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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.

Picture showing a rainstorm in Manitoba, Canada. Rain does not fall in the same amounts throughout the world or even in different parts of the United States. Here in Georgia it rains fairly evenly all during the year (especially during weekends, holidays, and picnics), around 40-50 inches per year. But the rain we get in one month might be more than Las Vegas, Nevada gets all year. The world's record for rain belongs to Mt. Waialeale, Hawaii, where it averages about 450 inches per year, and it even received 642 inches during one twelve-month period (that's almost 2 inches every day!). Contrast that to Iquique, Chile, where no rain fell for 14 years (no mosquito problem, at least).

On average, the 48 continental United States receives enough precipitation in one year to cover the land to a depth of 30 inches. That works out to about 1,430 cubic miles of water, which would have a weight (at about 62.4 pounds per cubic foot) or about 6.6 billion tons. The United States is fortunate to be a country where ample rainfall occurs over much of the land area.

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.

Amount of water received when an inch of rain occurs
Area	Area (square miles)	Area (square kilometers)	Amount of water (gallons)	Amount of water (liters)
1 acre	.000156	.004	27,154 gallons	102,789 liters
1 square mile	1	2.6	17.38 million gallons	65.78 million liters
Washington, DC	61.4	159	1.07 billion gallons	4.04 billion liters
United States	3,537,438	9,161,922	61,474 billion gallons	232,700 billion liters

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.

Amount of water received when an inch of rain occurs
City	Area (square miles)	Amount of water (million gallons)
Atlanta, GA	131.7	2,289
Baltimore, MD	80.8	1,404
Chicago, IL	227.1	3,947
Cincinnati, OH	78.0	1,356
Denver, CO	153.4	2,666
Detroit, MI	138.8	2,412
Honolulu, HI	85.7	1,489
Houston, TX	579.4	10,069
Jacksonville, FL	757.7	13,168
Louisville, KY	62.1	1,079
Milwaukee, WI	96.1	1,670
New Orleans, LA	180.6	3,139
New York, NY	303.3	5,271
Philadelphia, PA	135.1	2,348
Salt Lake City, UT	109.1	1,906
Seattle, WA	83.9	1,458
Washington, DC	61.4	1,067

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). 1 inch of rain falling on 1 acreis equal to about 27,154 gallons of water, and 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:

Are raindrops tear-shaped?

Why are raindrops different sizes?

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

Can you guess how many baths you can get from a rainstorm?
Visit our Activity Center and find out.

Sources and more information

• Rain: A Water Resource, USGS water-information pamphlet
• The water cycle: Precipitation

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U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu//earthrain.html
Page Contact Information: Howard Perlman
Page Last Modified: Wednesday, 13-Aug-2008 07:19:05 EDT