Lecture 07 Chapter 10- Water: Hydrologic Cycle and Human Use Droughts are striking worldwide • California, and much of the southwest United States, is in a mega-drought—one of the worst since AD 1000 – Decreasing water is making it hard to irrigate crops – Water comes from far-away rivers, ancient groundwater, desalinization, conservation, dams • China and Australia are also suffering from drought • Freshwater is liquid gold – Finding enough water for the millions of people being added will be a top priority © 2017 Pearson Education, Inc. Earth’s water Although Earth has a lot of water, accessible freshwater makes up only 0.77% of the total © 2017 Pearson Education, Inc. Humans control water • Humans construct infrastructures to control water: – Dams, canals, reservoirs, aqueducts, sewer systems – Treatment plants, water towers, pipelines, irrigation – Desalinization plants – Waterborne diseases have been controlled • Developed nations control water – Decrease diseases, build cities in deserts, irrigate, create electricity • Millions in developing nations lack safe drinking water and sanitation, and die from diseases © 2017 Pearson Education, Inc. Water will become more scarce • Because of infrastructure to control water: – – – – – – Seas and rivers are being lost Reservoirs have displaced millions of people Aquifers are being pumped dry Tensions increase for access to water It will be harder to feed a growing population Cities and industries compete with agriculture • Climate change will affect the hydrologic cycle – Floods and droughts will devastate croplands © 2017 Pearson Education, Inc. Differences in water availability Some people can luxuriate in water—others have to walk long distances to get enough water just to survive © 2017 Pearson Education, Inc. Two ways to consider water issues • Quantity: the water cycle, controlling and managing water, policies for water use • Quality: pollution and its consequences • Earth’s water cycles through the hydrologic cycle – 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 © 2017 Pearson Education, Inc. The hydrologic cycle Green arrows represent green water (water vapor), blue arrows represent blue water (liquid) © 2017 Pearson Education, Inc. Animation: The Hydrologic Cycle © 2017 Pearson Education, Inc. Animation: Water Cycle Scramble © 2017 Pearson Education, Inc. Global air circulation The equator’s two Hadley cells (a) and six cells on either side of the equator (b), and (c) global wind patterns © 2017 Pearson Education, Inc. What causes a rain shadow? • When moisture-laden trade winds encounter mountain ranges, air is deflected and rises – Precipitation occurs on the windward side • Air crossing the mountains warms and picks up moisture from the soil – Deserts occur on the leeward sides of mountains – Rain shadow: the dry region downwind (leeward) of a mountain range • The Sierra Nevada Mountains of California support lush forests on the windward side – Death Valley, on the other side, is extremely dry © 2017 Pearson Education, Inc. Rain shadow Moisture-laden air cools as it rises over a mountain range, causing precipitation on the windward side. The leeward side has desert conditions. © 2017 Pearson Education, Inc. Global precipitation Note the high rainfall near the equator, and regions of low rainfall north and south © 2017 Pearson Education, Inc. Precipitation ends up in several places • Precipitation can either soak into the ground (infiltration) or run off the surface • Infiltration-runoff ratio: the amount of water that infiltrates 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 © 2017 Pearson Education, Inc. 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: not held in the soil – Percolation (blue water flow): trickling of water through pores or cracks in the soil due to gravity – Once it hits an impervious layer, it accumulates © 2017 Pearson Education, Inc. What happens to gravitational water? • 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 – Groundwater fills the well to the level of the water table – It can also seep laterally as it seeks its lowest level © 2017 Pearson Education, Inc. Recharge of an aquifer • Aquifers: layers of porous material through which groundwater moves – It is hard to determine the location of aquifers – Layers of porous rock are found between layers of impervious rock • Recharge area: the area where water enters an aquifer – May be far away from where water leaves the aquifer • Aquifers hold 99% of all liquid freshwater © 2017 Pearson Education, Inc. Underground purification • As water percolates through the soil, debris and bacteria are filtered out • Water may dissolve or leach out minerals – Most are harmless (limestone) – Some 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 – But dry up if the water table drops © 2017 Pearson Education, Inc. Yearly water balance in the hydrologic cycle This figure shows how water moves between the oceans and the land © 2017 Pearson Education, Inc. 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 (diversion) for human use © 2017 Pearson Education, Inc. Changes to the surface of the Earth • In natural systems, vegetation intercepts precipitation – Water infiltrates into porous topsoil • In cleared forests and overgrazed land, plants do not intercept rainfall – Water shifts from infiltration and recharge into runoff – Causing floods and erosion, sedimentation – Less infiltration, evapotranspiration, and groundwater recharge result in dry, barren, lifeless streambeds © 2017 Pearson Education, Inc. More surface changes… • Wetlands store and release water – Destruction causes flooding, droughts, polluted waterways • Water is moved from land to rivers to the ocean – Drainage pipes (tiles) in fields lower water tables – Channelizing (straightening) rivers • Roads, buildings, parking lots prevent infiltration • Dams store water and slow movement to oceans – No recharge of groundwater, evaporation