When a natural area—that is, an area undis­turbed by humans—is inundated with heavy pre­cipitation, the plant-protected soil absorbs much of the excess water. What the soil cannot absorb runs off into the river, which may spill over its banks in the flood plain. However, because rivers meander (that is, they aren't straight), the flow is slowed, and the swollen waters rarely cause significant dam­age to the surrounding area. (For a discussion of a river whose natural course was altered, see Focus On: Untwisting and Twisting the Kissimmee River.)

    When an area is developed for human use, much of the water-absorbing plant cover is re­moved. Buildings and paved roads do not absorb water, so runoff (usually in the form of storm sewer runoff) is significantly greater (Figure 13-10). Peo­ple who build homes or businesses on the flood plain of a river will most likely experience flooding at some point.

It is easier and more economical to ban or re­strict development in a flood plain than to build on it and then try to prevent flooding with such struc­tures as retaining walls and levees. Increasingly, local governments, both in the United States and in the rest of the world, now zone flood plains to curtail development.

Too Little Water

Arid lands, or deserts, arc fragile ecosystems in which plant growth is limited by lack of precipita­tion. Semiarid lands receive more precipitation than deserts but are subject to frequent and pro­longed droughts. Forty percent of the world's popu­lation lives in arid or semiarid lands, primarily in Asia and Africa. These people spend substantial amounts of time and effort obtaining water. Each day they may have to walk many miles to a stream or river and carry back the heavy water.

    Overpopulation in arid and semiarid regions intensifies the problem of water shortage. The immediate need for food prompts people to remove natural plant cover in order to grow crops on mar­ginal land (land subject to frequent drought and subsequent crop losses), and their livestock over­graze the small amount of plant cover in natural pastures. As a result of the lack of plants, when the rains do come, runoff is greater, for the soil cannot absorb the water. Because the soil is not replen­ished by the precipitation that docs fall, crop pro­ductivity is poor and the people are forced to culti­vate food crops on additional marginal land.

Overdrawing Surface Waters

Removing too much fresh water from a river or lake can have disastrous consequences in local ecosystems. Humans can remove perhaps 30 percent of a river's flow without greatly affecting the natural ecosystem. In some places, however, considerably more than that amount is withdrawn for human use. In the Ameri­can Southwest, for example, it is not unusual for 70 percent or more of surface water to be removed.

    When surface water is overdrawn, wetlands dry up. Natural wetlands play many roles, not the least of which is serving as a breeding ground for many species of birds. Estuaries, where rivers empty into seawater, become saltier when surface waters are overdrawn, and this change in

salinity greatly affects the productivity that is asso­ciated with estuaries.

    Mono Lake, a salty lake in eastern California, is one of die most striking examples of the effects of humans removing too much surface water. This lake is replenished by rivers and streams that are largely formed from snowmelt in the Sierra Nevada range. Evaporation provides the only natural outflow from the lake.

    Much of the surface water that would naturally feed Mono Lake is now diverted to Los Angeles. As a result, Mono Lake's water level has subsided about 12.2 m (40 ft), and its salinity has increased dramatically. The organisms living in and around Mono Lake—including rabbits, shrews, muskrats, mink, mule deer, porcupines, and

shoreline vegetation—have been adversely af­fected by these changes. If Mono Lake becomes much more saline as a result of further drops in water level, it will be unable to support the brine shrimp and aquatic brine flies that are, in turn, con­sumed by a large migratory bird population that includes Arctic loons, grebes, cormorants, egrets, sandpipers, gulls, and bitterns. In all, there are 293 different species of birds in the Mono Lake eco­system. If Mono Lake becomes more saline, it will be dead lake, unable to support wildlife. The state of California is currently reviewing the Mono Lake mm, ii ion and should rule on its own water rights in 1993.

    The Aral Sea in Kazakhstan, part of the former Soviet Union, is suffering from the same problem as Mono Lake. Like Mono Lake, it has no outflow other than evaporation. In recent years, much of the inflow has been diverted for irrigation of fertile farmland around the lake. Since 1960, the Aral Sea has declined in area by 40 percent (Figure 13-12). Much of its biological diversity has disappeared, and, like Mono Lake, it runs the risk of becoming a lifeless brine lake. Millions of people living near the Aral Sea have developed health problems rang-

ing from allergies to throat cancer, presumed to he caused by winds that whip the salt on the receding shoreline into the air, causing blinding salt storms. Moreover the salt is carried by the wind hundreds of miles from the Aral Sea, and where it is deposited it causes soil pollution, which reduces the productivity of the land.

Aquifer Depletion: The removal by humans of more groundwater than can be recharged by precipitation or melting snow—called aquifer depletion— has several serious consequences. Prolonged aquifer depletion drains an aquifer dry, effectively elimi­nating it as a water resource. The depth to which one must go to find water in the Ogallala aquifer in Texas, for example, has increased steadily since the 1940s, a result of aquifer depletion. In addition, aquifer depletion from porous rock causes subsid­ence, or sinking, of the land on top. For example, the San Joaquin Valley in California has sunk almost 10 m (32.8 ft) in the past 50 years due to aquifer depletion. Salt water intrusion, the movement of seawater into a freshwater aquifer, can occur along coastal areas when groundwater is depleted faster than it can he replenished.

Irrigation: Semiarid lands usually provide marginal crops, but farmers can increase the agricultural productivity of semiarid lands with irrigation. Almost any crop can be grown in the desert if enough water is supplied to the soil.

Although irrigation improves the agricultural productivity of arid and semiarid lands, it can also cause salt to accumulate in the soil, a process called salinization. In a natural scenario, as a result of precipitation runoff, rivers carry salt away. Irrigation water, however, normally soaks into the soil and does not run off the land into rivers, so when it evaporates, the salt remains be­hind and accumulates in the soil. Salty soil results in a decline in productivity and in extreme cases render the soil completely unfit for crop produc­tion. Chapter 21 discusses the problem of soil salinization in greater detail.

