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Rain: A valuable resource

 Care to take a guess at how many gallons of water fall when 1 inch of rain falls on 1 acre of land?

Rain and snow are key elements in the Earth's water cycle, which is vital to all life on Earth. Rainfall is the main way that the water in the skies comes down to Earth, where it fills our lakes and rivers, recharges the underground aquifers, and provides drinks to plants and animals.

Fortunately for everyone, water is a renewable resource that moves in a cycle with neither beginning nor end. Water vapor (evaporated from oceans, lakes, forests, fields, animals, and plants) condenses and returns to Earth as precipitation, once again replenishing reservoirs, lakes, rivers, and other sources of water and providing the moisture required by plants and animals.s

The amount of precipitation that falls around the world may range from less than 0.1 inch per year in some deserts to more than 900 inches per year in the tropics. One of the driest spots on Earth is Lquique, Chile, where no rain fell for a period of 14 years. The world's wettest spot, as shown by data collected from a rainfall gage operated by the U.S. Geological Survey, is on Mt. Waialeale, Hawaii, where an average of more than 451 inches of rain falls each year, and where more than 642 inches fell from July 1947 to July 1948. Although Mt. Waialeale averages slightly more rain per year, Cherrapunji, India, holds the single year record of 905 inches measured in 1861.

For example, the direct runoff in a highly urbanized area is relatively great, not only because of the density of roofs and impermeable pavements permits less rain to infiltrate the ground, but also because storm-sewer systems carry more water directly to the streams and lakes. In a more natural or undeveloped area, the direct runoff would be considerable less.

In the United States, an average of some 70 percent of the annual precipitation returns to the atmosphere by evaporation from land and water surfaces and by transpiration from vegetation. The remaining 30 percent eventually reaches a stream, lake, or ocean, partly by overland runoff during and immediately after rain, and partly by a much slower route through the natural ground-water reservoir.

Much of the rain that enters the ground filters down into subsurface water-bearing rocks (aquifers) and eventually reaches lakes, streams, and rivers where these surface-water bodies intercept the aquifers. The portion of the precipitation that reaches the streams produces an average annual streamflow in the United States of approximately 1,200 billion gallons a day. By comparison, the Nations's homes, farms, and factories withdraw and use about 400 billion gallons a day.

How much water falls during a rainstorm?

Have you ever wondered how much water falls onto your yard during a rainstorm? Using a 1-inch rainstorm as an example, the table below gives example of how much water falls during your storm for various land areas.

There are 640 acres in a square mile.

Once on the land, rainfall either seeps into the ground or becomes runoff, which flows into rivers and lakes. What happens to the rain after it falls depends on many factors such as:

• The rate of rainfall - A lot of rain in a short period tends to run off the land into streams rather than soak into the ground.
• The topography of the land - Topography is the lay of the land -- the hills, valleys, mountains, and canyons. Water falling on unlevel land drains downhill until it becomes part of a stream, finds a hollow place to accumulate, like a lake, or soaks into the ground.
• Soil conditions - There is a lot of dense clay in the southeastern United States that rain has a hard time soaking into. Contrast that to the sandy soils in more desert areas, which allow water to quickly be absorbed, at least initially.
• Density of vegetation - It has long been known that plant growth helps decrease erosion caused by flowing water. If you look at hills without vegetation you'll see gullies dug out by running water. Land with plant cover slows the speed of the water flowing on it and thus helps to keep soil from eroding.
• Amount of urbanization - As a city is being built, a lot of money and construction goes into moving water out of built-up areas. Roads, pavement, and parking lots create impervious areas where water can no longer seep into the ground. Rather, water is funneled into creeks and streams that were never meant by nature to handle so much runoff. This can cause problems in urban areas.

The table below gives example of how much water (in millions of gallons) falls within the city limits of selected cities when one inch of rainfall occurs.

City areas are provided by the U.S. Census Bureau, County and City Data Book: 2000, Table C-1 (http://www.census.gov/statab/ccdb/cit1010r.txt).

Consider for a moment how much rainwater some cities may receive during a year. for example, Atlanta, Ga. averages about 45 inches of precipitation per year; multiplying this by the 2.36 billion gallons shown in the table as the number of gallons in 1 inch reveals that some 106 billion gallons of water fall on Atlanta in an average year. In a city the size of Atlanta, the per capita water use is about 110 gallons per day or 40,150 gallons per year. Thus, the water from a year's precipitation, if it could be collected and stored without evaporation loss, would supply the needs of about 2,640,000.

Water Equivalents (approximate)

The following equivalents show the relationship between the volume and weight of water and between the volume and speed of flowing water.

Volume and weight

• One inch of rain falling on 1 acre of ground is equal to about 27,154 gallons and weighs about 113 tons.
• An inch of snow falling evenly on 1 acre of ground is equivalent to about 2,715 gallons of water. This figure, however, based upon the "rule-of-thumb" that 10 inches of snow is equal to 1 inch of water, can vary considerable, depending on whether the snow is heavy and wet, or powdery and dry. Heavy, wet snow has a very high water content—4 or 5 inches of this kind of snow contains about 1 inch of water. Thus, an inch of very wet snow over an acre might amount to more than 5,400 gallons of water, while an inch of powdery snow might yield only about 1,300 gallons.
• One acre-foot of water (the amount of water covering 1 acre to a depth of 1 foot) equals 326,000 gallons or 43,560 cubic feet of water, and weighs 2.7 million pounds.
• One cubic mile of water equals 1.1 trillion gallons, 147.2 billion cubic feet, or 3.38 million acre-feet, and weighs 9.2 trillion pounds (4.6 billion tons).

Rate of flow (in a stream)

• Water flowing at the steady rate of 1 gallon per minute is equivalent to: 1,440 gallons per day; 0.00223 cubic foot per second; 192.7 cubic feet per day; or 0.00442 acre-foot of water per day.
• Water flowing at the steady rate of 1 cubic foot per second is equivalent to: 449 gallons per minute; 646,000 gallons per day; 86,400 cubic feet per day; or 1.98 acre-feet of water per day.
• Water flowing at the steady rate of 1 acre-foot per day is equivalent to: 226 gallons per minute; 326,000 gallons per day; 0.504 cubic foot per second; or 43, 560 cubic feet of water per day.
• Water flowing at the steady rate of 1 cubic mile per day is equivalent to: 764.6 million gallons per minute; 1.1 trillion gallons per day; 1.7 million cubic feet per day; or 3.38 million acre-feet of water per day.

Information on this page is from Rain, A Water Resource (Pamphlet), U.S. Geological Survey, 1988

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Sources and more information

• The water cycle: Precipitation

Related topics:

100-year floods  Rivers  Stormflows  Streamflow patterns
Measuring streamflow  Impervious surfaces and flooding  Floods Q&A
 Investigate the water cycle (in many languages!)