The USGS Water Science School
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.
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.
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
|Amount of water
|1 acre||.00156||.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 trillion gallons||232,700 trillion 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 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|
|New Orleans, LA||180.6||3,139|
|New York, NY||303.3||5,271|
|Salt Lake City, UT||109.1||1,906|
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.
The following equivalents show the relationship between the volume and weight of water and between the volume and speed of flowing water.
Information on this page is from Rain, A Water Resource (Pamphlet), U.S. Geological Survey, 1988
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