THE VOICE OF THE DESERT 



18 



either comparatively or absolutely. The region around 

 Paris, for instance, gets little more than half that amount. 

 Forty inches is, in absolute terms, more than most people 

 imagine. One inch of rain falling on an acre of ground 

 means more than 27,000 gallons of water. No wonder that 

 irrigation in dry regions is quite a formidable task even for 

 modern technology. 



In terms of what vegetation can use, forty inches is 

 ample for the kind of agriculture and natural growth 

 which we tend to think of as "normal." It means luxuriant 

 grass, rapid development of second growth woodland, 

 a veritable jungle of weeds and bushes in midsummer. In 

 inland America the rainfall tends to be less than in the 

 coastal regions. As one moves westward from the Missis- 

 sippi it declines sharply and begins to drop below twenty 

 inches a year at about the one hundredth meridian or, 

 very roughly, at a line drawn from Columbus, Ohio 

 through Oklahoma City. This means too little water for 

 most broad-leaved trees and explains why the southern 

 Great Plains were as treeless when the white man first saw 

 them as they are today. 



Our true deserts — the Great Basin Desert in Utah and 

 Nevada, the Chihuahuan in New Mexico, the Sonoran in 

 Arizona, and the Mohave in California — all lie still further 

 to the west. The four differ among themselves but they 

 are all dry and hot and they all fulfill what is probably 

 the most satisfactory definition of "desert" — namely a re- 

 gion where the ground cover is not continuous; where, 

 that is, the earth remains bare of vegetation between such 

 plants as manage to grow. Over these American desert:s 

 the rainfall varies considerably and with it the character 

 and extent of the vegetation. In southern Arizona, for in- 



