430 



SCIENCE 



[N. S. Vol. XXXIV. No. 875 



" duty of water " considered in relation to 

 human population may be stated roughly as 

 the maintenance of a human life a year for 

 each 5 acre-feet used effectively in agriculture. 



Now mainland United States {i. e., the 

 chief body of our territory, exclusive of 

 Alaska and the insular possessions) comprises 

 something over 3,000,000 square miles, or 

 somewhat less than 2,000,000,000 acres of 

 land; yet the annual rainfall — the sole orig- 

 inal source of fresh water — averages barely 

 2 J feet (30 inches), or hardly 5,000,000,000 

 acre-feet. So while the land area, if peopled 

 to the density of Belgium (over 640 per 

 square mile), would carry a population of 

 2,000,000,000, the water supply suffices for only 

 1,000,000,000. 



Of course all these figures are but approxi- 

 mations ; yet they indicate that the method of 

 measuring capacity for population in terms 

 of land area is adapted only to countries in 

 which the water supply is ample, and that in 

 this and most other countries estimates can 

 safely be based only on the quantity of water 

 available for the production of those staples 

 of life used in food and clothing. Water is 

 indeed the primary resource. In plant life it 

 is essential to germination, to tissue-making, 

 to all growth; and far the greater part of the 

 average growing plant consists of water, 

 chiefly in circulation. For men and other 

 animals water is the leading food; the average 

 human ration is some 6 pounds daily, 4J 

 liquid and IJ nominally solid, but actually 

 more than one third water — i. e., fully five 

 sixths of the sustenance (and indeed a like 

 proportion of the bodies) of human beings is 

 water. Within the body there is no assimila- 

 tion or metabolism in the absence of water, 

 nor does germination or any other vital proc- 

 ess take place without it or apparently other- 

 wise than as a manifestation of its inherent 

 properties. The measure of water is the 

 measure not merely of productivity but of 

 vitality; and disregarding other climatal fac- 

 tors, the habitability of every country on the 

 globe is determined by the presence or absence, 

 and finally by the quantity, of water distilled 



from the oceans, circulating through the at- 

 mosphere, and descending on the land. 



Considered in relation to natural water sup- 

 ply, mainland United States comprises three 

 divisions : (1) the humid section, or eastward 

 states — 31 in number — extending from the 

 Minnesota-Louisiana tier to the Atlantic, com- 

 monly viewed as the chief part of the country 

 though forming only two fifths of its area; 

 (2) the sub-humid section, or 6 median states 

 from the Dakotas to Texas, containing a fifth 

 of the area of the country; and (3) the semi- 

 arid section, or westward states — 11 in num- 

 ber, including Arizona and New Mexico — 

 making up the remaining two fifths of the 

 territory. 



Over the humid section the mean annual 

 rainfall ranges from about 25 inches in Min- 

 nesota to 55 in Mississippi and over 70 in the 

 southern Appalachians, averaging some 48 

 inches, or four fifths that required for full 

 productivity. In round figures, the 800,000,- 

 000 acres receive annually over 3,000,000,000 

 acre-feet of rainfall, or nearly two thirds of 

 the entire supply of the country, and now sus- 

 tain a population of 75,000,000. The pros- 

 pective population, reckoned on the basis of 

 5 acre-feet of water supply per capita an- 

 nually, may reach 600,000,000, or 8 times that 

 of the present ; so far as may be foreseen, that 

 population could best be sustained by inten- 

 sive cultivation to such degree that each ten- 

 acre lot would yield materials for food and 

 clothing for a family of five direct producers, 

 and perhaps an equal number of urban resi- 

 dents living by secondary production or inci- 

 dental industries. 



Over the 400,000,000 acres comprised in the 

 median states the rainfall averages scant 30 

 inches, or half the water required for full 

 productivity (though as shown by Gannett 

 from 60 per cent, to over 80 per cent, of it 

 falls during the six summer months). While 

 adapted only to extensive agriculture, the ca- 

 pacity of this section for production of staples 

 is far beyond the present yield; if the entire 

 water supply (including the natural sub-irri- 

 gation from the Rocky Mountains) were ef- 

 fectively used, it would sustain a family to 



