WHAT'S HAPPENING TO WATER? — ROBINOVE 377 



air is lifted high over the mountains and becomes cooler. The cool 

 air cannot contain as much water vapor as warm air and so the 

 moisture falls as rain or snow on the w^estern slopes of the coastal 

 ranges and the Sierra Nevada. East of the coastal ranges and the 

 Sierra Nevada the air is drier ; there is less moisture to precipitate and 

 fall; and as a result, the region between the coastal areas and the 

 Rocky Mountains is arid or semiarid. The lifting and drying effect 

 of the continental mass is again shown as the air reaches the Rocky 

 Mountains, Precipitation is greatest on the western slopes of the 

 Roclcy Mountains; the drier air moves across the mountains to the 

 eastern slope, where the precipitation is less. 



Precipitation in the central part of the United States increases as 

 additional air masses moving from the Gulf coast, the Atlantic, and 

 the Arctic region bring moisture-laden air over the continent. 



Water is abundant in the United States. If all the precipitation 

 that fell within the limits of the 48 conterminous States during an 

 average year were to be spread evenly over the country, it would 

 stand 30 inches deep. However, as the pattern of air movement 

 indicates, this precipitation is not spread evenly over the United 

 States. Some areas receive only a few inches of rainfall during the 

 year, while others receive as much as 100 inches. 



The water represented by the average 30-inch depth is about 4,800 

 million acre-feet per year, or about 4,400 billion gallons per day. 

 This is an enormous amount of water, but unfortunately not all of it 

 is available for our use. Of the 30 inches of water, about 211/^ inches 

 is evaporated from open water areas or is transpired from the soil and 

 the leaves of plants and thus returned to the atmosphere. Only part 

 of this 211/2 inches supports cultivated crops, native grass, and forests; 

 the rest is evaporated or used by nonbeneficial plants. 



The remaining 8i/^ inches of rainfall moves over the ground to 

 streams as "direct runoff," or seeps to the water table to become 

 "ground water," later to discharge into streams as "ground-water 

 runoff." Of the 81/^ inches, man withdraws from streams, lakes, 

 reservoirs, springs, and wells the equivalent of about 2 inches (but in 

 part this represents the same water used over again) and uses it for 

 municipal and rural water supplies, industry, and irrigation. About 

 half an inch evaporates or is transpired, in part as a result of the 

 activities of man (principally irrigation) and in part by natural 

 means in the Great Basin. The remainder joins the "unused" water 

 to make a total of about 8 inches flowing into the oceans. Actually, 

 the "unused" water is used too, though not "withdrawn" — for hydro- 

 power (to an extent equivalent to nearly twice the average stream- 

 flow), for dilution of sanitary and industrial wastes, for navigation, 

 and for recreation and fish and wildlife. 



