Rain: A Water Resource

Water is vital to existence on our planet. Fortunately, it 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.

By contrast, the conterminous (48) United States receives enough precipitation during an average year to cover the States to a depth of about 30 inches. This is equivalent to about 1,430 cubic miles of water each year and would weigh about 6.6 billion tons. What happens to the water after it reaches the ground depends upon many factors such as the rate of rainfall, topography, soil condition, density of vegetation, temperature, and the extent of urbanization.

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.

The following table lists several major cities in the United States and the approximate amount of water each would receive in an evenly distributed inch or rain.

*Areas within corporate limits of cities based upon Bureau of Census 1969 Statistics.

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 people or about 4 times the present population of Atlanta.

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.

• 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.

This information is from USGS Water Science for Schools.