NATURAL MOVEMENT OF WATER IN 

 SEMI^ARID REGIONS* 



By W J MCGEE, LL,D 

 Secretary, U. S. Inland Waterways Commission 



THE average annual rainfall on 

 mainland United States is esti- 

 mated at thirty inches ; the aggre- 

 gate at 200,000,000,000,000 cubic feet. 

 This is the yearly boon from the heav- 

 ens, the virtual source of every indus- 

 try, indeed of habitability. Without it 

 the land would be bare rock or sun- 

 scorched sand ; the lower atmosphere 

 without the tempering influence of the 

 moisture would range from below 

 freezing to boiling temperature from 

 night to day ; and no living thing could 

 exist. 



This annual water supply, as our 

 primary resource, may be counted our 

 capital. Without it no other resource 

 would possess the slightest value. Of 

 it the amount varies slightly from year 

 to year; yet there is no more. It may 

 be used or wasted ; in the end, if we are 

 wise, it will all be used^ regulated, 

 and controlled in such manner as to 

 yield the greatest good to the greatest 

 number for the longest time. 



Taking mainland United States as a 

 whole, our 200,000,000,000,000 cubic 

 feet of liquid capital may be expressed 

 as ten Mississippis. About one-fifth of 

 all, equivalent to two Mississippis, flows 

 through waterways into the sea, while 

 five or six Mississippis in quantity are 

 evaporated to temper the air, form 

 dews, and in part redescend as rains or 

 snows. A part of the remainder seeps 

 into the soil below the surface, or per- 

 meates deeper rocks ; and it may be es- 

 timated that something like a tenth of 

 the whole, or an equivalent of a Mis- 

 sissippi, is utilized in plant growth and 

 in animal assimilation. It is this frac- 



tion which is of most immediate and 

 general use to mankind. 



The distribution of the natural sup- 

 ply varies with seasons ; it varies still 

 more widely with regions. Approxi- 

 mately two-thirds of the aggregate 

 amount falls on the eastern two-fifths 

 of the country ; the remaining third on 

 the lion's share of our territory lying 

 west of the ninety-fifth meridian — for 

 the average west of that meridian is 

 only about eighteen inches, while that 

 of the eastern fraction is some forty- 

 eight inches. Within the larger west- 

 ern area the range is greater than in 

 the east — from over sixty inches on the 

 northwestern coast down to less than 

 one-tenth of that in certain arid spots 

 of the southwest. The distribution by 

 what may be called function is also 

 wider in the larger western area. 



In defining the natural functions of 

 the water resulting from rainfall, it 

 should be noted that scarcely anywhere 

 in the United States is the rainfall ex- 

 cessive, i e., greater than can be used 

 beneficially by growing plants, living 

 animals, and intelligent men (as is the 

 case in certain tropical countries) ; 

 nearly everywhere it falls below the 

 aggregate best adapted to organic de- 

 velopment. Thus eastern United 

 States might produce more abundant 

 crops, develop greater industries, and 

 sustain a larger population with a well- 

 distributed rainfall of sixty inches than 

 it is able to do with the mean precipi- 

 tation of forty-eight inches. With a 

 rainfall of sixty inches, and with other 

 climatical features as they are, probably 

 half of the aggregate would evaporate. 



*Read before the National Irrigation Congress at the sixteenth session, held in Albu- 

 querque, September 23-29, 1908. 

 596 



