Despite the overall abundance of fresh water in the United States, many areas have serve water shortages because of geographical variations. For example, arid and semiarid areas of the western United States normally receive little rain­fall. Annual fluctuations in precipitation also occur; even areas of the country that usually re­ceive adequate precipitation sometimes experience droughts.

    In the United States there are eleven major rivers or river systems that either flow into U.S. coastal waters or cross U.S. boundaries. The largest river system by far is the Mississippi River, which receives water from seven other large rivers and empties into the Gulf

of Mexico. An estimated 1.5 trillion liters (400 bil­lion gallons) of water enter the Gulf of Mexico from the Mississippi River each day.

Global Water Supply

Data on global water availability and use indicate that, overall, the amount of fresh water on the planet is adequate to meet human needs, even tak­ing population growth into account. These data do not, however, consider the distribution of water resources in relation to human populations. Citi­zens of Bahrain, the tiny island nation in the Per­sian Gulf, for example, have no fresh water supply and must rely completely on desalinization (remov­ing salt) of salty ocean water for their fresh water.

    Large variations in per-capita use of water exist from country to country and from continent to continent, depending on the size of the human population and the available water supply. South America and Asia are the two continents with the greatest total water supply; together they receive more than one-half of the world's re­newable fresh water (by precipitation). Although South America has more available water per person than Asia does, it does not have the potential to support as many people as its water supply would suggest. That is because most of the precipitation received by South America falls in the Amazon Rivet basin, which has poor soil and is therefore unsuitable for agriculture. In contrast, most of the precipitation in Asia falls on land that is suitable for agriculture; therefore, the water supply can sup­port more people.

    Humans need an adequate supply of water year round. Global water supply is complicated by the fact that stable runoff, the portion of runoff (from precipitation) that is available throughout the year, can be low despite the fact that total runoff is quite high. India, for example, has a wet season—June to September—during which 90 percent of its annual

precipitation occurs. Most of the water that falls during India's wet season quickly drains away into rivers and is unavailable during the rest of the year; thus, India's stable runoff is low.

    Variation in annual water supply is an impor­tant factor in certain areas of the world. The African Sahel region has wet years and dry years, for example, and the lack of water during the dry years limits human endeavors during the wet years.

    Global water supply is also complicated by the fact that surface water is often an international re­source. The management of rivers that cross inter­national boundaries requires international coopera­tion. The river basin for the Rhine River in Europe, for example, is in Switzerland, Germany, France, and the Netherlands. Traditionally, Switzerland, Germany, and France used water from the Rhein for industrial purposes and then dis­charged polluted water back into the river. The Dutch then had to clean up the water so they could drink it. Today, these countries recognize that in­ternational cooperation is essential if the supply and quality of the Rhine River is to be conserved and protected.