Environmental Problems, Their Causes, and Sustainability 1

and wildlife could trigger “water wars”—intense politicaland legal conflicts—in the next 20 years.15-3 TOO LITTLE WATERWhat Causes Freshwater Shortages? Climateand DemandDry climate, drought, dry soil, and too many peopleusing the reliable supply cause water scarcity.According to Swedish hydrologist Malin Falkenmark,there are four causes of water scarcity: dry climate,drought (a prolonged period in which precipitation isat least 70% lower and evaporation is higher than normal),desiccation (drying of exposed soil because ofactivities such as deforestation and overgrazing bylivestock), and water stress (low per capita availabilityof water caused by increasing numbers of people relyingon limited runoff).Figure 15-8 shows the degree of stress on theworld’s major river systems, based on the amount ofwater available compared to the amount used by humans.A country is said to be water stressed when thevolume of reliable runoff per person drops belowabout 1,700 cubic meters (60,000 cubic feet) per year.This usually occurs when water withdrawal is morethan 20% higher than the reliable supply. A countrysuffers from water scarcity when per capita water availabilityfalls below 1,000 cubic meters (35,000 cubic feet)per year.According to the United Nations, about 41% of theworld’s population lives in river basins located in 20countries that suffer from water stress or water scarcity.Look at the red and orange areas in Figure 15-8 to seewhere these areas are located. The number of countriessuffering from water stress or water scarcity couldgrow to 40 countries by 2020 and 60 countries by 2050.Some areas have lots of water, but the largest riverscarrying most of the runoff are far from agriculturaland population centers. For example, South Americahas the largest annual water runoff of any continent,but 60% of the runoff flows through the Amazon Riverin remote areas where few people live.In some areas, overall precipitation is plentiful butarrives mostly during short periods or cannot be collectedand stored because of a lack of storage capacity.For example, only a few hours of rain provide overhalf of India’s rainfall during a four-month monsoonseason.The volumes of some of the world’s lakes andrivers have shrunk drastically, mostly because of humanwithdrawals of water for irrigation and industry.Siberia’s Aral Sea, once the world’s fourth-largestfreshwater lake, has shrunk in area to less than half itsformer size and lost 83% of its volume of water sinceEuropeNorthAmericaAsiaAfricaSouthAmericaStressAustraliaHighNoneFigure 15-8 Natural capital degradation: stress on the world’s major river basins, based on a comparison ofthe amount of water available with the amount used by humans. (Data from World Commission on Water Use inthe 21st Century)http://biology.brookscole.com/miller14311