CONDITIONS CONTRIBUTORY TO DESERTS. gi 
CONDITIONS CONTRIBUTORY TO DESERTS. 
The term desert may be applied to areas of the earth’s surface which 
support a sparse vegetation of a more or less specialized character, owing 
to inadequate rainfall, or to the unsuitable composition of the soil, while 
the two may be present in such combination that limited areas may be 
devoid of any but possibly microscopic forms of plants. Of these factors 
scanty water-supply may be regarded as of the greatest importance, and 
it is to this that most deserts owe their existence. The essential and 
resultant features include undeveloped drainage, activity of wind erosion, 
great diurnal variation in temperatures, low relative humidity, and nearly 
humus-free soils, with comparatively small vertical increase in the propor- 
tion of soil moisture. Landscapes of this character are readily recognizable 
by the xerophytic aspect of the vegetation inhabiting them. 
Desert conditions come about in any region in which the rainfall 
is markedly less than the amount of water that might evaporate from a 
free water surface in the openair. Asthe amountof evaporation increases 
from the poles toward the tropics and is affected by winds, it follows that 
no arbitrarily fixed amount of rainfall may be designated as an invariable 
cause of aridity. Thus in certain portions of the tropics a rainfall of 
less than 70 inches results in aridity, while some of the most fertile agri- 
cultural 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, although 
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 vol- 
ume, 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 minimum 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 condi- 
tions 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 