Water Problems in the United States

Americans consume less than one-fourteenth of the fresh water that is available in the continental United States. However, this general picture of our water supply overlooks the regional and seasonal differences in distribution of water, amount of groundwater, climate, and consumption rates that make acquisition of water a challenge for many re­gions. Droughts, higher-than-average precipitation rates, and other natural conditions cause problems in water availability throughout the country, and human activities sometimes exacerbate the difficul­ties.

Surface Water

The increased use of U.S. surface water for agriculture, industry, and personal con­sumption during the past 35 years has caused mum water supply and quality problems. Some U.S. re­gions that have grown in population during ilii¹ period (for example, California, Arizona, and Flor­ida) have placed correspondingly greater burdens on their water supplies. If water consumption in these and other areas continues to increase, the availability of surface waters could become re­gional problem in many places that have never he-fore experienced water shortages. Areas that derive their water from the Arkansas River, for example, may soon experience critical water shortages.

    Nowhere in the country are water problems as severe as they are in the West and Southwest. Much of this large region is arid or semiarid and receives less than 50 cm (about 20 in) of precipita­tion annually. The West and Southwest consume an average of 44 percent of their renewable water, as compared to an average consumption of 4 per­cent of renewable water elsewhere in the United States. Historically, water in the West was used primarily for irrigation. However, with the rapid expansion of population in that re­gion during the past 25 years, municipal, commer­cial, and industrial uses now compete heavily with irrigation for available water.

    Until recently, the development of new sources of water met expanding water needs in the West and Southwest. Water was diverted from distant sources and transported via aqueducts—large conduits to areas that needed it. As long ago as 1913. for example, Los Angeles started bringing in water from an area of California 400km (250 miles) north, along the east side of the Sierra Nevada. Dams were built and water-holding basins created to ensure a year-round supply. These solutions are no longer viable, because the closest, most practical water sources have already been used and because the public, which has come to expect inexpensive water, opposes paying for costly solutions such as dams and aqueducts.

The Colorado River Basin

    In certain coastal areas of Louisiana and Texas, the removal of too much groundwater has resulted in the intrusion of salt water from the Gulf of Mexico. Salt water intrusion from the Pacific Ocean has incurred along parts of the California coast, along coastal areas of Puget Sound in northwestern Washington, and in certain areas of Hawaii. Florida and many coastal regions in the Northeast and Mid-Atlantic states also have salt water intrusion.

    The quality of an area's groundwater varies from fresh to saline depending on the soil and rock characteristics of the area as well as the age of the water and its rate of recharge. Generally, ground-water is more saline in areas that have high evaporation rates and where saline water infiltrates (penetrates) through rocks such as carbonate and sandstone, as it does in parts of North Dakota and Minnesota. Certain areas in the United States have poor groundwater quality due to naturally high lo­calized concentrations of salts of such chemicals as fluoride and arsenic. Some areas of Nevada and Utah have unusually high levels of toxic minerals in their groundwater as a result of natural condi­tions, rather than pollution caused by humans. (Chapter 21 discusses groundwater contamination caused by pollution.)

Global Water Problems

Many developing countries have insufficient water to meet the most basic drinking and household needs of their people. The World Health Organization (WHO) estimates that 1 bil­lion people lack access to safe drinking water and almost 2 billion are without access to a satisfactory means of domestic wastewater and fecal waste dis­posal. They risk disease because the water they con­sume is contaminated by sewage or industrial

wastes. Many of these people have to travel great distances to secure the water they need; this prac­tice consumes large amounts of time, particularly for women and children, and tends to perpetuate poverty. WHO also estimates that 80 percent of human illness results from insufficient water sup­plies and poor water quality caused by lack of sani­tation. Although many developing countries have installed or are installing public water systems, pop­ulation increases tend to overwhelm efforts to im­prove the water supply.

    The United States is involved in efforts to im­prove water quality and supply in countries with critical water problems. The Agency for Interna­tional Development (AID) manages projects in areas vulnerable to prolonged drought, such as the Sahel region in Africa. AID has assisted in well digging and other measures that alleviate the ef­fects of drought in the Sahel. In addition to contri­butions by individual governments such as the United States, both the United Nations and the World Bank sponsor water management projects in developing countries.

As the world's population continues to in­crease, global water problems will become more serious. Population growth is already outstripping water supplies in countries such as India, where approximately 8,000 villages have no local water. The water supply to some Indian cities—Madras, for example—has been so severely depleted that water is rationed from a public tap. Water supplies are also precarious in much of China, owing to pop­ulation pressures. One-third of the wells in Beijing, for example, have gone dry, and the water table continues to drop. Mexico is facing the most seri­ous water shortages of any country in the Western Hemisphere. The main aquifer supplying Mexico City, for instance, is dropping by as much as 3.5 m (11.2ft) per year.

Shortages in global water supplies may also affect humans by limiting the amount of food they can grow. Recall that the main use of fresh water is for irrigation. As fresh water supplies are depleted by a growing human population, less water will be available for crops. The availability problem is compounded by salinization of agricultural soil as a result of irrigation practices. Local or even wide­spread famines from water shortages are a very real danger.

    The decade of the 1990s may well see countries facing one another in armed conflict over water rights. One particularly troublesome spot is be­tween Israel and Jordan, neighboring arid countries that have never been friendly. Both countries ob­tain fresh water from the Jordan River basin. Israel and Jordan both anticipate large population in­creases during the 1990s, which could make their water situation critical. Neither country approves of the other increasing its allotment of water from the Jordan River, because an increase for one coun­try would mean a smaller supply for the other.