EVAPORATION AT THE SURFACE 141 
highest percentage of water in the first foot lost 
13.30 pounds of water, while the other lost only 8.48 
pounds per square foot. This great difference was 
due no doubt to the fact that direct evaporation 
takes place in considerable quantity only in the upper 
twelve inches of soil, where the sun’s heat has a 
full chance to act. 
Any practice which enables the rains to sink 
quickly to considerable depths should be adopted 
by the dry-farmer. This is perhaps one of the great 
reasons for advocating the expensive but usually 
effective subsoil plowing on dry-farms. It is a very 
common experience, in the arid region, that great, 
deep cracks form during hot weather. From the 
walls of these cracks evaporation goes on, as from 
the topsoil, and the passing winds renew the air so 
that the evaporation may go on rapidly. (See Fig. 
33.) The dry-farmer must go over the land as often 
as needs be with some implement that will destroy 
and fill up the cracks that may have been formed. 
In a field of growing crops this is often difficult to 
do; but it is not impossible that hand hoeing, ex- 
pensive as it is, would pay well in the saving of soil 
moisture and the consequent increase in crop yield. 
How soil water reaches the surface 
It may be accepted as an established truth that 
the direct evaporation of water from wet soils occurs 
