6 THE FUTURE OF ARID LANDS 
sand covered by vegetation, the conclusion might well have been 
reached that 1937 was a less arid year than 1915. 
On the basis of soi/s about 43% of the land area of the earth is 
estimated to be of pedocals. These soils have dry subsoils. The 
moisture received at the surface hangs like a blanket above the 
dry subsoil. Moisture will pass down into the dry soil only if the 
water content of the surface soil has been raised above the field 
carrying capacity. This soil moisture is lost to the air by two 
methods. (1) Soil moisture in the first few inches can be evaporated 
into the air, but moisture does not move up by capillarity to take 
the place of the water lost, and the dry soil, be it of dust or of 
hard soil, protects the moisture below. (2) If there are any growing 
plants on this soil they absorb the soil moisture in contact with the 
roots and pass it out into the air by transpiration. 
The soluble carbonates are carried down by the percolating soil 
moisture but they are not returned to the surface and gradually 
accumulate at the bottom of the moist layer. This carbonate 
layer marks the depth to which the soil is generally moistened by 
the precipitation. In much of the more arid region the carbonates 
are present in the surface soil, but the layer on the more humid 
side of the chernozems may be 3 feet below the surface. Three 
feet of soil would hold enough soil moisture to produce a luxuriant 
grass cover and such an area would not be called arid or semi-arid. 
These pedocals are not leached and are usually productive. The 
plant growth is generally terminated by drought and the dry 
vegetation is often burned off by fires started by man or by 
lightning. Lightning fires have repeatedly been observed in the 
short grass in eastern Colorado. Fire is also a characteristic of the 
sclerophyll brushland of the Mediterranean type where the hot 
and the dry periods come together and rainfall is confined to the 
cool period of the year. 
A world map of interior drainage has been drawn by de Mar- 
tonne and Larfrére (g) which outlines very clearly the arid zone 
as discussed above if we eliminate such areas as the upper Volga 
and the upper Shari which although they do not drain into the 
ocean come from relatively humid areas. The pedocal soils also 
extend beyond the arid zone. 
