24 



DESERT BOTANICAL LABORATORY 



which the rainfall is markedly less than the amount of water that evap- 

 orates from the surface of this liquid in the open air. As the 

 amount of evaporation naturally increases from the polar regions 

 toward the tropics and is affected by winds and elevation it follows 

 that no arbitrary amount of rainfall may be designated as an invari- 

 able cause or accompaniment of arid or desert conditions. Thus in cer- 

 tain portions of the tropics a rainfall of less than 70 inches results in 

 aridity, while some of the most fertile agricultural districts in the north 

 and south temperate zones receive scarcely one third this amount. 



Regions in which precipitation is less than evaporation are charac- 

 terized by a lack of running streams, or of a permanent run-off, al- 

 though in some instances these districts may be traversed by large 

 rivers which have their sources in distant mountain ranges as in the 

 case of the Nile in Africa and of the Colorado river in America. The 

 rainfall in a desert may be so heavy at certain seasons as to produce 

 torrents of great volume, which rushing downward over the slopes 

 and mountain sides, wear distinct streamways extending out into the 

 plains below in some instances for miles ; but the flow soon ceases after 

 the rains have passed and the stream beds become dusty channels until 

 the next rainy season. Striking examples of such streamways are to be 

 seen in the great Sonoran desert in northwestern Mexico. It is evident 

 that districts in which the average rainfall is not much greater than the 

 evaporation are in a very critical condition, since in seasons of mini- 

 mum precipitation the amount of water received may be less than that 

 lost, and drought may result, often with direful effects on agricultural 

 operations and economic conditions in general. 



The seasonal distribution of the rainfall is a matter of importance in 

 regions where evaporation is nearly as great as precipitation. If the 

 rainfall occurs within a brief period the remainder of the year must be 

 extremely dry and the region will show distinct desert conditions with 

 a tendency on the part of the native plants to develop marked storage 

 capacity for water. The distribution of the scanty rainfall throughout 

 the year in any region will favor the development of slowly growing 

 xerophytic forms. These factors are further considered on page 28. 



Arid deserts occur in all of the great land divisions and reach an 

 enormous extent in Africa, Asia, and Australia. The most pronounced 

 desert conditions of South America are found on the western slopes and 

 benches of the Andes, one locality, that of Copiapo, having an average 

 precipitation of 0.4 inch per year, and, so far as known, is the driest 

 spot on the earth's surface. The deserts of North America are confined 

 to the Cordilleran region and occupy plateaus and plains east and west 

 of the main ranges to an extent of more than a million square miles. 



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 N. C, State College 



