MOVEMENT OF WATER IN SEMI-ARID REGIONS 



597 



when its function would be to temper 

 the atmosphere, form dews, etc.; prob- 

 ably another quarter would flow into 

 the seas, and might be utilized for navi- 

 gation, power, etc. ; while the greater 

 part of the remaining quarter might be 

 utilized in vital and chemical functions 

 (including plant growth), all subject to 

 more or less definite human control. 



Throughout the western three-fifths 

 of the country the precipitation is far 

 less than that required for the highest 

 organic growth except in so far as 

 human genius and creative capacity can 

 improve on the processes of nature, 

 Here probably no more than a tenth of 

 the rainfall runs back to the sea; ex- 

 eluding the northwesternmost districts, 

 in which the precipitation is high, prob- 

 ably no more than a twentieth so leaves 

 the land. The evaporation is larger 

 than in humid lands, probably exceed- 

 ing three-fifths and possibly three- 

 fourths of the rainfall ; it would be far 

 greater were there more water to evapo- 

 rate ; and the quantity evaporated is not 

 enough to temper the atmosphere com- 

 pletely, so that clews are scant and the 

 daily range of the thermometer is wide, 

 Probably less than a tenth, perhaps no 

 more than a twentieth, of the total rain- 

 fall remains available for the most im- 

 portant function of all, i. e., the suste- 

 nance of organisms, including plants, 

 animals, and men. 



Neglecting the loss by deep seepage 

 and a few other minor factors, the 

 great functions of water are (i) evapo- 

 ration to temper the atmosphere; (2) 

 returning to the sea, thereby removing 

 excess of earth-salts and affording fa- 

 cilities for navigation and power; and 

 (3) entering into chemical combina- 

 tions largely in organic growth. The 

 portions of the water involved in these 

 functions are sometimes denoted the 

 fly-off, the run-off , and the cut-off ; and 

 other things equal the benefit to 

 plants, animals, and men. is proportion- 

 atc to the cut-off. \o\\- . in all portions 

 of the country the fly-off and the run- 

 off are greater, and the cut-off propor- 

 tionately less, than required for highest 

 productivity: and in the larger western 

 portion the distribution is much less fa- 



vorable than in the smaller eastern por- 

 tion. Such are the primary natural 

 conditions; yet, especially in deserts, 

 both nature and man have improved on 

 these conditions in ways affording 

 greater promise to prospective human 

 development than can be found in any 

 other direction the promise that the 

 desert shall blossom as the rose, and 

 that the people of our vast semi-arid 

 regions shall in good time guide Un- 

 ship of state and direct our moral 

 emprise. 



In a state of nature, the running 

 waters of arid regions comport them- 

 selves differently from those of 

 humid lands. In the latter it is the 

 law of running water to gather into 

 streams which make for themselves 

 channels and gradually shape the entire 

 land in forms permitting the water to 

 flow down to the sea with greater ease 

 and swiftness. Not so in arid districts ; 

 here it is the law of running water not 

 to gather into streams, but to spread 

 into sheets facilitating soakage and 

 evaporation into the thirsty soil and air, 

 so that the water tends to remain on 

 the land instead of flowing seaward ; 

 and during the ages these sheet-floods 

 tend so to shape the land surface as 

 progressively to reduce the run-off 

 seaward, and progressively increase the 

 ratio of water retained. It is this pe- 

 culiar law of running water, coupled 

 with the slow but ever acting work of 

 the winds that yields the distinctive 

 forms of arid landscapes those combi- 

 nations of far-stretching plains with 

 rugged and precipitous ranges wide- 

 scattered between, giving the effect of 

 lost mountains buried to shoulders in 

 their own debris. At last railway en- 

 gincers are learning the law of the 

 desert water they are finding that to 

 make culverts useful on the gently 

 sloping plains they must build out 

 wing-dams to concentrate the sheet- 

 floods, which, on passing the obstruc- 

 tions. quickly spread again Such is 

 nature's device in arid regions for re- 

 taining water on the land rather than 

 throwing it quickly hack into the sea: 

 plants and animals have unwittingly 

 learned of it : and it behooves mankind 



