Chapter 6 Water Resources PDF

Chapter 6 WILL WE HAVE ENOUGH USABLE WATER? Freshwater Is an Irreplaceable Resource That We Are Managing Poorly  Freshwater is relatively pure and contains few dissolved salts.  Earthhas a precious layer of water—most of it saltwater—covering about 71% of the earth’s surface.  Water is an i...

Chapter 6 WILL WE HAVE ENOUGH USABLE WATER? Freshwater Is an Irreplaceable Resource That We Are Managing Poorly  Freshwater is relatively pure and contains few dissolved salts.  Earthhas a precious layer of water—most of it saltwater—covering about 71% of the earth’s surface.  Water is an irreplaceable chemical with unique properties that keep us and other forms of life alive. A person could survive for several weeks without food, but for only a few days without water. Concerns regarding water include: Access to freshwater is a global health issue. Every day an average of 3,900 children younger than age 5 die from waterborne infectious diseases by WHO. An economic issue – vital for reducing poverty and producing food and energy. A women’s and children’s issue in developing countries because poor women and girls often are responsible for finding and carrying daily supplies of water. A national and global security issue because of increasing tensions within and between nations over access to limited water resources that they share. An environmental issue because excessive withdrawal of water from rivers and aquifers results in dropping water tables, lower river flows, shrinking lakes, and losses of wetlands. Most of the earth’s freshwater is not available to us About 0.024% is readily available to us as liquid freshwater in accessible groundwater deposits and in lakes, rivers, and streams. The rest is in the salty oceans, in frozen polar ice caps and glaciers, or in deep underground and inaccessible locations.  The world’s freshwater supply is continually collected, purified, recycled, and distributed in the earth’s hydrologic cycle, except when:  Overloaded with pollutants.  We withdraw water from underground and surface water supplies faster than it is replenished.  We alter long-term precipitation rates and distribution patterns of freshwater through our influence on projected climate change. HYDROLOGIC CYCLE – the movement of water in the seas, in the air, and on land, which is driven by solar energy and gravity.  Freshwater is not distributed evenly. Differences in average annual precipitation and economic resources divide the world’s continents, countries, and people into water haves and have- nots. Canada, with only 0.5% of the world’s population, has 20% of the world’s liquid freshwater, while China, with 19% of the world’s people, has only 7% of the supply. Groundwater and surface water are critical resources  Some precipitation infiltrates the ground and percolates downward through spaces in soil, gravel, and rock until an impenetrable layer of rock stops this groundwater—one of our most important sources of freshwater. The zone of saturation is where the spaces are completely filled with water. The top of this groundwater zone is the water table. Aquifers: underground caverns and porous layers of sand, gravel, or bedrock through which groundwater flows—typically moving only a meter or so (about 3 feet) per year and rarely more than 0.3 meter (1 foot) per day. Watertight layers of rock or clay below such aquifers keep the water from escaping deeper into the earth. Natural recharge – the process where most of aquifers are replenished naturally by precipitation that percolates downward through exposed soil and rock. Lateral recharge – from nearby lakes, rivers and streams.  Surface water is the freshwater from precipitation and snowmelt that flows across the earth’s land surface and into lakes, wetlands, streams, rivers, estuaries, and ultimately to the oceans. Precipitation that does not infiltrate the ground or return to the atmosphere by evaporation is called surface runoff. The land from which surface water drains into a particular river, lake, wetland, or other body of water is called its watershed, or drainage basin.  We use a large and growing portion of the world’s reliable runoff  According to hydrologist, two-thirds of the annual surface runoff in rivers and streams is lost by seasonal floods and is not available for human use.  The remaining one third is reliable surface runoff, which we can generally count on as a source of freshwater from year to year.  Duringthe last century, the human population tripled, global water withdrawals increased sevenfold, and per capita withdrawals quadrupled. We now withdraw about 34% of the world’s reliable runoff of freshwater. We use a large and growing portion of the world’s reliable runoff  Worldwide, about 70% of the water we withdraw each year comes from rivers, lakes, and aquifers to irrigate cropland, industry uses another 20%, and residences 10%.  Affluent lifestyles require large amounts of water. Groundwater overdrafts in the United States Freshwater shortages will grow  The main factors that cause water scarcity in any particular area are a dry climate, drought, too many people using a water supply more quickly than it can be replenished, and wasteful use of water.  More than 30 countries—mainly in the Middle East and Africa—now face water scarcity.  By 2050, 60 countries, many of them in Asia, with three-fourths of the world’s population, are likely to be suffering from water stress.  The world’s major river basins differ in their degree of freshwater-scarcity stress  In 2009, about 1 billion people in the world currently lack regular access to enough clean water for drinking, cooking, and washing.  By 2025, at least 3 billion people are likely to lack access to clean water.  We can increase freshwater supplies by: withdrawing groundwater; building dams and reservoirs to store runoff in rivers for release as needed transporting surface water from one area to another; and converting saltwater to freshwater (desalination) Groundwater is being withdrawn faster than it is replenished in some areas  Aquifers provide drinking water for nearly half of the world’s people.  