MOVEMENT OF WATER IN SEMI-ARID REGIONS 597 



when its function would be to temper vorable than in the smaller eastern por- 



the atmosphere, form dews, etc. ; prob- tion. Such are the primary natural 



ably another quarter would flow into conditions ; yet, especially in deserts, 



the seas, and might be utilized for navi- both nature and man have improved on 



gation, power, etc. ; while the greater these conditions in ways affording 



part of the remaining quarter might be greater promise to prospective human 



utilized in vital and chemical functions development than can be found in any 



(including plant growth), all subject to other direction — the promise that the 



more or less definite human control. desert shall blossom as the rose, and 



Throughout the western three-fifths that the people of our vast semi-arid 



of the country the precipitation is far regions shall in good time guide the 



less than that required for the highest ship of state and direct our moral 



organic growth — except in so far as emprise. 



human genius and creative capacity can In a state of nature, the running 



improve on the processes of nature, waters of arid regions comport them- 



Here probably no more than a tenth of selves differently from those of 



the rainfall runs back to the sea ; ex- humid lands. In the latter it is the 



eluding the northwesternmost districts, law of running water to gather into 



in which the precipitation is high, prob- streams which make for themselves 



ably no more than a twentieth so leaves channels and gradually shape the entire 



the land. The evaporation is larger land in forms permitting the water to 



than in humid lands, probably exceed- flow down to the sea with greater ease 



ing three-fifths and possibly three- and swiftness. Not so in arid districts ; 



fourths of the rainfall ; it would be far here it is the law of running water not 



greater were there more water to evapo- to gather into streams, but to spread 



rate ; and the quantity evaporated is not into sheets facilitating soakage and 



enough to temper the atmosphere com- evaporation into the thirsty soil and air, 



pletely, so that dews are scant and the so that the water tends to remain on 



daily range of the thermometer is wide, the land instead of flowing seaward ; 



Probably less than a tenth, perhaps no and during the ages these sheet-floods 



more than a twentieth, of the total rain- tend so to shape the land surface as 



fall remains available for the most im- progressively to reduce the run-off 



portant function of all, i. e., the suste- seaward, and progressively increase the 



nance of organisms, including plants, ratio of water retained. It is this pe- 



animals, and men. culiar law of running water, coupled 



Neglecting the loss by deep seepage with the slow but ever acting work of 

 and a few other minor factors, the the winds that yields the distinctive 

 great functions of water are (i) evapo- forms of arid landscapes — those combi- 

 ration to temper the atmosphere; (2) nations of far-stretching plains with 

 returning to the sea, thereby removing rugged and precipitous ranges wide- 

 excess of earth-salts and affording fa- scattered between, giving the effect of 

 cilities for navigation and power ; and lost mountains buried to shoulders in 

 (3) entering into chemical combina- their own debris. At last railway en- 

 tions largely in organic growth. The gineers are learning the law of the 

 portions of the water involved in these desert water — they are finding that to 

 functions are sometimes denoted the make culverts useful on the gently 

 fly-off, the run-off, and the cut-off ; and sloping plains they must build out 

 other things equal the benefit to wing-dams to concentrate the sheet- 

 plants, animals, and men, is proportion- floods, which, on passing the obstruc- 

 ate to the cut-off. Now, in all portions tions, quickly spread again Such is 

 of the country the fly-off and the run- nature's device in arid regions for re- 

 off are greater, and the cut-off propor- taining water on the land rather than 

 tionately less, than required for highest throwing it quickly back into the sea ; 

 productivity ; and in the larger western plants and animals have unwittingly 

 portion the distribution is much less fa- learned of it ; and it behooves mankind 



