The land of giant refrigerators
Every winter I go skiing on our city's water supply. As I swoosh down the slopes, I marvel at the fact that much of the water we use in the summer - for drinking, bathing, watering the lawn, or washing the car - was put into storage during the winter.
How? Nature does the work.
In the winter, the Rocky Mountains and the Sierra Nevadas become giant refrigerators, jokes Robert Hartman of the California-Nevada River Forecast Center in Sacramento, Calif. These mighty peaks store frozen water in the form of ice and snow, to be used by people in the valleys below all the next summer.
Mr. Hartman is a hydrologist - a person who studies water cycles and predicts the amount of water that will be available in specific regions.
"It's great, all the ways we benefit from snow," he says. "We can play in it, ski on it, enjoy its beauty, and then put it to use when it melts in the spring."
If you're in an area where winters are cold, you might be able to ski or skate on your water supply, too.
In warmer areas, you could be walking over the stored H2O while it remains hidden underground in an aquifer (right) - a layer of porous rock or sand that contains water.
Areas that get cold in the winter have aquifers, too. Some parts of the United States rely on water that is stored in these underground layers during the winter.
Rain and melting snow sink into the ground and often collect in these aquifers. The soil or rock is loosely packed in these places, so water can seep in between the particles.
Out on the great plains, which lack snow-covered mountains, pumps are used to bring this water up to the surface. Other groundwater slowly sinks farther into the ground, until it seeps out into a river or lake.
In the Eastern and Southern states, winter also supplies water that is collected in lakes and ponds. But less storage is needed because rain falls more often during the summer.
Mountain snow is an important source of summer water in the Western part of the United States. Unlike the Eastern regions, where rain falls more frequently during the spring and summer growing seasons, much of the rain and snowfall in the West happen during the winter.
For example, while Tennessee averages more than 48 inches of rain each year, with several inches falling each month, Nevada averages fewer than 8 inches a year.
Most of the precipitation out West falls between October and April. To water crops and provide drinking water in the summer, Western states rely on the snow that gathers on the mountains each winter.
As this snow melts and becomes runoff in the spring and summer, it flows into valleys. As much as 70 percent of the water that flows into the valleys during the year comes from melting snow.
As the population has grown, however, more water has been needed. People realized that a lot of water was running down into lakes or to the ocean that would be helpful to people in other parts of the valleys. So they began building dams along the rivers to help store the water for later use.
The dams hold the water that collects from spring runoff, and it is gradually sent down the river to the people below throughout the summer. Dam construction also provides a few other benefits, such as a new lake above the dam where people can fish and swim.
Dams can also use the water to create electricity. Water falling through the dam turns giant turbines, just the way a waterwheel is turned when water flows through it. So the dam (below) not only stores and provides water, it sends electricity down into the valleys as well.
While dams have been very helpful in supplying water to dry areas, they also bring changes to the environment. That can cause problems that scientists are still working to solve. One spectacular solution was used at the Grand Canyon in November 2004.
In 1956, Glen Canyon Dam was built on the Colorado River, at a point before the river flows through the Grand Canyon. Before the dam was constructed, the river had always carried mud, sand, and silt down through the canyon.
When the river was wild, floods would leave this sediment in sandbars and on the sides of the river, forming beaches and habitat for fish. Because the dam now prevented the natural floods, these materials were no longer being deposited. As a result, the river would carry away sand from the beaches, and some types of fish were disappearing.
In an attempt to restore the natural system, scientists decided to create a man-made flood that would send sediment down the river. Conducted in 1996, this experiment was partially successful.
The scientists tried again last November. They opened up the drain pipes at the dam so that huge amounts of water poured into the river - up to 41,000 cubic feet per second - to create a flood of water and silt. They hoped to move 800,000 tons of sediment down onto the beaches below the dam. The water poured out of the pipes for 90 hours.
Scientists are studying the results of the artificial flood, and so far the results are promising. But more study is needed to find ways to make sure that the dams so helpful to humans do not harm creatures and habitats that depend on the rivers.
Scientists track how much water there will be in the summer by measuring mountain snowfall. They often use the old-fashioned method of poking a yardstick into the snow until it hits the ground, but there are some high-tech snow-measuring tools as well.
Ultrasonic devices that stand about 10 feet above the ground determine snow depth by sending down a pulse. They measure the height of the snow by counting the amount of time it takes the pulse to bounce back from the snow's surface, and transmit this information to a weather station.
But knowing the snow level isn't enough. The snow's density, or weight, is another key factor. Light, fluffy, powder snow (a favorite of skiers) contains very little water. Heavy, wet snow creates much more water when it melts.
The modern way to measure snow density is to weigh it with a snow pillow(left), explains hydrologist Brian McInerney of the National Weather Service office in Salt Lake City, Utah.
Imagine a sealed plastic bag full of jelly, with a straw sticking out. If you pushed on the bag, jelly would squirt up the straw. A snow pillow works on the same principle. Four stainless steel panels are filled with antifreeze. The weight of the snow on the pillow forces the antifreeze up into a pressure transducer, or sensing device. The transducer then determines the snow's density by measuring the antifreeze, and transmits this data to a weather station.
All winter long, scientists carefully monitor snowfall so they will be able to predict drought or floods. Their work helps people wisely use the water stored in their mountainous refrigerators.
• To learn more about weather and climate, go to http://www.education. noaa.gov/students.html