Fresh Water Shortage
A water cr is about to explode．Fresh water is a finite resource．The amount of fresh water supply provided by the hydrological cycle does not increase．Water everywhere on the planet is an integral part of the global hydrologic cycle．Precipitation (降水) originates as evaporation from land and the oceans．Soil moisture is used by plants, which return more moisture to the atmosphere, which then returns to the Earth as rain or snow．Barlow and Clark add, 'Unless we dramatically change our ways, between one-half and two-thirds of humanity will be living with severe freshwater shortages within the next quarter-century.'
Humans share the Earth with other creatures who also need water, therefore, a water shortage is also a cr for wildlife．Of the 25-biodiversity hot spots designated by Conservation International, 10 are located in water-short regions.
Fresh Water in Man's Life
Sy percent of all the fresh water is used for irrigation.
? Agriculture uses the largest amount of freshwater (70%).
? Forty percent of the world grain harvest is produced on irrigated land, therefore, a water shortage will become a food shortage.
? Countries are importing grain as a way to import water．It takes 1,000 tons of water to grow one ton of grain．On the other hand, exporters of grain are exporting water．(The U.S．annual grain exports of 90 million tons of grain represent 90 billion tons of water, an amount that exceeds the 67-billion-ton annual flow of the Missouri River．)
? Producing one ton of grain requires 1,000 tons of water, but producing one ton of beef requires 15,000 tons of water (and nearly that much is required to produce a ton of cotton)．Producing wheat or soybeans requires only 2% of the water required by beef.
Twenty percent of fresh water is used by industry.
As water becomes scarce, demand for water in cities and by industry is satisfied by taking water from a country's agriculture, with imported grain offsetting the shortfall．Conservation programs are not applied to industry.
Ten percent of fresh water is used for residential purposes.
Residential use accounts for 10 percent of fresh water use and about three-fourths of the water demand．Each day in the U.S., more than 4.8 billion gallons of drinking water is flushed down toilets.
Showers account for about 20 percent of total indoor water use．The EPA says that by replacing standard 4.5-gallon-per-minute showerheads with 2.5-gallon-per-minute heads, which cost less than $5 each, a family of four can save approximately 20,000 gallons of water per year．Outdoor residential water use varies greatly, but on average, nationally, lawn care accounts for about 32 percent of the total residential outdoor use．Other outdoor uses include washing automobiles, maintaining swimming pools, and cleaning sidewalks and driveways.
Signs of Stress as the Demand for Fresh Water
Rivers are running dry.
Many major rivers—including the Colorado, Ganges, Indus, Rio Grande, and Yellow—are so over-tapped that they now run dry for part of the year．Freshwater wetlands have shrunk by about half worldwide．In 1972, the Yellow River in China failed to reach the sea for the first time in history．That year it failed on 15 days; every year since, it has run dry for a longer period of time, until in 1997, it failed to reach the sea for more than half a year．
Water tables are falling on every continent.
Aquifer (蓄水池) depletion is a new problem．Water tables are falling from the overpumping of groundwater in large portions of China, India, Iran, Mexico, the Middle East, North Africa, Saudi Arabia, and the United States．India has the highest volume of annual groundwater overdraft of any nation in the world．In most parts of the country, water mining is taking place at