The Supply of Water 



In an average year, about 40 trillion gallons a day 

 pass over the conterminous United States as water 

 vapor. About 10 percent is precipitation in the form 

 of rain, snow, sleet, or hail, which equals an average 

 annual amount of 30 inches nationwide. About two- 

 thirds of this precipitation returns to the atmos- 

 phere via evaporation and transpiration. The re- 

 maining 1.4 trillion gallons a day of precipitation 

 (average of 9 inches) flows to the ocean or across 

 U.S. boundaries, accumulates in storage, or is con- 

 sumptively used. 



Precipitation is enough to meet current and pro- 

 jected needs if it were available for use where and 

 when needed. However, there is wide variation in 

 precipitation by region. The normal annual precip- 

 itation over the contiguous States generally ranges 

 from an average of less than 4 inches in parts of 

 Great Basin and Lower Colorado regions to more 

 than 200 inches in coastal areas of the Columbia- 

 North Pacific Region (fig. 7.4). There are specific 

 localities that even fall outside this range. About 26 

 inches of the total of 30 is from rainfall; the re- 

 mainder is snow or other frozen form. The area east 

 of the Mississippi River averages about 18 inches. In 

 the Alaska region, the normal annual precipitation 

 ranges from about 5 inches in the extreme north to 

 more than 200 inches in the southeast, with a State 

 average of about 20 inches. 



A large portion of the precipitation in the United 

 States falls on forested land because forests are 

 typically located at higher elevations, initially cap- 

 turing and gradually releasing water to downstream 

 areas. Also, forest and range vegetal cover usually 

 provides excellent protection for streams because it 

 maintains good water quality and helps stabilize 

 flow. 



Just as precipitation varies greatly from place to 

 place, season to season, and year to year, so do runoff 

 and streamflow. For example, even in a normal 

 year, the ratio of maximum flows to minimum flows 

 may be 500 to 1 or greater. As a result, adverse 

 impacts of drought are intensified, especially in 

 areas that use a high proportion of normal stream- 

 flow or where storage is minimal. The range in 

 variation in streamflow in the humid East tends to be 

 less from year to year and from month to month than 

 in other regions. Average annual runoff based on 

 data from 1931 to 1960 is shown in figure 7.5. More 

 than 60 percent of the annual runoff originates on 

 forest lands, which comprise about one-third of the 

 total land area. In the 1 1 Western States, more than 

 90 percent of the usable precipitation originates on 

 high-altitude watersheds, which are typically forested. 



In 1975, the conterminous United States withdrew 

 a total of 393 billion gallons per day from surface and 

 ground sources. Of this total, 254 billion was from 

 fresh surface water sources, 58 billion from saline 

 surface sources, and 8 1 billion gallons per day from 

 ground water. Surface and ground sources are gen- 

 erally highly interactive; consequently, significant 

 impacts upon one is likely to affect the other. 



Water supply problems stem from the high varia- 

 tion in both the geographic and temporal distribution 

 of water. Some regions have an abundance of water, 

 while others receive very little precipitation. Still 

 others have problems because precipitation largely 

 occurs during certain seasons so that other parts 

 of the year are very dry. Only a small portion of 

 the potential 1 .4 trillion gallons a day can be devel- 

 oped for intensive use. 



The temporal problem can often be reduced 

 through storage, either in reservoirs or as ground 

 water. Total reservoir storage capacity in the United 

 States is about 700 million acre-feet; about 35 percent 

 of this capacity was built for flood control and the 

 remainder for water supply, hydropower, recreation, 

 fire protection, and esthetics value. 



It is estimated that 100 bilHon acre-feet of ground 

 water is within 2,500 feet of the surface in the con- 

 terminous United States, about 50 percent of this 

 volume is economically and environmentally avail- 

 able. This amount is nearly 150 times the amount 

 of our total reservoir storage capacity, or more than 

 the Mississippi River has discharged into the Gulf 

 of Mexico over the last 200 years. About half of 

 the country is underlain by rock material that could 

 yield at least 50 gallons per minute from wells. 



Ground water also provides the base flow of 

 streams; in some regions, ground water flows pro- 

 vide streams with a continuity of flow that they would 

 not otherwise possess. The water supply information 

 presented in the section on surface water includes 

 considerable water that enters from ground water 

 aquifers. Part of this ground water resource does 

 not get into surface water supply naturally, and can 

 be developed only by drilling. 



The Atlantic and Gulf Coastal Plains contain the 

 largest reserve of ground water in the Nation (fig. 

 7.6). Present pumpage is but a small fraction of 

 the supplies that could be developed. Even so, salt- 

 water encroachment along the Gulf and Atlantic 

 coasts is a limiting factor in ground water develop- 

 ment. 



It is estimated that 100 billion acre-feet of ground 

 water is within 2,500 feet of the surface in the 

 conterminous United States; about 50 percent of 

 this volume is economically and environmentally 

 available. This amount is nearly 150 times the amount 



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