Environmental Problems, Their Causes, and Sustainability 1

Our liquid planet glows like a soft blue sapphire in the hardedgeddarkness of space. There is nothing else like it in thesolar system. It is because of water.JOHN TODDThis chapter discusses the water problems we face andways to use this wonderful but irreplaceable resourcemore sustainably. It addresses the following questions:■■■■■■■■■What are water’s unusual physical properties?How much fresh water is available to us, and howmuch of it are we using?What causes freshwater shortages, and what can bedone about this problem?What are the advantages and disadvantages of usingdams and reservoirs to supply more water?What are the advantages and disadvantages oftransferring large amounts of water from one placeto another?What are the advantages and disadvantages ofwithdrawing groundwater and converting saltwater to fresh water?How can we waste less water?What are the causes of flooding, and what can bedone to reduce the risk of flooding and flooddamage?How can we use the earth’s water more sustainably?15-1 WATER’S IMPORTANCEAND UNIQUE PROPERTIESWhy Is Water So Important? LiquidNatural CapitalWater keeps us alive, moderates climate, sculptsthe land, and removes and dilutes wastes andpollutants.We live on the water planet, with a precious film ofwater—most of it salt water—covering about 71% ofthe earth’s surface. Look in the mirror. What you see isabout 60% water, most of it inside your cells.No species can do without water. You could surviveseveral weeks without food but only a few dayswithout water. It takes huge amounts of water to supplyyou with food, shelter, and other needs and wants.Water also plays a key role in sculpting the earth’s surface,moderating climate, and removing and dilutingwastes and pollutants.What Are Some Important Propertiesof Water? An Amazing MoleculeWater’s unique and important properties arise mostlyfrom attractive forces between its molecules.Water is a remarkable substance with a unique combinationof properties:■ There are strong forces of attraction (called hydrogenbonds, Appendix 3, Figure 4) between molecules of water.These attractive forces are the major factor determiningwater’s distinctive properties.■ Water exists as a liquid over a wide temperature rangebecause of the strong forces of attraction between watermolecules. Without its high boiling point the oceanswould have evaporated a long time ago.■ Liquid water changes temperature slowly because it canstore a large amount of heat without a large change in temperature.This high heat capacity helps protect livingorganisms from temperature fluctuations. It also moderatesthe earth’s climate and makes water an excellentcoolant for car engines and power plants.■ Evaporating liquid water takes large amounts of energybecause of the strong forces of attraction between its molecules.Water absorbs large amounts of heat as itchanges into water vapor and releases this heat as thevapor condenses back to liquid water. This helps distributeheat throughout the world and determine theclimates of various areas (Figure 6-9, p. 107 andFigure 6-10, p. 107. This property also makes evaporationcooling process—explaining why you feel coolerwhen perspiration evaporates from your skin.■ Liquid water can dissolve a variety of compounds. Thisenables it to carry dissolved nutrients into the tissuesof living organisms, flush waste products out of thosetissues, serve as an all-purpose cleanser, and help removeand dilute the water-soluble wastes of civilization.This property also means that water-solublewastes can easily pollute water.■ Water filters out wavelengths of the sun’s ultraviolet(UV) radiation that would harm some aquatic organisms.■ Attractive forces between the molecules of liquid watercause its surface to contract and to adhere to and coat asolid. These strong cohesive forces allow narrowcolumns of water to rise through a plant from its rootsto its leaves (capillary action).■ Unlike most liquids, water expands when it freezes.This means that ice floats on water because it has alower density (mass per unit of volume) than liquidwater. Otherwise lakes and streams in cold climateswould freeze solid and lose most of their currentforms of aquatic life. Because water expands uponfreezing, it can break pipes, crack a car’s engine block(which is why we use antifreeze), break up streets,and fracture rocks that end up as soil particles.Without these unique properties of water, you andmost other forms of life on this planet would not exist.As water endlessly recycles through the biosphere, it306 CHAPTER 15 Water Resources