increases – Can cause regional water loss and serious problems © 2017 Pearson Education, Inc. Impervious surfaces Roofs, roads, and parking lots decrease the water that goes into the soil and recharges groundwater © 2017 Pearson Education, Inc. The Three Gorges Dam • Completed in 2006, it is the world’s largest dam – Generates an amount of electricity equal to burning 50 million tons of coal a year – Controls flooding of the Yangtze River • Is an example of the problems dams cause – Displacing 1.3 million people—4 million more will be moved to make room for the 410-mile long reservoir – Lowering the Yangtze let saltwater into the river, stranded ships, and accumulated pollutants – Lower fish population, and more earthquakes © 2017 Pearson Education, Inc. Moving water to the ocean • Almost all human actions increase water going to the oceans—but less goes into groundwater – Cultivation, impervious surfaces, wetland loss, and so on – Causes floods, droughts, and erosion – Deforestation reduces the “sponge” effects of forests • We need changes in requirements for construction – Flood retention ponds stop floods and allow infiltration – Protect wetlands and river vegetation – Grow vegetation in cities (urban gardening) – Environmentally friendly surfaces © 2017 Pearson Education, Inc. Flooding in the Balkans, 2014 People had to evacuate their homes when this region experienced its heaviest rainfall in 100 years © 2017 Pearson Education, Inc. Climate change affects the water cycle • The Earth is warming—the rise in greenhouse gases is speeding up the water cycle • Evaporation increases with a warmer climate – A wetter atmosphere increases precipitation and floods – More hurricanes and droughts – Water-stressed areas will get less water • The greater volume of warm water, along with melting ice and glaciers, raises sea levels – Affecting ocean currents and 630 million people © 2017 Pearson Education, Inc. Atmospheric particles • Aerosol particles form nuclei that let water condense into droplets that form clouds – Reduced solar radiation has a cooling effect • Particles (sulfates, soot, dust) are increasing – Industrial areas, tropical burning, and dust storms • Aerosols’ smaller droplets suppress rain (even though clouds form) and atmospheric cleansing – Leads to dry conditions, dust, smoke, more aerosols • Aerosols work differently from greenhouse gases – They have local impacts and last only days © 2017 Pearson Education, Inc. 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 water uses: water may be contaminated, but is still available to humans – In homes, industries, and electric power production • Consumptive uses of water: the applied water does not return to the water resource – It percolates into the ground or evapotranspirates © 2017 Pearson Education, Inc. U.S. trends in population and freshwater withdrawals Since 1980, Americans have used less water, even though the population has increased © 2017 Pearson Education, Inc. What are major uses of water? • Worldwide, the largest use is for irrigation – Then industry and direct human use • Use varies by region, depending on: – Precipitation and degree of regional development • Most increases in withdrawal are due to agriculture – Irrigation uses 80% of freshwater in the United States • Each fracking well (for natural gas) uses 4–6 million gallons of water, which becomes contaminated – But, shale deposits are located in arid regions! – It takes water away from other uses © 2017 Pearson Education, Inc. Regional usage of water The percentage used in each category varies with the climate and development of the region © 2017 Pearson Education, Inc. Water is often polluted • 25% of U.S. water comes from groundwater – 75% from surface water (rivers, lakes, reservoirs) • Rural areas in poor 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 wastes (animal waste, human sewage) – 1.1 billion people use polluted water – 1.6 million (mostly children) die each year • One Sustainable Development Goal: – Increase water sanitation © 2017 Pearson Education, Inc. Water in the developing world In many areas of the developing world, water sources are often contaminated with pathogens and other pollutants © 2017 Pearson Education, Inc. Developed nations collect, treat, and distribute water • Larger municipalities rely mostly on surface water – Dams are used to generate power, and for recreation, flood control, and irrigation • Water used in homes, schools, industries is first piped to a treatment plant • Wastewater is collected by a sewage system and treated before being released downstream – Water is reused many times on major rivers • Smaller public drinking-water systems depend on groundwater, which is purified by percolation © 2017 Pearson Education, Inc. Municipal water use and treatment © 2017 Pearson Education, Inc. Is bottled water the answer? • Half of Americans use bottled water – 10 billion gallons sold for $12 billion in 2013 • Using bottled water has negative effects – Bottling occurs where locals also need the water – In 2009, the courts decided Nestle Waters, North America, could not pump 400 gallons/minute • Bottled water uses a lot of energy for plastic bottles and transportation • Bottled water may be good for emergencies – But not as a source of drinking water generally © 2017 Pearson Education, Inc. Dams keep and use surface water • Over 48,000 large dams (over 50 feet tall) exist – They provide 19% of the world’s total electricity • Only 31% of annual runoff is available for withdrawal – Rivers are too remote – Navigation, flood control, hydropower also need water • Large dams have enormous social impacts – Displacing 40–80 million people – Preventing access to goods and services of the now-buried ecosystems © 2017 Pearson Education, Inc. Number of dams in the United States Each point represents a dam; the colors tell the height © 2017 Pearson Education, Inc. Dams have enormous