What Is Water? - Sometimes It Rains; Sometimes It Doesn't

The amount of precipitation that falls around the world can range from less than one-tenth of one inch per year in the deserts to hundreds of inches per year in the tropics. In 2000 the National Climatic Data Center reported that the lowest average annual precipitation occurs in Arica, Chile, with 0.03 inches. The world's wettest spot is Mawsynram, India, with an average annual rainfall of 467.4 inches. The single wettest year recorded was in Cherrapunji, India, with 905 inches in 1861. By contrast, the Central Valley in California, which produces a large percentage of U.S. vegetable crops, averages only fifteen inches of rainfall per year. Without supplemental water from irrigation, this area FIGURE 1.5
Comparison of the amount of fresh water in storage
SOURCE: "Comparison of the Amount of Fresh Water in Storage," in Groundwater, U.S. Geological Survey General Interest Publication, 1999, http://capp.water.usgs.gov/GIP/gw_gip/ (accessed March 25, 2005) would not be productive. The lower forty-eight states receive enough rain in an average year that, were it to fall all at one time, it would cover the entire country to a depth of about thirty inches of water, weighing 6.6 billion tons.

What happens to rainwater after it reaches the ground depends on such factors as the rate of rainfall, topography, soil conditions, density of vegetation, temperatures, and the extent of urbanization. For example, the direct runoff in a highly urbanized area is relatively great, not only because of impermeable roofs and pavements, but also because storm-sewer systems carry water directly to streams and lakes. Figure 1.6 illustrates this point. Urbanization leads to fewer floodplains and wetlands, fewer riparian areas (for example, riverbanks), less ground cover, and compacted soil. A large winter storm could lead to increased turbidity (cloudiness or discoloration of the water), flooding, runoff, and erosion.

Variations in precipitation occur not only seasonally but also annually. For example, southern Florida has a rainy season (May to October) followed by a dry season (November to April). Most of the forty-five to sixty inches of annual rain that falls (under normal conditions) in this area occurs in the rainy season. In exceptionally dry years, droughts occur because the area receives little or no precipitation in the rainy season; in exceptionally wet years, flooding may occur.

Natural phenomena known as El Niño and La Niña influence weather and precipitation. El Niñois a naturally occurring disruption of the ocean-atmosphere system in the tropical Pacific Ocean, which has important consequences for weather around the globe. It is characterized by an unusually warm current of water that appears every three to five years in the eastern Pacific Ocean. Unusually warm sea surface temperature results in a decline in primary productivity (microscopic plants and animals) that in turn brings sharp declines in commercial fisheries and bird populations that are also dependent on fish. Unusual weather conditions occur around the globe as jet streams, storm tracks, and monsoons are shifted. Some other consequences are increased rainfall across the southern United States and Peru that has caused destructive flooding in the past, and drought in Australia and Indonesia. El Niño brings warmer than normal temperatures to the north-central states and cooler than normal temperatures to the southeastern and southwestern United States.

La Niña global climate impacts tend to be the opposite of El Niño since La Niña is characterized by unusually cold ocean temperatures in the Equatorial Pacific. In the United States, winter temperatures are warmer than normal in the Southeast and cooler than normal in the Northwest. La Niña events occur after some, but not all, El Niño events. Generally, La Niña occurs half as frequently as El Niño.

FIGURE 1.6
Impacts on water quality during a large winter storm
SOURCE: "Figure 2. Impacts on Water Quality During a Large Winter Storm," in Many Activities Contribute to Turbidity During Large Storms, U.S. General Accounting Office, July 1998