry and direct human use • Use varies by region, depending on: • Natural precipitation • Degree of development • Most increases in withdrawal are due to increases in agriculture • Irrigation accounts for 65% of freshwater consumption in the U.S.

  24. Water: management and control • Humans use 27% of all accessible freshwater runoff • Global withdrawal will increase 10% each decade • Americans use less water than in 1980 • Nonconsumptive uses of water: water may be 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 • Water for irrigation

  25. 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 where 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 • Millennium Development Goal 7: increase access to safe drinking water

  26. Water in developed and developing nations • Developed countries benefit from controlling water • Controlling diseases, building cities in deserts, irrigation, electricity • Water in developing countries is costly or inaccessible • They lack access to safe drinking water and sanitation • People die from waterborne diseases • Because of infrastructure to control water • Seas and rivers are being lost • Millions have been displaced to make room for reservoirs • Tensions increase for access to water

  27. Technologies in developed nations • Industrialized countries collect, treat, and distribute water • Larger municipalities rely mostly on surface water • Dams create reservoirs to generate power and for recreation, irrigation, flood control • Water is piped to a treatment plant • It is then distributed to homes, schools, industries • After use, it is collected by the sewage system and treated • It is then released downstream into the same river • Water is reused many times on major rivers (e.g., Mississippi)

  28. Technologies in other areas • In developing nations, wastewater is often discharged with little or no treatment • Problems? • Smaller public drinking-water systems depend on groundwater • Water is purified by percolation • Surface and groundwater represent a sustainable or renewable (self-replenishing) resource • But they can be overdrawn

  29. Dams and the environment • Over 45,000 large dams (over 50 feet) exist • Many reservoirs store billions of liters (billions of gallons) • Covering 49 million hectares (120 million acres) • Only 31% of annual runoff is available for withdrawal • Rivers are too remote • Navigation, flood control, hydropower also need water • A) Large dams have enormous social impacts • 1) Displacing 40 million people • 2) Preventing access to goods and services of the buried ecosystems

  30. Dams have enormous impacts • 3) Valuable freshwater habitats (waterfalls, rapids, fish runs) are lost • 4) The waterway below the diversion is deprived of water • 5) Fish and other aquatic organisms are directly impacted • 6) Wildlife is adversely affected (e.g., food chains) • 7) Wetlands dry up and waterfowl die • 8) 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

  31. So… let’s get rid of ‘em!... Dams gone • People are recognizing the unacceptable costs of dams • 714 dams have been removed in the U.S. • 1) removal is not easy • There are legal complexities to dam removal • 2) Existing uses conflict with expected advantages (establishing fish, recreational and aesthetic uses) • There are practical problems to dam removal • 3) Built up sediment in the reservoir can be contaminated

  32. Floods • A. Dams help prevent devastating floods • B. Many areas have increased flood frequency and severity • 1. Erosion and reduced infiltration from deforestation and cultivation • “Sponge effects” of forests helps prevent floods • 2. Deforestation in the Himalayan foothills causes extreme flooding in Bangladesh • India’s Koshi River broke through dirt levees • Caused massive floods and stranded a million people • The breach came because of massive silt deposition

  33. Impacts on estuaries • Estuaries: very productive ecosystems in bays and rivers … we divert that water for irrigation and municipal use • We need them because.. • Where fresh river water mixes with seawater • Breeding grounds for birds, fish, shellfish • Decreased fresh water increases the water’s salinity • 1) Devastated the bay • 2) Fish populations have disappeared or been reduced • Tidal wetlands have been reduced by 92%

  34. Groundwater levels • The Ogallala aquifer supplies irrigation water to 4.2 million hectares (10.4 million acres) in seven U.S. states • 20% of the irrigated land in the U.S. • It has “fossil water” recharged during the last ice age • Water is being withdrawn twice as fast as it recharges • Water tables in the Ogallala aquifer have dropped 30–60 m (100–200 ft) • Irrigated farming has stopped in some areas • Within 20 years, 1.2 million hectares (3 million acres) will be abandoned or converted to dryland farming

  35. Some aquifers are nonrenewable • Nonrenewable groundwater: more than 75% of aquifers have recharge rates of centuries • Renewable groundwater is replenished by percolation • We are tapping large, but not unlimited, natural reservoirs • Sustainability depends on balancing withdrawal rates with recharge rates • Most groundwater in arid regions has no recharge • AQUIFERS must be considered nonrenewable, like oil

  36. Impacts of falling water tables • 1) Decreased crop production • 2) Diminishing surface water • a. Dries up wetlands, springs and seeps, streams, rivers • b. Excessive groundwater removal creates the same results as diverting surface water

  37. 3) Land subsidence • Groundwater fills spaces in the ground • Helping 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 • Results in flooding in coastal areas • People in developing areas drill their own wells • Causing subsidence—a case of tragedy of the commons

  38. In extreme cases: sinkholes • A sinkhole: another kind of land subsidence • Results when an underground cavern is drained of its supporting groundwater and suddenly collapses • Sinkholes can be 91 m (300 ft.) 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

  39. 4) Saltwater intrusion • In many coastal areas, springs of outflowing groundwater lie under the ocean • High water tables keep pressure in the aquifer • Fresh water flows into the ocean • Wells near the ocean yield fresh water • Lowering the water table reduces pressure, allowing salt water to flow into the aquifer and wells • This is a serious problem in many European countries along the Mediterranean coast