4 THE FUTURE OF ARID LANDS 
TABLE 1 
Area of Arid Lands Based on Vegetation 
Semi-arid 
Sclerophyll brushland I, 180,000 
Thorn forest 340,000 
Short grass 1,200,000 
Total semi-arid land — 2,720,000 
Arid 
Desert grass savanna 2,300,000 
Desert grass-desert shrub 10, 600,000 
Total arid land a 12,900,000 
Extreme arid 
Desert 2,430,000 
Total extreme arid — 2,430,000 
Total 18,050,000 
Rercentas 35 (Land area 15,970,000) 
safely to utilize this area. Moreover, the methods of use best 
suited to arid lands are the safest means of utilizing the semi- 
arid lands. 
The area of the arid lands may be estimated on the basis of 
vegetation, of climate, of soils, and by the area of interior drainage. 
Estimates based on vegetation, expressed in square miles, are 
given in Table 1. 
The arid zone is designated sharply by vegetation. All plants 
are adjusted to drought conditions, are either xerophytes or the 
short-lived annuals. There are long rest periods during which 
there is little or no growth. The open plant cover with much of the 
soil surface bare is a characteristic of arid zone vegetation. How- 
ever, drought rest periods are characteristic of many of the 
grasslands and monsoon forests beyond the boundary of the arid 
zone. Much of the tall grass or prairie and the tall grass and high 
grass savannas in the tropics have drought rest periods late in 
the season which stop growth and prepare these areas for the 
characteristic fires which sweep over them. However, drought is 
not the chief characteristic, and the plants are not xerophytes but 
are mesophytes. 
The extreme arid, the arid, and the semi-arid climates of the 
earth have been mapped by Meigs (11), using the method de- 
veloped by Thornthwaite (18). These estimates, expressed in 
square miles, have been supplied by Dr. Meigs and are shown in 
ablear 