Most aquifers are renewable resources unless their water becomes contaminated or is removed faster than it is replenished by rainfall.  Water tables are falling in many areas of the world because the rate of pumping water from aquifers (mostly to irrigate crops) exceeds the rate of natural recharge from rainfall and snowmelt. Groundwater is being withdrawn faster than it is replenished in some areas The world’s three largest grain producers—China, India, and the United States—as well as Mexico, Saudi Arabia, Iran, Yemen, Israel, and Pakistan are overpumping many of their aquifers. Withdrawing groundwater has advantages and disadvantages Overpumping of aquifers has several harmful effects  As water tables drop, farmers must drill deeper wells, buy larger pumps, and use more electricity to run those pumps. Poor farmers cannot afford to do this and end up losing their land.  Withdrawing large amounts of groundwater causes the sand and rock in aquifers to collapse. This causes the land above the aquifer to subside or sink (land subsidence), referred to as a sinkhole. Once an aquifer becomes compressed by subsidence, recharge is impossible. In addition, land subsidence can damage roadways, water and sewer lines, and building foundations. Groundwater overdrafts near coastal areas can pull saltwater into freshwater aquifers. The resulting contaminated groundwater is undrinkable and unusable for irrigation. Deep water aquifers hold enough freshwater to support billions of people for centuries. Littleis known about the geological and ecological impacts of pumping large amounts of freshwater from deep aquifers. Some deep aquifers flow beneath more than one country and there are no international treaties that govern rights to them. Without such treaties, water wars could break out. The costs of tapping deep aquifers are unknown and could be high. There are four (4) major concerns about tapping these ancient deposits of water.  They are nonrenewable and cannot be replenished on a human timescale.  Little is known about the geological and ecological impacts of pumping water from deep aquifers.  Some deep aquifers flow beneath more than one country, and there are no international treaties that govern rights to use them. Without such treaties, water wars could break out.  The costs of tapping deep aquifers are unknown and could be high. SOLUTIONS FOR GROUNDWATER DEPLETION BUILDING MORE DAMS THE ANSWER? Large dams and reservoirs have advantages and disadvantages  Dams are structures built across rivers to block some of the flow of water.  Dammed water usually creates a reservoir, a store of water collected behind the dam.  A dam and reservoir: capture and store runoff and release it as needed to control floods. generate electricity (hydroelectricity).  supply water for irrigation and for towns and cities. provide recreational activities such as swimming, fishing, and boating.  The world’s 45,000 large dams have increased the annual reliable runoff available for human use by nearly 33%.  Negative effects of dams include: displaced 40–80 million people from their homes.   floodedan area of mostly productive land totaling roughly the area of California.  impaired some of the important ecological services that rivers provide.  Reservoirs eventually fill up with sediment, typically within 50 years, eventually making them useless for storing water or producing electricity.  Around 500 small dams have been removed in the U.S. but removal of large dams is controversial and expensive. Large dams and reservoirs have advantages and disadvantages Provides Flooded land irrigation destroys water above forests or and below dam cropland and displaces people Large losses of water through evaporation Provides water for drinking Deprives downstream cropland and Reservoir estuaries of useful for nutrient-rich recreation silt and fishing Risk of Can produce failure and cheap devastating electricity downstream (hydropower) flooding Reduces down- stream flooding of cities and Disrupts farms migration and spawning of some fish Fig. 11-12a, p. 247 IS TRANSFERRING WATER FROM ONE PLACE TO ANOTHER THE ANSWER? Water transfers can be wasteful and environmentally harmful  In many cases, water has been transferred into various dry regions of the world for growing crops and for other uses.  Such water transfers have benefited many people, but they have also wasted a lot of water and they have degraded ecosystems from which the water was taken.  Such water waste is part of the reason why many products include large amounts of virtual water. Removing salt from seawater is costly, kills marine organisms, and produces briny wastewater  Desalination involves removing dissolved salts from ocean water or from brackish water in aquifers or lakes for domestic use.  Distillationinvolves heating saltwater until it evaporates (leaving behind salts in solid form) and condenses as freshwater.  Reverse osmosis (or microfiltration) uses high pressure to force saltwater through a membrane filter with pores small enough to remove the salt.  Today,about 13,000 desalination plants operate in more than 125 countries, especially in the arid nations of the Middle East, North Africa, the Caribbean, and the Mediterranean.  There are three major problems with the widespread use of desalination  Thehigh cost, because it takes a lot of increasingly expensive energy to desalinate water.  Pumping large volumes of seawater through pipes and using chemicals to sterilize the water and keep down algae growth kills many marine organisms and also requires large inputs of energy to run the pumps.  Desalinationproduces huge quantities of salty wastewater that must go somewhere.  Some scientists have hopes for using solar energy as the primary power source for desalination. Reducing freshwater waste has many benefits  Reasons so much freshwater is wasted: Government subsidies that keep the cost of freshwater low. Lack of government subsidies for improving the efficiency of freshwater use. We can cut freshwater waste in irrigation  About 60% of the irrigation water worldwide does not reach the targeted crops.  