Environmental Problems, Their Causes, and Sustainability 1

physically connects us to one another, to other formsof life, and to the entire planet.Despite its importance, water is one of our mostpoorly managed resources. We waste it and pollute it.We also charge too little for making it available. Thisencourages still greater waste and pollution of this resource,for which we have no substitute. As BenjaminFranklin said many decades ago: “It is not until thewell runs dry that we know the worth of water.”15-2 SUPPLY, RENEWAL, AND USEOF WATER RESOURCESHow Much Fresh Water Is Available?Natural Recycling to the RescueOnly about 0.01% of the earth’s water supply isavailable to us as fresh water, but this supply isrecycled.Only a tiny fraction of the planet’s abundant water isavailable to us as fresh water (Figure 15-2). Study thisfigure carefully to see where the world’s water isfound. About 97.4% of the world’s total volume ofwater is found in oceans and saline lakes and is toosalty for drinking, irrigation, or industry (except as acoolant). Most of the remaining 2.6% that is freshwater is locked up in ice caps or glaciers or in groundwatertoo deep or salty to be used.Thus only about 0.014% of the earth’s total volumeof water is easily available to us as soil moisture,usable groundwater, water vapor, and lakes andstreams. If the world’s water supply were only 100liters (26 gallons), our usable supply of fresh waterwould be only 0.014 liter, or 2.5 teaspoons!Fortunately, the world’s fresh water supply is continuouslycollected, purified, recycled, and distributedin the solar-powered hydrologic cycle (Figure 4-28,p. 76). You might want to review this figure to trace thecirculation of the earth’s water as it evaporates fromwater, land, and organisms into the atmosphere; condensesand falls as precipitation back to the earth’sland and water; and flows across the earth’s surfacesas runoff into streams, rivers, lakes, wetlands, and theseas and infiltrates underground.This magnificent water recycling and purificationsystem works only as long as we do not overload watersystems with slowly degradable and nondegradablewastes or withdraw water from undergroundsupplies faster than it is replenished. Bad news. In partsof the world we are doing both of these things.Differences in average annual precipitation dividethe world’s continents, countries, and people into waterhaves and have-nots. Some places get lots of rain (thedark green and light green areas in Figure 6-6, p. 106)and others very little (the yellow areas in Figure 6-6,p. 106). For example, Canada, with only 0.5% of theworld’s population, has 20% of the world’s fresh water,whereas China, with 21% of the world’s people,has only 7% of the supply.What Is Surface Water? Water on TopWater that does not sink into the ground or evaporateinto the air runs off into bodies of water.One of our most precious resources is fresh water thatflows across the earth’s land surface and into theworld’s rivers, streams, lakes, wetlands, and estuaries.The precipitation that does not infiltrate the ground orreturn to the atmosphere by evaporation (includingtranspiration from plants) is called surface runoff. Theregion from which surface water drains into a river,lake, wetland, or other body of water is called its watershedor drainage basin.The renewable supplies of freshwater we dependon consist of accessible surface runoff and freshwaterthat infiltrates into aquifers found fairly near theearth’s surface. About two-thirds of the world’s annualrunoff is lost by seasonal floods and is not available forhuman use. The remaining one-third is reliable runoff:the amount of runoff that we can generally count on asa stable source of water from year to year.All waterOceans andsaline lakes97.4%Fresh water2.6%Fresh waterIce capsand glaciers1.984%Groundwater0.592%0.014%Readily accessible fresh waterLakes0.007%Soilmoisture0.005%Biota0.0001%Rivers0.0001%Atmosphericwater vapor0.001%Figure 15-2 Natural capital: the planet’s water budget. Only a tiny fraction by volume of the world’s watersupply is fresh water available for human use.http://biology.brookscole.com/miller14307