impacts • Habitats (waterfalls, rapids, fish runs) are lost • The river downstream is deprived of water • Fish and other aquatic organisms are impacted – Wetlands dry up, and waterfowl and wildlife die • 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 • Decaying plants change the water chemistry – Releasing pollutants (e.g., mercury) from the soil © 2017 Pearson Education, Inc. An example of a dam’s impacts • The Glen Canyon Dam spans the Colorado River – It stores excess water in Lake Powell (the secondlargest U.S. reservoir) and generates hydropower • The dam had seriously damaged the downstream ecology of the Colorado River – Recreational resources (hiking, rafting) were hurt • The Grand Canyon Protection Act (1992) required “adaptive management” in rates of water release – To improve its ecological and recreational values © 2017 Pearson Education, Inc. Dams gone • People are recognizing the costs of dams • 850 dams have been removed in the United States – But removal is not easy • There are legal complexities to dam removal – Existing uses (flood control, irrigation water, recreation) – Conflict with expected advantages (establishing fish, restoring recreational and aesthetic uses) © 2017 Pearson Education, Inc. Removal of a dam The 2014 removal of the Glines Canyon Dam on the Elwah River in Washington State allowed salmon to spawn upstream © 2017 Pearson Education, Inc. Surface water is often diverted • Small-scale diversions include ditches and canals to irrigate fields – Increases evaporation, water leaks out of the ditch, mosquitoes can breed in the ditches • Large-scale diversions move water long distances – In China, the South-to-North Water Diversion Project will bring water to the arid north – They can bring pollution and affect original rivers and lakes © 2017 Pearson Education, Inc. Impacts on estuaries • Estuaries: productive ecosystems in bays and rivers, where fresh river water mixes with seawater – Breeding grounds for fish, shellfish, and birds – Less fresh-water increases the water’s salinity • In the San Francisco Bay, over 60% of freshwater has been diverted for irrigation and municipal use – Salt-water intrusion has devastated the bay – Fish populations have disappeared or been reduced – Tidal wetlands have been reduced by 92% – Land sinks due to compaction © 2017 Pearson Education, Inc. The CALFED Bay-Delta Program • This program was supposed to develop a plan to restore ecological health and improve water management • The program is not working – Farmers, government agencies, environmental groups do not agree with scientists’ recommendations • Other estuaries around the world are suffering – The Aswan Dam prevents the Nile River from flushing the Mediterranean Sea © 2017 Pearson Education, Inc. Some aquifers are nonrenewable • 99% of all freshwater is in aquifers • Nonrenewable groundwater: more than 75% of aquifers have recharge rates of centuries • Percolation replenished renewable groundwater – Is vulnerable to variations in precipitation • We are tapping large, but limited, natural reservoirs – Sustainability depends on balancing withdrawal rates with recharge rates • Most groundwater in arid regions has no recharge – It must be considered nonrenewable, like oil © 2017 Pearson Education, Inc. The Ogallala aquifer • The Ogallala aquifer supplies irrigation water to 10.4 million acres in 7 U.S. states – 20% of the irrigated land in the United States – It has “fossil water” that was recharged during the last ice age – Water is being withdrawn twice as fast as it recharges • Water tables have dropped 30–60 m (100–200 ft.) – Irrigated farming has stopped in some areas – Within 20 years, 3 million acres will be abandoned or converted to dryland farming © 2017 Pearson Education, Inc. Exploitation of an aquifer Water from the Ogallala has made the arid part of the United States into productive farmland, using center-pivot irrigation— but this is going to end © 2017 Pearson Education, Inc. Exploitation of an aquifer Sustainably used aquifers have a withdrawal rate less than 1. Footprints show the land needed to replace the water being withdrawn. © 2017 Pearson Education, Inc. Impacts of falling water tables • Falling water tables indicate that groundwater withdrawals exceed recharge rates – Common throughout the world – Will decrease crop production • Falling water tables dry up wetlands, springs and seeps, streams, rivers • Excessive groundwater removal creates the same results as diverting surface water © 2017 Pearson Education, Inc. Land subsides from removing groundwater • Groundwater fills spaces in the ground – Dropping water tables removes this support • Land subsidence: settling of the land – Causes cracked building foundations, roadways, and sewage pipes, and flooded coastal areas • A sinkhole: another kind of land subsidence – Results when an underground cavern is drained of its supporting groundwater and suddenly collapses – Can be 300 feet across and 150 feet deep – They’ve “consumed” buildings, livestock, highways © 2017 Pearson Education, Inc. Sinkhole Removing water drains an underground cavern and causes the roof to collapse © 2017 Pearson Education, Inc. 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 © 2017 Pearson Education, Inc. Saltwater intrusion Freshwater is maintained in an aquifer by the pressure of the freshwater inland—until excessive water removal reduces the pressure © 2017 Pearson Education, Inc. Water stewardship: supply and policy • The water cycle can meet human needs – But water is often not located where it is most needed, resulting in persistent water scarcity • The developing world lacks infrastructure (wells, water-treatment systems, dams) for water – Increasing populations increase water demand – Water for food dominates demand • We need a Blue Revolution: a radical change in the way we use water – Five options exist to meet future water needs © 2017 Pearson Education, Inc.