In most irrigation systems, water is pumped from a groundwater or surface water source through unlined ditches and about 40% is lost through evaporation, seepage, and runoff.  Flood irrigation delivers far more water than needed for crop growth and typically loses 40% of water through evaporation, seepage, and runoff.  With existing irrigation, could be reduced to 5–10%. Drip or Trickle irrigation (microirrigation). - The most efficient way to deliver small amounts of water precisely to crop. Ways to reduce freshwater waste in irrigation  We can cut freshwater waste in industry and homes  Producers of chemicals, paper, oil, coal, primary metals, and processed food consume almost 90% of the water used by industry in the United States.  Some of these industries recapture, purify, and recycle water to reduce their water use and water treatment costs.  Most industrial processes could be redesigned to use much less freshwater. We can cut freshwater waste in industry and homes  Flushing toilets with freshwater is the largest use of domestic water in the US.  Standards have required that new toilets use no more than 6.1 liters (1.6 gallons) of water per flush.  Studies show that 30–60% of the freshwater supplied in nearly all of the world’s major cities in less- developed countries is lost, primarily through leakage in water mains, pipes, pumps, and valves. We can cut freshwater waste in industry and homes  Fixing leaks should be a high government priority, would cost less than building dams or importing water.  Homeowners and businesses in water-short areas are using drip irrigation and replacing lawns with native plants that need little freshwater.  About 50–75% of the slightly dirtied water from bathtubs, showers, sinks, dishwashers, and clothes washers in a typical house could be stored in a holding tank and then reused as gray water to irrigate lawns and nonedible plants, to flush toilets, and to wash cars.  Therelatively low cost of water in most communities causes excessive water use and waste. Many water utility and irrigation authorities charge a flat fee for water use, and some charge less for the largest users of water. About one-fifth of all U.S. public water systems do not have water meters and charge a single low rate for almost unlimited use of high-quality water. Many apartment dwellers have little incentive to conserve water, because water use charges are included in their rent. We can cut freshwater waste in industry and homes We can use less water to remove wastes  Largeamounts of freshwater good enough to drink are being flushed away as industrial, animal, and household wastes.  Within 40 years we may need the world’s entire reliable flow of river water just to dilute and transport the wastes we produce each year.  Savewater by using systems that mimic the way nature deals with wastes by recycling them.  Rely more on waterless composting toilets. HOW CAN WE REDUCE THE THREAT OF FLOODING? Some areas get too much water from flooding  Some areas sometimes have too much water because of natural flooding by streams, caused mostly by heavy rain or rapidly melting snow. A flood happens when water in a stream overflows its normal channel and spills into the adjacent area, called a floodplain.  Floodplains, which usually include highly productive wetlands, help to provide natural flood and erosion control, maintain high water quality, and recharge groundwater.  People settle on floodplains to take advantage of their many assets, such as fertile soil, ample freshwater and proximity to rivers for transportation and recreation.  To reduce the threat of flooding for people who live on floodplains:  Rivers have been narrowed and straightened (channelized), equipped with protective levees and walls, and dammed to create reservoirs that store and release water as needed.  Greatly increased flood damage may occur when prolonged rains overwhelm them.  Floods provide several benefits. Create the world’s most productive farmland by depositing nutrient-rich silt on floodplains. Recharge groundwater and help to refill wetlands, thereby supporting biodiversity and aquatic ecological services.  Since the 1960s, human activities have contributed to a sharp rise in flood deaths and damages, meaning that such disasters are partly human-made. We can reduce flood risks  To improve flood control, we can rely less on engineering devices such as dams and levees and more on nature’s systems such as wetlands and natural vegetation in watersheds.  Channelization reduces upstream flooding, but:  It eliminates aquatic habitats, reduces groundwater discharge, and results in a faster flow, which can increase downstream flooding and sediment deposition.  Channelization encourages human settlement in floodplains, which increases the risk of damages and deaths from major floods.  Levees or floodwalls along the sides of streams contain and speed up stream flow, but they increase the water’s capacity for doing damage downstream.  No protection against unusually high and powerful floodwaters.  In 1993, two-thirds of the levees built along the Mississippi River were damaged or destroyed.  Dams can reduce the threat of flooding by storing water in a reservoir and releasing it gradually, but they also have a number of disadvantages.  An important way to reduce flooding is to preserve existing wetlands and restore degraded wetlands to take advantage of the natural flood control they provide in floodplains.  We can sharply reduce emissions of greenhouse gases that contribute to projected climate change, which will likely raise sea levels and flood many coastal areas of the world during this century.  We can think carefully about where we choose to live.  Many poor people live in flood-prone areas because they have nowhere else to go. Most people, however, can choose not to live in areas especially subject to flooding or to water shortages. We need to use water more sustainably We need to use water more sustainably END